ALBERT

Notes: The decay rate of gamma radiation emitted by a Jamaican mineral spring was measured using portable rate meters. The results of these experiments, supported by auxiliary tests, suggested that the main radioelement causing the radioactivity was radon 222. Fluctuations in the intensity of the radiation were studied in situ. An inverse relation between the intensity and the discharge rate of water was established and the effect of rainfall and of earthquakes was investigated. Some suggestions are made concerning the possible source of radon.

Notes: The iron ores of the Siegerland-district are found in lodes occurring in schists of Lower Devonian age. In the course of the geophysical exploration for these ores, an attempt was also made to clarify the structure and the tectonics of the substratum of the sedimentary rock formations by seismic reflection measurements from the surface. Reflections were recorded in great numbers from layers of Silurian and Devonian age. The form of the folding of the strata derived therefrom corresponds well with that deduced from geological investigations. Beyond that, correlatable reflections were recorded from horizons down to a depth of nearly 25 000 m. It turned out that the iron ore lodes known from the mine workings occur almost vertically above those places where greater dislocations appear in the deepest substratum, and which geologically could be interpreted as fracture zones. The results so far obtained seem to be indicative of the fact that the tectonics of the deepest substratum and the occurrence of ore deposits are related to each other.

Notes: Precision calibration tests carried out with a Worden gravimeter in environment conditions as varied as possible have led to the following conclusions. The secular variation curve of the calibration shows that the secular relative variation has been very appreciable in the first year of life of the instrument, precisely of 1.3 × 10−3 in 13 months, while it has been practically zero in the second year. This must be ascribed to the fact that at the beginning of the operations the gravity-meter had just arrived from the factory with its measuring body renewed.On the other side the effect of 24°C of temperature difference between two series of determinations reveals itself in a variation of 0.0022 of the value of the calibration, so that it can be established that the variation of the calibration is I‰ for each 10°C of temperature variation.

Notes: Due weight must be given to three factors (1) its purpose, (2) the limitations imposed by nature, industry and environment, (3) the effect of errors in auxiliary measurements. Together the last two factors give an estimate of the accuracy which can be achieved. If this is sufficient for the instrument to fulfil its purpose design and construction can proceed. If it is not, a detailed survey of the last two factors is necessary in order to estimate the research effort that will be required when directed into the most effective channels. If the project is considered worth the effort it can proceed. These factors are discussed in relation to a borehole gravimeter. It is shown that the purpose will be served only if the accuracy is comparable with the best surface gravimeters. Borehole versions of some surface instruments are considered and it is concluded that none is suitable without considerable modifications. A successful borehole gravimeter is likely to be a completely new instrument.

Notes: Recently, quite good determinations of interfaces have been made by means of seismic well surveys. These were carried out for various oil companies in Northwest Germany, especially on salt-domes, and in Southern Germany, and in the area of the Upper Rhine Valley. It was the purpose to determine the configuration and position of salt-flanks and important planes of stratification in the neighbourhood of deep wells, and thus to reduce the risk of expensive wells.For the delineation of the boundary surfaces the method described by Gardner in “Geophysics 1949” was applied.The problems arising and the results obtained are discussed in connection with some interesting examples. From them we see, above all, that a knowledge as precise as possible of the seismic velocities of all media involved in the measurement is very important. A good idea of the general stratification in the area of deep wells seismically surveyed will make possible a rational and adequate planning of the survey program and will facilitate the interpretation.

Notes: A method is described in which the major part of gravimetric terrain corrections may be estimated by digital computing methods.

Notes: The electric and magnetic field of a direct-current element, embedded in a space consisting of two halfspaces differing in conductivity and permeability, is given in a form suitable for integration along a cable. From these formulae is obtained the field when one end of a D.C. cable is grounded in a highly conducting, pencilformed ore body, considered as a line electrode with uniform current leakage. Numerical examples are given for a horizontal line electrode. The formulae should be useful as a first approximation to the A.C. case. The validity of this is shortly discussed.

Notes: An analogy is established between the propagation of an electromagnetic field in a horizontally stratified conductive medium and that of currents in a linear electric network. This analogy will allow us to know in which case measurements executed at the surface of the ground can provide information about the properties of deeper layers. The situation is the same as when one tries, by means of impedance measurements at the input terminals, to know the properties of electric lines or lumped networks.A symmetrical linear network establishes between:Voltages V1 and V2 (respectively at input and output terminals)Currents I1 and I2 (respectively at input and output terminals), the relationship 〈displayedItem type="mathematics" xml:id="mu1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR77:GPR_77_mu1"/〉 is called the “phase constant” and Z the characteristic impedance The table 〈displayedItem type="mathematics" xml:id="mu2" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR77:GPR_77_mu2"/〉 is called “characteristic matrix”If one then takes the horizontal components of an electromagnetic field, one can compare the voltage V with the horizontal electric field E (in volts m-1) and the current I with the horizontal magnetic field H (in Amp m−1)For plane waves in vacuum, one gets (Schelkunoff) Z=C (velocity of light)A layer whose thickness is D has a phase constant⌈= 2 πD/λ (λ= wavelength)For plane waves in a conductive, non magnetic medium whose conductivity is δ, one gets 〈displayedItem type="mathematics" xml:id="mu3" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR77:GPR_77_mu3"/〉 For the system of waves created by a point-source one expresses the solution, since it is classical, as a sum or integral of different modes. For each of these modes, one can still write a bilinear relationship similar to (1), and calculate Z and ⌈.A generalization of this result is given.Boundary conditions that exist at the surfaces of separation are met by writing the continuity of E and H. This condition exactly corresponds to the fact that there one V and one I at the junctions of various sections of lines, or networks.The characteristic matrix of a cascade of networks-or here that of a horizontally stratified ground– is the product of the matrixes of each network. If, in addition, one knows the end impedance–here the conductivity λn of the last layer, supposed to be infinite–one can compute the input impedance Re of the whole system. In electromagnetic prospecting, it is Re that conditions the observable field.Inversely, if measurements were infinitely accurate, the. knowledge of the surface field would give all the thicknesses D and conductivities δ of intermediate layers, in the same way that impedance measurements at the input terminals of a cable would allow to locate any failure. The imperfection of measurements causes the elements located too far away (from the point of view of wave attenuation) to escape detection.

Notes: The iron ores of the Siegerland district are found in lodes occurring in schists of Lower Devonian age. These lodes, normally dipping almost perpendicularly, could be traced only by seismic reflection measurements underground, up to distances of approximately 500 m. The geophones were attached to the walls of the galleries. The frequencies of the recorded longitudinal and transverse waves ranged from 200 to 500 c.p.s. By this method a new siderite lode was discovered.

Notes: Book Reviewed in this article:Lehrbuch der angewandten Geophysik, Teil II, H. Haalck.

