ALBERT

Notes: In a previous paper, we have shown that estimates of both the mean conductivity and the azimuthal anisotropy of the sea floor can be obtained by measuring the diffusion times of an electromagnetic disturbance from a transmitter on the sea floor to a set of receivers located around it. For that seminal study, the model considered was a simple double half-space, an upper half-space representing sea water in contact with a lower, more resistive half-space representing laterally anisotropic crustal material, material for which the electrical conductivity in one horizontal direction x is different from the conductivity in the other two principal directions y and z.A uniform half-space is rarely a realistic model of the sea floor. We extend the theory here to include the effects of several crustal layers which may have differing conductivity and anisotropy. We develop in two-dimensional wavenumber domain the general solution of the governing Maxwell equations in terms of two scalar functions, the x-components of electric and magnetic fields. The theory is valid for any finite source.We show that for static fields, the general solution simplifies greatly and expressions for measured magnetic and electric fields may be obtained through processes of upward and downward recursion and, for the magnetic field, surface integration over layer interfaces.At non-zero frequencies, when the effect of electromagnetic induction is included in the formulation, the solution is more pedantic because elementary recursion rules could not be found and may not exist. Fundamental solutions for both the perpendicular magnetic and electric components must be written down for each layer and determined simultaneously as the solution of a large set of linear equations.

Notes: In a typical marine controlled-source electromagnetic (CSEM) experiment, one or more self-contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea-floor. A remote horizontal electric dipole is towed close to the sea-floor by a surface vessel. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The variations contain information about the distribution of electrical conductivity of the subsurface rocks. The electrical conductivity is related to such critical physical parameters as porosity, temperature and fluid content.Published studies of the response of the sea-floor to electromagnetic excitation have not included any systematic variation in the sea-floor conductivity with horizontal direction. A horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge. Here, for example in the near-surface layers, basalts are extruded onto the sea-floor within the very narrow, fissured, neo-volcanic zone. The fissures may be filled with sea water and sediment which are better conductors than typical basalt. Alternatively, as time progresses, they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks. In both instances, the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction. Measures of anisotropy on sea-floor of varying age are an indication of the history of the tectonic activity at the ridge.The electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half-space model. An upper half-space representing sea water is connected to a lower half-space representing anisotropic crustal material. A general theory describing the excitation of this model by any transient electromagnetic source is developed and the step-on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed.The results of previous work indicate that following a change in current in the transmitter, two distinct changes in horizontal electric field strength separated in time are observed at the receiver. The first is caused by the diffusion of the electromagnetic field through the relatively resistive sea-floor, the second is caused by diffusion through the relatively conductive sea water. The times at which the two events occur are robust measures of the isotropic conductivity of the sea-floor and the sea water respectively.The results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea-floor. For some orientations of the transmitter and the receiver, the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity. The initial event is delayed. However, there are orientations for which increasing the anisotropy actually results in an earlier arrival. Natural Science and Engineering Research Council in Canada.

Notes: The eastern part of Middle Valley, on the northern Juan de Fuca Ridge, is characterized by locally high heat flow, evidence of hydrothermal activity, and near-surface mineralization. A magnetometric offshore electrical sounding (MOSES) survey was conducted in this area to determine the electrical structure of the sediment and upper crust. Two perpendicular lines of data were obtained across a ‘mound’, a 500 m diameter uplifted hill where sulphide mineralization and hydrothermal activity have been observed. The best-fitting model for the east-west regional line consists of a sediment layer of resistivity 0.70 ± 0.06 Ωm and thickness 500 ± 100 m overlying basaltic basement of resistivity 2.3 ± 0.6 Ωm. Basement porosities are estimated to be of the order of 10-14 per cent, using measured vent fluid temperatures of 275-300d̀C, consistent with previous estimates of the porosity in fractured oceanic crust. Results for the north-south line along the mound indicate the presence of a 1.35 ± 0.15 Ωm, 100 ± 20 m thick resistive cap overlying deeper sediments of much lower resistivity, of the order of 0.1 Ωm. The resistivity structure across a localized high heat flow anomaly is similar but the resistive cap is absent. The phase lags calculated from the resistivity model deviate significantly from the measured lags and provide the clearest evidence for transverse isotropy, the interbedding of very conductive with less conductive layers. The conductive thin layers are two orders of magnitude less resistive than the resistive layers because of massive sulphide mineralization, a result now confirmed by drilling. Additional data were gathered at a site centred about the southern end of a vent field 3–4 km northwest of the hydrothermal mound; the resistivities can be fit using either the regional model or a 1.1 Ωm halfspace. There are some anomalous values, which appear to be spatially correlated with basement topographic highs.