Notes: A series of repeat observations during a period of 5 years reveal a variation with time of the Worden gravimeter No. 142 small dial spring system. A possible change in the gravity difference concerned is ruled out. It is shown that the small dial scale factor has diminished by about 0.25 % within the last two years against about 0.5 % four to five years ago. The ratio of L.D. to S.D. has been analysed and the results obtained show confirmation of the change in the small dial system and prove a consistency in the large dial system.

Notes: A study is made of the possibility of calculating electric potentials, and therefore of interpreting apparent resistivity measurements, when the ground presents structural conditions more complicated than plane, parallel stratification.For this work a schematic type of structure has been chosen, in which the discontinuity planes for electric resistivity are both parallel and perpendicular to the level of the ground.The problem has been examinated in its general aspect, and then some criteria for the numerical calculation have been indicated.Some calculated examples are presented, relevant to vertical electric soundings accomplished by conventional technique, for particular types of structure.

Notes: The elastic impulse-from an explosion is propagated through the earth as an expanding shell bounded by the wavefront. This wavefront and its motion are a physical reality while rays or trajectories are much less sharply defined and cannot actually be observed because the wavelengths involved in seismic processes are too long. This means that the visualization and the interpretation of seismic processes can be carried out in a more satisfactory, simple and comprehensible way with wavefronts than with rays.H. R. Thornburgh (1930) introduced the very lucid and simple method of interpreting seismic refraction sections, using Huygen's principle to construct wavefronts from two symmetrical shotpoints by working back from their observed arrival times at the surface. The depths are found first with only a knowledge of the velocities down to the refractor and after that the velocities in the refractor can be determined.The pattern of the two wavefronts at equal time intervals, obtained when carrying out Thornburgh's construction, leads to the very simple, approximative “Plus-Minus” method. The additions of the travel times from two symmetrical shotpoints to-each geophone give the relative depths and their subtractions give the velocities of the refractor. The method extrapolates exact quantities from the refractor boundary up to the surface by the “Plus”- and “Minus” lines. This extrapolation introduces errors that can, more or less, be corrected for. A number of hypothetical cases are presented to give an insight into the applicability and reliability of the “Plus-Minus” method. Its main applications are for fairly shallow refraction investigations and for determining weathering corrections for refraction and reflection work.

Notes: Major savings for operators of seismic parties have been achieved by the use of prilled ammonium nitrate as a substitute for gelatin dynamite in areas where shot holes are dry. Pound for pound, the prills appear to have the same energy yield as 60% gelatin dynamite, and some improvement of records has been noted where prills are used. Safety problems are less with ammonium nitrate than with dynamite, and the development of efficient field handling techniques prevents the loss of production.

Notes: Some mathematical inequalities are proved and their relevance to the interpretation of local magnetic and gravity anomalies is discussed. They can be applied to the readings of either a vertical magnetometer or a total field magnetometer or a gravimeter and yield a maximum possible value for the depth below the earth's surface of the top surface of the body producing the anomaly.

Notes: A new logging method, which makes use of specific chemical reactions to locate petroleum or other minerals has been developed. The method has been named “Differential Electric Log” (CED).In this paper the application to the CED method of redox (reduction/oxidation) reactions is discussed. A series of organic oxidants was found, capable of undergoing reaction with crude oil, sulphur, mineral sulphides, lignite. A small amount of an oxidant of this type is added to the drilling fluid and pumped into the borehole where reaction takes place in correspondence with layers containing oxidizable minerals. Two SP logs, recorded before and after the oxidation, are compared and the presence of such minerals is thus detected.A theoretical hypothesis explaining the mechanism of the potential change due to the chemical reaction is proposed both in the case of electronically conducting and non conducting minerals. The influence of redox potential on SP is discussed.The results of laboratory experiments on several different oxidizable ores and those of field tests performed on pyrite, lignite, and oil wells are reported on and discussed on the basis of the theoretical hypothesis proposed.

Notes: Anisotropy measurements were carried out in two wells 300 km apart in the Northern Sahara, in order to improve the interpretation of seismic refraction surveys.These measurements were based on; the shortening of experimental oblique travel-times with respect to theoretical, travel-times computed by disregarding anisotropy. A perfect knowledge of the velocity distribution is required in order to eliminate all influence of refraction; therefore a continuous velocity log is indispensable.The results in the two wells are in agreement, and show that anisotropy is essentially a function of lithology. The following values were obtained: 〈displayedItem type="mathematics" xml:id="mu1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR202:GPR_202_mu1"/〉 Consequences pertaining to seismic refraction are as follows:— Depths computed without anisotropy are too small by 8 % to 9 %, i.e. about 300 meters.— The error on offsets reaches 70 %.— Depth computations carried out by assigning anisotropy factors at the sight of lithology gave very satisfactory results.— High velocity, highly anisotropic formations such as anhydrite play a very important part in the propagation of refracted waves and in interpretation.Consequences pertaining to seismic reflection are as follows:— Velocity profiles yield velocities closer to horizontal velocities than to vertical velocities.— Depth and migration computations are significantly influenced by high velocity, highly anisotropic formations such as anhydrite.

Notes: Book Reviewed in this article:Handbuch der Physik. Herausgegeben von S. Flügge.Der geologische Bau des Untergrundes von Schleswig-Holstein und seine Erdöllager-stätten.

Notes: Conventional or continuous velocity logging generally fail to solve the problem of velocities in the whole area of a seismic survey. Surface measurements often bring a large amount of additional information: effect of shallow layers, change of facies, correlations. The authors present a theoretical view on the physical meaning of interval velocities. After a short synthesis of the diverse formulae formerly proposed (Faust, West), the practical manner of using these formulae and the accurate interpretation obtained with the help of geological and geophysical data are shown. Several characteristical examples are finally explained and commented on.

Notes: A uniform field of the normal geomagnetic vertical intensity and its normal dependence on the first and second powers of latitude and longitude are derived for Europe and represented by sketches. The epochs of 1938.5, 1941.5, 1944.5, 1947.5, 1950.5, 1953.5, and 1956.5 have been chosen.It is suggested that anomalies of the vertical intensity should be given according to this field in order to obtain, in course of time, a coherent image of the magnetic effects of the European continent.

Notes: Solutions for the propagation of elastic waves at the surface of a cylindrical hole of infinite length are derived from the wave equation for a perfectly elastic and isotropic medium. It is found that the phase and group velocities depend on the ratio between the wave length and the circumference of the cylindrical hole. These surface waves can be classified into different orders. They differ in the calculated dispersion curves, in the range of possible wave lengths (cut-off-frequencies), and in the amplitude proportion of the components of the displacement vector. For very short wave lengths these waves converge to normal Rayleigh waves.These results are used to explain some of the multiple onsets and disturbances on seismograms, obtained in practical seismic investigations in the mines of Siegerland.The conformity of theoretical and practical results is limited, because the idealised suppositions on which the computation is based, such as perfect elasticity, homogeneous and isotropic medium, and circular cross section of the mine gallery are met only approximately in practice.

Notes: Gravity anomalies due to given mass distributions have been determined by model optical experiments, which utilise the fact that both the light intensity and the gravitational field due to point sources obey the Inverse Square law. The technique is somewhat different from that used by Gerrard et al (1957), and the results seem to be more accurate (maximum discrepancy 2 percent). The applicability of the anology for the determination of magnetostatic fields is also pointed out.

Notes: Book Reviewed in this article: The Earth and its Gravity Field, W. A. Heiskanen and F. A. Vening Meinesz.KRAUS, HUNT and RAMSDELL, Mineralogy.

Notes: The present paper deals with various causes giving rise to potential differences caused by induced polarization. In order to obtain the induced polarization effect of ore bodies it is necessary to determine the effect of the barren rock and substract it from the total observed effect.Results of field measurements carried out by this method in the mercury mine of Idrija, as well as those in the lead-zinc mine of Mežica and the Kankberg ore district are in good agreement with the geological data. A theoretically calculated profile is compared with an actual field profile.

Notes: A 3-component flux gate magnetometer has been built for use in 1½ inch drill-holes. It has an accuracy of 1 mgs. It requires 2–3 men to operate it and they can take measurements in up to 3 100 metre holes in one day. The instrument has definitely proved its worth in describing the‘general’ geology of an ore-body, and it is now used on a routine basis.

Notes: To obtain inferences useful in predicting the overburden velocities, a statistical study of velocity data was undertaken on 46 well velocity surveys confined mainly to Eastern England. Five formations were particularly studied: Keuper, Bunter, Permian, Coal Measures and Millstone Grit. Their interval velocities plotted against the mean depth of the interval below the surface supplied five equations of the form V=k. H1/n, which cover the range of depth explored down to 7000 ft. Comparison of these results shows that the interval velocities increase with depth and in the Keuper and Permian attain a maximum value nearly twice that in the remaining three formations. The interval velocities of these formations are confined within a relatively narrow band of 9000–12000 ft/sec. and the formations cannot be distinguished satisfactorily by their interval velocities alone. More practical results were obtained considering the relation between the vertical time and depth to the top of the Permian, Coal Measures, Millstone Grit and Carboniferous Limestone. For each of these a linear relation T=k. H+ To, was obtained from which the vertical time T can be predicted from the known depth with a probable error of ± 4 millisecs.

Notes: Using a resonance method, determinations have been made of the longitudinal bar velocity and of Poisson's ratio of thirteen different chemically pure limestones. The relation between the velocity and the density showed far less scatter than in the determinations previously published; this is primarily attributed to the high purity of the specimens used. The measurements further showed a tendency of the values of Poisson's ratio to increase with increasing density.

Notes: In nine small areas in the Black Forest measurements were made of the CO2 content of ground air, using a method which is based on the difference in heat conductivity between CO2 on one side and N2 and O2 on the other. Measurements during four days yielded four strong CO2 anomalies. At one of these a well was sunk, which gave a considerable discharge of mineral water.

Notes: The attention of everyone has already been drawn to the part which accident reports must play (Geophysical Prospecting, March, 1957, PP- 1 to 8). We intend to show here, with supporting examples, that the systematic analysis of the causes, does allow one to obtain practical conclusions.To begin with, it is advisable to widen the notion of geophysical accident as much as possible. This being done, about 200 reports distributed over a long period and a sufficient number of parties, are dealt with. The collected reports are sufficiently numerous to draw conclusions in a general way bearing on the headings: drilling, transport, outbreaks of fire, explosives, falls, stings, shocks, miscellaneous causes.All the quoted figures are referred to the number of accidents reported. Action must be taken to encourage european geophysicists to study accident reports and to analyse the causes. This may be done in such a way that secret information, such as whereabouts of their personnel, is not disclosed.Efforts to promote “Security” must in the first place bear on the quest for the causes, the definitions of ‘accident’ and of ‘geophysicist’ being taken at their widest possible meaning.

Notes: Seismic model experiments are described in which long strips of plexiglass were used as models. One end of the strip was sawn off at an oblique angle and, at the opposite end, the strip was excited by means of a barium titanate transducer. The experiments showed that, if the width of the strip was sufficiently small, an anomalous reflection against the oblique end occurred which travelled in the longitudinal direction of the strip. This anomalous reflection did not occur when the width of the strip was large. These results are explained on the basis of Fresnel's theory. It is inferred that, in the subsurface, refracted waves may be reflected against fault planes without the law of reflection being satisfied, provided that the refracted wave is propagated in a sufficiently thin high velocity layer.

Notes: Changes of shape of seismic waves provide information on the properties of the material in which the waves propagate. Ricker (1953) has attempted to explain the changes of shape on the basis of a simple viscoelastic theory. His conclusions are at variance with those of others who find a dependence of the attenuation on frequency which could be explained only by a much more complicated linear theory or by nonlinear theories.To provide a basis for discussion, the essentials of the theory of viscoelasticity are briefly reviewed. If a relaxation spectrum, rather than one or very few relaxation times, is admitted, a great variety of experimental results can be described by the linear theory of viscoelasticity. A linear theory is indicated when no obvious violations of the principle of superposition occur.Ricker's theory is presented with some modifications which allow for a finite duration of the initial pulse and for the approximate character of his basic assumptions. There do not appear to be serious discrepancies between his theory and his experimental results. Some of the objections to his theory can be met by assuming a finite duration of the initial pulse. However, more direct measurements made under similar circumstances by McDonal et al. (1958) at the same location lead to a conclusion on the nature of the material not in accordance with Ricker's. This casts doubt on the sensitivity of his method.Laboratory measurements usually yield results which are not explainable in terms of simple viscoelastic models. Whether a linear theory with a relaxation spectrum or a nonlinear theory should apply depends much on the experimental conditions. We must also consider the possibility of nonlinear mechanisms which are active at small amplitudes. No stand is taken in this controversy, but it is pointed out that the question linear or nonlinear could be decided experimentally without considering the details of the theories.

Notes: A method of refraction computing is described which establishes the two points of incidence at the surface of the ray-paths from any point on the refractor when observed from opposing directions. The distance between these points is used to determine the radius and the centre of a circle to which the refractor must be tangential. The matching of forward and reverse data is intrinsic in the method so that the resulting profile is automatically positioned in depth. It is shown that if the effects of dip are neglected, errors are introduced in the derived radius and also in the position of the centre but these errors are shown to cancel each other almost exactly.