Notes: New oceanic crust is being formed along active segments of the global mid-ocean ridge (MOR) system. The presence of an axial magma chamber and associated zones of partial melt and hydrothermal activity located up to several kilometres beneath the seafloor is central to almost all recently proposed theories of crustal formation. Seismic images of the top few kilometres beneath the fast spreading East Pacific Rise (EPR) have already been obtained. Reflection profiles place strong constraints on the geometry of the axial magma chamber but refraction data provide only coarse estimates of the subsurface temperature, distribution of partial melt and porosity, parameters required to distinguish between the proposed petrological models. Electrical conductivity is directly related to all these critical parameters and therefore electromagnetic experiments should be designed to help characterize the ridge environment.We determine the magnetic field B(t) on the seafloor caused by a sudden change in current in a 2-D electric dipole aligned perpendicular to the strike of the ridge. The finite element technique is used to solve the governing differential equation numerically in the Laplace s-domain. The transformation to the time domain is by the Gaver-Stehfest method. We show that the extraction of two basic parameters from the response curve Ḃ(t) can provide sufficient information to identify the more important features of the petrology. The parameters are the response amplitude Ḃmax, which is the maximum derivative of δtB(t), and the diffusion time γT, the time at which this maximum occurs. The behaviour of γ as a function of distance from the source is analogous to that of first arrival time in refraction seismology. The value of γ is a weighted integral of the conductivity along the most resistive path between the source and the receiver.A highly conductive, partially molten magma chamber beneath the ridge axis slows the rate of diffusion of electromagnetic fields across the ridge, increasing γ but also reducing Ḃmax at sites on the side of the ridge opposite the transmitter. A melt lens ponding as a thin layer on top of the chamber increases Ḃmax at the ridge crest and increases γ at sites on the far side. Hydrothermal fluid circulation in the uppermost 2 km of the crust reduces Ḃmax everywhere across the ridge but increases γ only at sites within 0–3 km of the ridge crest. Electromagnetic energy in this case can reach the more distant points via paths which by-pass the fluids.Inferences made from the results of 2-D modelling indicate that a practical experiment would require a 104 A m horizontal electric dipole (HED) transmitter located 5 km off-axis and receivers with a sensitivity of at least 1 pT s−1 over a time window up to 10 s.

Notes: Marine controlled-source electromagnetic experiments are designed to measure the electrical conductivity of the sea-floor. The apparatus consists of a transmitter, typically an electric current dipole, and a series of remote receivers. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The signals contain information about the electrical conductivity of the crustal rocks. Electrical conductivity is related to such critical physical parameters as porosity, temperature, composition, fluid content and texture.

Notes: A natural-source electromagnetic sounding of the earth made near a surface conductivity anomaly will resolve different features of the underlying conductivity structure than a sounding in a more uniform region. the surface-conductivity anomaly deflects horizontal electric currents induced by an external source into a vertical plane converting transverse-electric (TE) mode currents into the transverse (TM) mode. the resulting current distribution involves both vertical current flow and spatial variations with shorter wavelengths than the external field, providing increased resolution of resistive layers and of the conductivity structure at shallow depths. We exmine the sensitivity of the converted-mode response for the vertical-gradient sounding (VGS) method in order to plan electromagnetic soundings in a narrow ocean strait such as the Strait of Georgia between Vancouver Island and the Canadian mainland.An integral-equation method is used to model the current system induced by a mode converter, consisting of a known conductivity structure, such as a body of ocean water. It is shown that the depth of penetration of the secondary current distribution produced by the mode converter depends on both the horizontal scale of the feature and the distance from its edge. Within this depth range the current system is strongly perturbed by the existence of either conductive or resistive layers. the sensitivity of the VGS response (the ratio of the horizontal magnetic field at the base and surface of the mode converter) is examined using forward modelling of layered conductivity structures. the response is found to be dependent on both the TE and TM current systems. For a narrow ocean strait such as the Strait of Georgia, a measurement of the converted-mode VGS response along a line of sites on the floor of the strait, will provide resolution of conductive and resistive layers in the upper 10 km. the appropriate frequency range over which the VGS response should be measured in the strait is 10−2 Hz to 10 Hz.In our investigation of mode conversion we examine both the frequency- and time-domain response. Snap shots showing the current system evolving in the earth after a step or impulse illustrate the interaction of the EM signals with resistive and conductive layers. We show that the time-domain response can be used in a ‘geometrical sounding’analogous to seismic refraction to determine the conductivity structure. Finally we examine the limitations on the accuracy of the frequency and time-domain VGS response imposed by natural signal levels and instrument sensitivity.

Notes: We describe the conception, design, construction and testing of a towed electromagnetic system capable of mapping the near-surface electrical conductivity of the sea-floor. The transmitter and receiver coils are arranged coaxially, and dragged along the sea-floor. The transmitter coil is 2 m in length and 1 m in diameter, and contains 100 turns of wire. It is energized from the surface by a constant voltage, whose polarity is reversed every 5 ms. The resulting transient magnetic field is detected in the receiver coil. Received signals are amplified and sent back to the surface for processing and analysis.Following a transition in the transmitter current, two distinct transients are observed at the receiver. These events correspond to electromagnetic energy which has diffused through the sea-water and less conductive sea-floor, respectively. The onset, amplitude and decay of the first transient are primarily a function of the conductivity structure of the sea-floor.A successful survey with the system was carried out in shallow coastal waters east of Vancouver Island. The survey yielded 20 conductivity measurements along three lines. The data are stacked 512-fold, and the shape and amplitude of the resulting noise-reduced signal are compared with theoretical signals using a generalized linear inversion process. The shape, amplitude, and delay time of the received signal are indicative of the conductivity of the bottom sediments. The resulting model is a layer of mud of conductivity 1.2 S m-1 and variable thickness overlying rock or sediment with a conductivity of about 0.1 S m-1. The model is consistent with seismic log profiles obtained during the survey, and with conductivity values expected for surficial, marine sediments.

Notes: Abstract A newly developed towed transient electromagnetic (TEM) system is capable of measuring the electrical conductivity of the uppermost 5–10 m of sediments on the sea floor. The profiles of conductivity may be interpreted to give the porosity and likely texture of the bottom sediments. Recent tests of the system have demonstrated that data may be collected continuously in a surveying mode. Results from Knight Inlet, British Columbia, are in good agreement with sea floor samples from the area. Applications for the system include the rapid identification of sediment types for dredging operations, geotechnical surveys, or reconnaissance mapping of Quaternary geology.