Notes: The wells Parentis 10 and 12 which are located on the eastern edge of the Parentis anticline were found to be considerably lower at the top of the reservoir series than had been expected.Unfortunately the village of Parentis is located in this important zone and no more seismic results could be obtained which might help locate faults and give a better picture of the area. To help in our understanding of the area, we attempted to investigate the subsurface conditions surrounding the wells by shooting shaped charges or primacord in the well and recording the returning seismic energy along profiles located on the surface. Thus far, we have used this method in three wells: Parentis 10, Parentis 12 and Lugos 2.With the data given by a velocity survey in each well, time-distance curves have been calculated for several typical possibilities: horizontal layers, dipping layers and the effect of a fault. A comparison of the calculated and experimental curves shows anomalies which correspond to a change of the average velocity along the travel path, which can correspond to geological changes such as: a sudden change of dip, a fault or a thickening of the section.The anomalies have been studied on the different profiles from each well and a possible interpretation of the subsurface conditions has been given. The directions of the major accidents determined by this method are in general agreement with those found by the regular seismic shooting.

Notes: The directional sensitivities of an array of geophones depend as much on the size of the array as on the directional sensitivities of the single geophones forming the array. In a former paper (Celminš 1956) the effect of the size of the array was investigated. In addition to this the influence of the directional sensitivities of the individual geophones on the sensitivities of an array of geophones is investigated here.In δ 2 some general formulae for the directional sensitivities are derived. In addition the directional sensitivities of a single geophone according to P- and S-waves are investigated (δ3 and δ4). In δ5 the'directional sensitivities for the most important cases of arrays of parallel geophones are computed. The results of the computation (shown in fig. 4 and 5) give an idea of the types of directional sensitivities which occur in the use of geophone arrays.

Notes: As an introduction the various ray paths of a refraction wave, which is reflected at a fault, are discussed for the case of an arbitrary angle between the refracting horizon and the fault. Simple geometric considerations lead to the conclusion that the best chances for recording these pulses are encountered, if the angle between the refracting horizon and the fault is either 90° or the critical angle of refraction. In both cases identical travel times of the pulses are to be expected.The case of a fault perpendicular to the refracting horizon is considered in detail for dipping beds. Formulas for the shot point travel time curve and the time contour map are derived. Computed time contour maps show considerable differences between the direction of strike of the contour lines and the strike of the fault, as well as between the recorded apparent velocity and the true velocity of the refracting horizon. Finally, alignment charts and computing procedures are given by which the position of the fault and the velocity of the refracting horizon can be obtained from the recorded shot point travel times or the time contour map.

Notes: It is not easy to record a seismic pulse at distances of interest to oil prospectors without there being reflected or refracted pulses superimposed on the direct arrival. Accordingly the record illustrated is considered worth publishing, although it was taken fortuitously during a normal survey and cannot claim to be a controlled experiment. A comparison with the filtered pulse to be expected from a theoretical Ricker-type wavelet is presented.

Notes: In this paper a method of calculating synthetic seismograms from continuous velocity logs (CVL) is presented and described in detail.The seismic velocities on the CVL are frequently and rapidly changing. Therefore a seismic field-record consists of a great number of reflections with different sign and amplitude. The sum of all reflections forms the seismogram.The first part deals with some relations between the log, the reflection-layers, and the normal field-record, with special emphasis on the influence of the seismic amplifier.Then several assumptions are made for the construction of the synthetic seismograms. Finally some examples and applications are given.

Notes: The Karroo of Madagascar is a thick continental deposit of sandstone with some shale, but no characteristic beds. Where it outcrops, velocities have been measured through bore hole surveys. It seems that they depend primarily on the age, but long refraction soundings show a progressive increase of the velocity with depth and a weak differentiation of the marker beds. These results may possibly be brought in accordance by discriminating between the present position and the deepest position of a bed through its geologic history. Thus we must admit very important erosions which, according to regional geologic data, seem possible. If we make the appropriate correction due to this distinction, all the velocity measurements fall near the curve proposed by Faust for an average shale and sand section.

Notes: Some new formulae for the rapid estimation from the gravity field of the maximum possible depth to the top surface of gravitating bodies of positive mass are stated, discussed and proved.

Notes: The shothole drilling industry has had new developments and its evolution is tied to several causes. First, more and more difficult and varied formations have to be drilled. Second, new procedures have to be used in desert regions recently open to prospecting. Finally, modifications of the seismic methods themselves (multiplication of the charges, shallow holes, etc.) lead the driller to use more economical and lighter tools. In this paper the following types of drilling equipment are described: the Mouton Delmag for spudding through pebbles and loose gravels; a mud mixing water truck and an extensible mast used for drilling multiple medium depth holes through caving sands; a compressed air-drill for regions where the water supply is practically nil; rock drills, such as a Wagon-Jack and the Warsop hammer, for multiple shallow holes in various formations.

Notes: The receiving of seismic waves by multiple geophones (geophone-group) is described by a system of coupled differential equations considering only effective resistances. For so-called symmetrical connections of the geophone-group which are used in practice nearly in all cases, a method can be given for solving this system of differential equations. The solutions so derived are valid for seismic waves of any shape. The calculation takes into account the coupling of the geophones as well as the building-up transient oscillations.A suitable measure of superposition is defined, based on the energy transferred during the receiving of the seismic waves. By this means effects similar to interference are investigated which occur at the geophone-group. As a numerical example calculation is carried out for a geophone-group consisting of three geophones which is excited by impulse-shaped waves of the form of a damped sinusoidal oscillation. The occurring effect of superposition and its dependence on direction are discussed. In the appendix formulae are presented for investigation of further numerical examples.

Notes: The Geophysical Journal of the Royal Astronomical Society, Volume 1, No -, March 1958. Published by the Royal Astronomical Society, Price $ 1, Annual Subscription $ 3. Grundlagen der Geoelektrik by A. P. Krajew. herausgegeben unter der Gesamtredaktion von Prof. Dr. Otto Meisser, VEB-Verlag Technik, Berlin, 1957, Price D.M. 28. Grundlagen für die Auswertung von magnetischen Anomalien (insbesondere Z-Kompo-nente)– (Fundamentals of the Interpretation of Magnetic Anomalies, Especially of the Vertical Component) by K. Köhler, Freiberger Forschungshefte C41 Geophysik, Akademie-Verlag, Berlin, 1958.

Notes: Electronic recorders have been used for recording borehole logs since several years.Some difficulties are however experienced in recording more than two curves at the same time.An experimental 4 curves electronic recorder has been constructed using up to date miniaturized servomechanism components.A general description together with performance data is given; applications of the same construction principles to pen recorders with a greater number of curves and to photographic recorders are also discussed.

Notes: The Kodur Manganese belt has been surveyed with a vertical magnetometer. It was generally observed that both high positive and negative anomalies were associated with some of the existing ore bodies. Other large manganese ore deposits gave little or no magnetic anomalies. Further it was observed that both types of manganese ore deposits giving rise to large and little magnetic anomalies, contained similar percentages of manganese and iron. In view of these discrepancies, an extensive study of the magnetic properties of these manganese ores was taken up. The results of these measurements showed that the susceptibilities of the manganese ore samples collected vary over a wide range.It was observed that the high susceptibilities are associated with samples containing manganmagnetite and vredenburgite, whereas samples rich in psilomelane and pyrolusite possess low susceptibilities. From the chemical analysis, normative values of manganmagnetite and vredenburgite–with a general formula of (Mn.Fe)3O4– were calculated and a linear relationship could be established between these normative values and susceptibilities.

Notes: Book reviewed in this articles: Principles of Geodynamics, by A. E. Scheidegger.

Notes: Theoretical work on seismic pulse propagation must, of necessity, use advanced mathematical methods. To the general reader these researches may seem far removed from the necessities of commercial geophysics; this paper attempts to show in non-mathematical terms that the basic potential of the seismic method permits the obtaining of considerably more information than is currently expected, and that, of all the advances which are required to realise that potential, a knowledge of the laws governing the transmission of the seismic pulse is most desired.

Notes: The present development of exploration in Gabon and the latest successes obtained by drilling have led the authors to publish a historical account of the geological and geophysical research concerning two particularly typical salt domes: those of N'Daminzé and Ozouri.Both of them are located in the western basin of Gabon: the first one in the zone of the savannahs, the second being a part of a series of salt domes in the delta of Ogooué.The western basin of Gabon forms, as a whole, a huge westward-inclined monocline and is characterized by a general phenomenon of diapirism the type of which, in the delta of Ogooué, greatly differs from that of the savannahs zone.Because of the scarcity of outcrops in these zones geophysical exploration has been of valuable help.

Notes: After a discussion of the underlying principles, some electromagnetic model experiments made primarily in connection with the development of the ABEM airborne method are described. The experiments were made on thin vertical and horizontal conductors of “infinite” extent with coil arrangements involving one transmitter and one receiver unit. The in-phase and out-of-phase components of the field picked up by the receiver were measured in percent of the amplitude of the normal field at the receiver. In this case 1° phase-shift corresponds to 1.75 % of the normal field amplitude. The experiments were made on a scale 600–2000 times smaller than the natural scale. The ores and overburdens normally encountered in the field were simulated by sheets of Cu, Al, Zn and Pb of varying thicknesses. The frequencies used were 500, 880 and 1500 c/s. The variation of the secondary fields with the thickness, resistivity and depths of the conductors causing them is discussed in some detail. The bearing of the laboratory work on ground and especially airborne electromagnetic methods is indicated. A few miscellaneous experiments are also described. The results of an airborne survey over a known ore body and those of the corresponding model experiments are given. The ABEM airborne electromagnetic method and the so-called “Canadian” method are briefly compared in the light of the model experiments described.

Notes: This paper deals with the theory of the gamma ray scattering method, which is applied in determinations of density, mainly in boreholes. In this method, the gamma radiation from a radioactive source is scattered round the borehole, and recorded by a detector which is protected against direct radiation by lead.The theory is based on the scattering and absorption properties of matter with regard to gamma radiation, as described by the Klein-Nishina formula in terms of the quantum theory. The single scattered portion of radiation, which is of particular importance, is calculated. In doing this an absorption dependent on the scattering angle, and a counter efficiency dependent on the energy of radiation and the angle of incidence, are taken into account. The range of the detector can also be considered.For homogeneous soil, the dependence of the counting rate on the distance between source and detector, on the density, and on the screening of radiation by the lead absorber, is investigated.Furthermore, the scattering range for various soil densities and distances between source and detector is also determined. The existence of a substantial degree of agreement between theory and experimental findings is demonstrated. Results are compared with those of the Djadkin diffusion theory, and are illustrated by means of examples of the gamma gamma probe and the gamma density meter for surface measurements.

Notes: A layer with parallel plane boundaries is assumed to have a constant density and exhibit a velocity of propagation of seismic waves which increases linearly with the distance from one boundary plane. The influence of this layer on the shape of seismic pulses is investigated in two different cases, in which:1. The layer is embedded between two media each of which has a constant density and velocity of propagation.2. The layer is adjacent to one medium of constant density and velocity; i.e. one boundary plane of the layer is the free surface of a two-layered elastic half space.Through one medium with constant velocity a plane compressional wave impinges at normal incidence on the layer complying with the linear velocity law. The incident seismic pulse is therefore split up into reflected and transmitted parts, the elastic motions of which are studied in the neighbourhood of the layer. The mathematical solution can be deduced for a general pulse by using the Laplace-Transformation. The general solution reveals that the layer following the linear velocity law influences the shape of the reflected and transmitted pulses. This influence is discussed in detail by demonstrating some numerical examples.

Notes: Reflection quality in the Netherlands is extremely variable, ranging from very good to very poor. It is believed that the variation is caused mainly by the changing surface conditions. This suggestion is discussed in the light of available geological data.Several examples are given showing the usefulness of making broad band recordings on normal reflection spreads.The combination of data obtained from the study of the surface conditions and from broad band seismograms is used to select the best shooting method.Some suggestions are made regarding seismometer and shot-hole patterns, shot medium and the choice of filters.

Notes: The introduction of the CVL and the “core speed tester” (or ultra sonic velocity meter) have enabled the geophysicist to complement large-interval, average velocity measurement of sedimentary formations (obtained by conventional well velocity surveys, “Gardner” velocity shots, etc.) by small interval, detailed study of the velocity of particular strata, in situ or in the laboratory. The significance of laboratory velocity measurements on cores is affected by irreversible alterations of the core. At first the integrated time curve of the CVL, calibrated with a skeleton conventional velocity survey, seemed to assure the precision of the interval velocity. Later it was realised that variations of the “Delay Time” (compensation for Mud Travel Time in the single receptor CVL), failure to identify the first energy arrival, etc., could cause errors in the interval velocity measured. These errors can be, detected and reduced by increasing the number of control points of the well survey, running logs down and up the hole, overlapping, and also by the comparison of the Continuous Velocity Logs for neighbouring wells and for various strata in a given well.Despite these errors, the C VL has been found extremely useful, not only for the determination of average velocity, the identification of reflecting strata, and of refraction markers, but also for geological correlations between wells, the determination of maximum porosity and detailed studies of the parameters affecting the velocity of sedimentary rocks.The relation between interval velocity and porosity is discussed and a hypothesis that the low velocity of argillaceous, low-porosity limestones may in part be due to the total fluid contained is put forward.

Notes: In 1956 a total profile length of about 10,000 kilometers was surveyed in Sweden by a new airborne electromagnetic method. The purpose of the survey was to direct ground parties to areas where conducting ore bodies (sulphides etc.) might be found and to eliminate barren ground. In this method an electromagnetic rotating field is used and the variations in both the real and imaginary components of the applied field are measured. The airborne work can be done with one plane towing a bird on a long cable or with two planes without cable connection. The two-plane method has proved to have definite advantages over other methods, as shown by practical examples. The relation between the magnitude of the anomaly over background noise in relation to the flying altitude and the transmitter-receiver distance is demonstrated, as well as different types of anomalies obtained. The use of this airborne method for the mapping of shallow structures in oil field terrains is discussed.The equipment for the ensuing electromagnetic ground investigations is briefly described and examples are given of anomalies obtained.

Notes: Faults or Uranium-bearing veins are often to be found by measuring the Radon-content of the air in the ground through a layer of porous material, even over some distance.It is shown by measurements in the laboratory and in the field, that this distance depends on the character of the porous material (composition of grains with different diameters and water content).A quantitative relation between the composition of grains with different diameters and the coefficient of diffusion of Radon is obtained by comparing the practical measurements with theoretical results.The result of the investigations is, that Uranium-bearing veins or faults do not cause any measurable anomaly even over a distance of only 5 cm, if they are hidden under fine-grained material with its natural water content.

Notes: Book reviewed in this articles: Elastic Waves in Layered Media by W. Maurice Ewing, Wenceslas S. Jardetski, and Frank Press Geology of Petroleum by A. I. Levorsen

Notes: Non-explosive seismic sources, notably the falling weight and the eccentric machine, have occasionally proved advantageous in experimental and practical seismology in spite of their relative feebleness.Another alternative is the pulsed vibrator, which offers the advantage of a completely controllable waveform. Recent theory shows that such a source is quite well adapted to the radiation of high frequency waves from the surface of the ground; it is estimated, for example, that in typical circumstances reflections should be obtainable from depths of the order of 200 metres. The theory also shows how resonances may be avoided so as to radiate the impulsive waveform unimpaired.

Notes: Book review in this articleGedenkboek F. A. Veiling Meinesz, Verhandelingen van het Koninklijk Nederlandsch Geologisch-Mijnbouwkundig Genootschap, Geologische Serie, Vol. XVIII, May, 1957.

Notes: A brief description of the procedure usually adopted for ascertaining the accuracy o aeromagnetic data is given as a background to the discussion of factors which affect this accuracy. These factors fall mainly into two groups: spatial positioning, i.e. relation to the ground surface, and effects contributing to the observed relative magnetic values. The evaluation of the observed anomalies in terms of the most probable causes and the anomalies themselves are directly influenced by the amount of the available control of factors mentioned above. The effects of various factors, such as plan positioning, height keeping, diurnal drift, are discussed, the arguments being based on theoretical and practical premises.

Notes: The effect of the composition of the tank walls on apparent resistivity determinations made in model experiments has been investigated for the two extreme cases of an insulating and a perfectly conducting tank wall: The resulting errors have been determined both by calculation and by experiment.Experiments are described which demonstrate that the magnitude of the effect can be considerably reduced by specially constructed tank walls.

Notes: Amplitude measurements have been made of the height of the first peak of an arrival refracted from a shallow refractor. If the amplitude is assumed to decay as the inverse mth power of the distance, the least squares value for m is found to be 2.16 ± .04. Because of this value and because of the character of the recorded event it is concluded that the arrival is a simple critical refraction. After applying the theoretical ‘spread’ factor for critical refraction there remains a residual attenuation of 1.96 ± 0.28 decibels per 1000 feet. The predominant frequency in the pulse is about 20 c.p.s. and this attenuation agrees with the losses found for such a frequency by extrapolation of the published results of other workers. Although no evidence could be seen on the records for a change of pulse frequency with distance, the quoted result would be consistent with a dependence of residual attenuation on the first power of the frequency, and would be inconsistent with a dependence on the second power of the frequency.It is concluded that studies of the amplitudes of refracted events will give useful estimates of the attenuation factors of rocks.

Notes: A method of constructing depth contour maps of arbitrarily curved horizons obtained from seismic reflection surveys is discussed. This method takes into account three dimensional refraction, avoiding the construction of seismic cross sections of any kind. It requires little work even if refraction at several horizons is taken into account. The multiple layer problem is traced back to the single layer case. Discontinuities in velocity are also taken into account.

Notes: 1. A Worden gravity meter has been tested for sinusoidal variations in the external temperature, by determining simultaneously the corresponding variations of temperature inside the Dewar vacuum bottle, and the variations in the readings.A second Worden gravimeter has been tested for rapid variations in external pressure.

Notes: The results of field and laboratory methods of density determination on a series of Coal Measure, Permian and Triassic rooks are presented and the different methods compared. It is concluded that the most satisfactory method is that of measuring the vertical change of gravity in a mine shaft. Nettleton's method is unsatisfactory to us, due to weathering of the rocks (particularly Magnesian Limestone) and possible effects from drift. Laboratory measurements are of variable value depending on the lithology and source of the samples.A method adopted to solve the problem of finding the true densities for use in a local gravity survey in N.E. England is given.

Notes: In order to eliminate the effect of smoothing due to the use of a finite number of grid points, the second derivative is computed by integrating the product of g with a convenient continuous function, which yields the second derivative to the extent to which the first terms of the Taylor expansion of g represent its value correctly. By applying this method to the anomaly caused by an isolated mass, and to that caused by a homogeneous half plane, it is shown that, if the result obtained is interpreted as if it really were a second derivative, erroneous values for the depth and the mass are obtained. If the real depth of the mass is small, a too large apparent depth is obtained. In the case of a half plane the use of a system of grid points gives the same result. These considerations permit the rational choice of the method of computing the second derivative, such that the effects of too shallow mass irregularities are attenuated.

Notes: From gamma ray absorption the density of soil to a depth of i m. can be determined in situ, without taking samples, by a method which has been described in a previous paper (Wendt 1954) and which has been checked with satisfactory results (mean error about 1%). A nearly vertical steel rod with a gamma ray source at its lower end is pushed into the ground, and the gamma radiation is measured at the surface with two counters placed symmetrically with respect to the rod.As a first step in interpreting the observed effects, the assumption had been made that counters are infinitesimally small. This paper shows how a theory can be developed without this assumption, the finite dimensions of the counters being taken into account by an integral expression. For this purpose a determination of the dependence of the counting rate on the angle of incidence of the gamma rays is required. A comparison of the experimental results with the theoretical ones demonstrates that the integral expression is correct.The influence of deviations from the normal geometry of the arrangement (e.g. caused by bending of the probe) on the results is investigated, as well as the influence of variations of the density with depth.The treatment of the basic problems has been carried through in such a manner that the results obtained can be applied to other absorption methods as well.

Notes: In this paper an attempt has been made to evaluate the possibilities of determining subsurface features from temperature measurements made at shallow depths. The temperatures at the observation depth used are subject to seasonable variations. The amount of this variation differs, and these differences have been attributed to structural features. Local differences in temperature are also caused by surface factors such as the thermal conductivity of the soil, the vegetation and the micro-climate. The differences in temperature due to surface factors are so great that the variation in heat flow caused by differing thermal conductivities of the rocks at depth cannot be detected at the surface. It is not possible therefore to use this method to determine the position of subsurface structures such as domes, anticlines or horsts.It is also shown that concentrations of radioactive elements in the rocks do not provide any measurable heating.The method has been successfully applied to the problem of finding fissures, cracks and similar features provided that convective heat transport from depth to the surface has taken place along these features. In order to compute the area through which a given amount of water at a given depth must pass to give a specific temperature increase a model has been considered. From the shape of the temperature anomaly above a fissure carrying such heated water, the places at which the rising water enters the groundwater stream can be seen immediately. It is possible to estimate the amount of rising water by computing the amount of heat energy transferred to the surface.The method is particularly suitable for determining the position of steam deposits in regions of recent vulcanicity. In such a region a location, hitherto unknown, was found where steam rising from depth condenses beneath the surface.The area and form of the anomaly indicate the extent in depth of this steam bearing zone. By means of observations made at the individual points, it was also possible to compute approximately the energy being released at the surface. This value can give an indication of the extent to which the steam deposits regenerate themselves.Finally it has been shown that percolating canal water can be detected thermally near the canal banks. Such anomalies are dependent on the season.

Notes: The following assumptions are made in the mathematical treatment of the problem. Below a plane earth's surface there is a three-layered elastic medium the interfaces of which are parallel to the earth's surface. The uppermost layer represents the weathered layer in which the velocity of propagation of seismic waves increases linearly with depth. The two lower layers, the so-called intermediate layer and the substratum each have a constant velocity. The surface of the earth is acted on simultaneously by a normal pressure N in the form of a Heaviside pulse. The seismic wave thus generated is propagated through the elastic media.The aim of the investigation is to study the shape of the wave1) in the intermediate layer, after the wave has entered it the first time2) at the earth's surface, after the wave has been reflected once at the interface between the intermediate layer and the substratum.The mathematical solutions can in both cases be expressed as series of Bessel functions. Some numerical examples illustrate the quasi-periodic nature of the solutions. The pseudo-frequency is determined by the gradient of velocity in the uppermost layer; it assumes a value of approximately 50 c.p.s. for a gradient of appr.〈inlineGraphic alt="inline image" href="urn:x-wiley:00168025:GPR328:GPR_328_mu1" location="equation/GPR_328_mu1.gif"/〉

Notes: The President of the E.A.E.G. Safety Committee has stated that European geophysicists fail to attach sufficient importance to accident statistics. Nevertheless these statistics are the best means at our disposal for the study of the factors influencing the number and severity of accidents.It appears essential that two points should be stressed, firstly that the accident reports must be completed correctly, and secondly that the interpretation of each report and of the statistical data must be done systematically.If these conditions are fulfilled it should be possible to create, at all levels, a technical and psychological atmosphere which would do much to protect geophysicists against accidents.

Notes: A strong late arrival, with several cycles, has been observed in line refraction shooting, for distances ranging from 5000 to 30,000 feet. Using equations given by Officer (1953) it has been possible to establish that this event is a multiply reflected refraction. The conclusive factor was its very large amplitude which was about 60 times that expected for the simple head wave, at the distances involved. The wave-guide was bounded by the surface of the earth and by the base of the Low Velocity layer, at a depth of about 80 feet.An earlier paper (O'Brien, 1957a) gave a study of the head wave pulse refracted from this interface and the conclusions in the two papers are altogether compatible.

Notes: For the interpretation of gravimetric and magnetic maps two types of master curves are proposed: firstly those which apply both to the vertical gradient of g and to the pseudo gravimetric gradient obtained by reduction to the pole of magnetic maps; and secondly those which apply direct to magnetic anomalies.With regard to the first type of master curve, the principle and the advantages of the reduction to the pole are discussed. In the last part of the paper it is shown how a set of master curves, calculated for two-dimensional east-west striking structures, can also be used for the interpretation of two-dimensional structures of arbitrary direction of strike. Practical examples of application are given.

Notes: “Cable” and other extraneous energy in well velocity surveys may be identified and distinguished from “formation” energy by using phase relationships. Conventional instruments with at least two different narrow band-pass filters provide sufficient data to achieve good results even in the presence of extreme amounts of interference. The method is illustrated by laboratory tests and an actual well survey.

Notes: In desert countries, reflection seismic prospecting is often very difficult due to bad surface conditions, which adversely affect the quality of the reflections. From the beginning the Northern Sahara proved to be particularly unfavourable. In 1954 a series of systematic tests led to a shooting method which gave satisfactory results. This method used a high multiplicity of the shotholes (up to 100 per shotpoint) combined with a high multiplicity of geophones (up to, and sometimes more than 100 per trace). The method has proved effective and relatively economical to run since 1954. The evident conclusion is, that, under special conditions, high multiplicity may be very helpful in the future.

Notes: Seismic two-dimensional model experiments have been made at the Laboratory of Fondazione Lerici in the Polytechnic School of Milan.For this purpose the Laboratory has developed and constructed a complete equipment including pulse generators, piezoelectric transducers and amplifiers for detecting the waves propagation through the models.We record here experiments concerning reflections from horizontal layers thin in relation to the wave lengths propagated through the media.Layers, with a thickness from a 50th of the total wave length to several wave lengths were used. These strata consist of materials sometimes with lower and sometimes with higher velocities than the homogeneous surrounding medium.The object of this investigation was to clarify the effects of the very thin reflecting layers, individually and in combination, on the reflected arrivals and to determine the influence of the layers upon the reflected energy in comparison with their thickness.We are also studying at what minimum distance, as a fraction of the wave length, the reflection of a horizon appears on the seismogram in an independent event.

Notes: This paper describes the two methods of magnetic tape recording used today in the seismic industry. It discusses objectively the characteristics of both methods including historical material, discussion of circuitry, magnetic tape, drive systems, effects of flutter and harmonic distortion, signal transfer and tape transport construction. The paper also includes discussion of noise and reproduction of transient signals. Diagrams illustrating the two recording systems, their frequency response and phase shift characteristics, distortion and noise levels as well as typical wave forms, are also attached.

Notes: The numerous well velocity measurements carried out in Italy by Agip Mineraria are considered, in an attempt to establish some correlations between the determined velocities and the stratigraphical and lithological characteristics of the formations in which the measurements were made.The distribution of the seismic velocity is discussed especially in relation to depth, age and lithologic composition of the Tertiary formations in the Po Valley.Finally, mention is made of some satisfactory results newly obtained on the basis of such considerations, particularly for the delineation of stratigraphic traps.

Notes: At the Niemegk Observatory geomagnetic balances with torsion ribbons have been constructed since a few years. They have well known advantages over balances resting on knife edges: greater stability of the zero position, ease of enlarging the measuring range by torsion, no serious damage when the instrument is not clamped. Both H- and Z- balances have proved highly satisfactory. Also a combined H-and Z-balance has been developed. To a certain extent, the torsion ribbon balances can be made insensitive to levelling errors. Torsion wire balances used as zero instruments have not been considered for field measurements for reasons of time saving, They are however being made for laboratory measurements.

Notes: Summary. A brief account of the material employed in the preparation of the Admiralty world charts for epoch 1955.0 is given together with the results of a spherical harmonic analysis made to assist in estimating the field values in poorly observed areas. The deduced Gaussian coefficients are tabulated.

Notes: Summary. A practical method is given for using observations of vertical intensity to improve world isomagnetic charts of declination and horizontal intensity. The method involves the assumption that the contribution of the external part of the Earth's magnetic field is not significant. Results of the first cycle of the method are given, together with an indication of the degree of improvement to be expected.

Notes: The Rayleigh-type motion at any depth in an elastic medium with a double surface-layer has been discussed in earlier papers. The ten boundary conditions at the free surface and the interfaces give ten linear simultaneous equations to determine the amplitudes of the horizontal and vertical displacements at all depths in terms of the amplitude of the horizontal motion, say, at any assigned depth. The vanishing of the ten-row determinant obtained by eliminating these ten constants gives an equation determining the wave velocity c as a function of wave-length 2°/K. This equation has previously been solved numerically for a series of values of K under each of the hypotheses T2=T1; T2= 1/2 T1; T2= 1/3 T1; where T1, T2 are respectively the thicknesses of the upper and lower of the two surface layers. For any pair of values (c, K) any set of nine of the equations can be solved for the ratios of the constants, and in this way the relative amplitudes of the displacements have been found at various depths for each of the pairs (c, K) obtained in an earlier paper. The numerical work is heavy, and has been effected on the SEAC machine of the National Bureau of Standards.

Notes: Summary. This paper describes the statistical techniques available to the experimenter in palaeomagnetic work. The theory of these methods is based on an assumed probability distribution of errors. It is shown that the mathematical requirements of this distribution are obeyed by the observations from rock samples which are known to possess a stable magnetization; observations on rocks with unstable magnetization however do not conform to it. A theoretical derivation is given for this probability distribution.The problem of estimating the mean direction of magnetization of a geological formation has in recent years become a matter of the greatest geophysical interest since it is from such estimates that the position of the pole of the Earth in past geological ages is determined. This problem is largely one of the judicious choice of samples and a procedure is suggested whereby such estimates may be achieved with the greatest sample economy.

Notes: Summary. Refraction diagrams are drawn for 6-sec microseisms when they approach the British Isles from the SW, WSW, W, WNW and NW directions. Refraction diagrams are also drawn for 4-s and 8-s microseisms approaching from a westerly direction. The diagrams show that all the westerly microseisms are very much reduced. This effect is confirmed by measurement of the vertical component of the microseisms due to sixteen storms which occurred during the period 1945–53.

Notes: Summary. Representative samples of Cretaceous and Jurassic arenaceous rocks and some Jurassic limestone samples were collected. The arenaceous rocks were generally either too weakly magnetized for the measurement of their remanent magnetization, or were unstable; higher intensities of magnetization were recorded in the limestones. The magnetic directions of the few reversed Jurassic limestones indicate a pole position near Omsk. X-ray analysis indicates that the detrital iron ore mineral present was goethite. It is concluded that the palaeomagnetic unsuitability of these Jurassic and Cretaceous deltaic deposits results from the weathering conditions in the source area.

Notes: The progress in the registration of refraction shots obtained in the last years permits a more systematic study of the secondary arrivals. In zones with disturbed tectonics, a noticeable portion of the received energy derives from diffraction phenomena caused by velocity anomalies in the marker bed or in the overburden (faults, local heterogeneities). Some characteristic examples are presented and schematically explained.A more advanced analysis of the branches of the travel time curve caused by diffraction leads, in favourable cases, to a new method of calculating the offsets and the vertical velocities, independent of the usual methods for the determination of these parameters.

Notes: The use of small scale models in the interpretation of results of electric prospecting involves two majors difficulties: (1) the dimensions of the models and of the tanks in which the experiment is performed must be of unwieldy large sizes if one wants to be quite free from side effects; (2) the precise geometrical relation, in respect to the model, of the successive points where the interesting data are measured, requires an intricate installation, and is costly in both time and money.The technique which is described in this paper eliminates these difficulties as well as some others. The surface of the ground is represented by the lower face of a thick horizontal plate of plexiglas which is laid at the surface of the liquid filling the tank. This plate is pierced by a great number of small holes whose coordinates have been measured initially with the necessary precision. The electrodes are then introduced into these holes without creating any perturbation in the distribution of the lines of current flow. A solution of copper sulphate is used as the liquid in conjunction with copper electrodes. In this way, a D.C. excitation is possible, combining precision with convenience.

Notes: A seismic field investigation on a reduced scale is described. A falling weight was used as the source of energy, with a recording system which permitted the display on a cathode ray oscillograph of two dimensional pictures of ground displacement in the vertical plane containing the source. These permit a quantitative study of the magnitude and direction of displacement and of the instantaneous velocity of motion.

Notes: The first part of the paper deals with theoretical considerations concerning the arithmetic mean of gravity values and its use with regard to the derivation of approximation formulae for the second derivative. In order to calculate the second derivative in practice the arithmetic mean. ḡ(r) of a continuum of gravity values on a circle of radius r is approximated by a Taylor polynomial and then replaced by the arithmetic mean gn(r) of n discrete gravity values. Because of the invariance of ġ(r) with regard to rotations of the coordinate system in the horizontal datum plane there exists a lower limit for the number n; this lower limit depends on the degree of the Taylor polynomial used in the formula for gzz.The general results of the first part yield routine formulae for the special case of a regular hexagonal grid; these formulae are given and discussed in the second part of the paper. Three formulae are applied to the gravity data of the Los Angeles Basin. Some remarks concerning the comparability of different approximation formulae and some hints with regard to routine calculations conclude the paper.