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  • 1
    Electronic Resource
    Electronic Resource
    Seasonal differences in the soil water balance under perennial and annual pastures on an acid Sodosol in southeastern Australia (2001)
    Oxford, UK : Blackwell Science Ltd
    European journal of soil science 52 (2001), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Replacement of native deep-rooted grasses by shallow-rooted ones has resulted in greater losses of water and nitrogen by drainage. To counter this effect we have tested the hypothesis that liming, and the conversion of annual grass pastures to perennial grass pastures, could improve the sustainability of grazing systems in the high rainfall zone (〉 600 mm per annum) in southeastern Australia, through better use of water and nitrogen. A field experiment consisting of sixteen 0.135 ha (30 m × 45 m) grazed paddocks representing four pasture combinations (annual pasture (mainly Lolium rigidum) without lime (AP–); annual pasture with lime (AP+); perennial pasture (mainly Phalaris aquatica) without lime (PP–), and perennial pasture with lime (PP+)) was carried out from 1994 to 1997 on an acid Sodosol (Aquic Hapludalf) in southern New South Wales, Australia. Measurements were made of surface runoff, subsurface flow (on top of the B horizon) and soil water content. The results showed that perennial grass pastures, especially PP+, extracted approximately 40 mm more soil water each year than the annual grass pastures. As a result, surface runoff, subsurface flow and deep drainage were at least 40 mm less from the perennial pastures. These measurements were further supported by a simulation of soil water deficit and deep drainage for AP– and PP+ paddocks, using 10 years' past meteorological records. Overall, the results suggested that well-grown, phalaris-based pastures could reduce recharge to groundwater and make pastoral systems more sustainable in the high rainfall zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2389.2001.00386.x
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  • 2
    Electronic Resource
    Electronic Resource
    PRINCIPLES AND APPLICATION OF MAXIMUM KURTOSIS PHASE ESTIMATION (1988)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 36 (1988), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Methods of minimum entropy deconvolution (MED) try to take advantage of the non-Gaussian distribution of primary reflectivities in the design of deconvolution operators. Of these, Wiggins’(1978) original method performs as well as any in practice. However, we present examples to show that it does not provide a reliable means of deconvolving seismic data: its operators are not stable and, instead of whitening the data, they often band-pass filter it severely. The method could be more appropriately called maximum kurtosis deconvolution since the varimax norm it employs is really an estimate of kurtosis. Its poor performance is explained in terms of the relation between the kurtosis of a noisy band-limited seismic trace and the kurtosis of the underlying reflectivity sequence, and between the estimation errors in a maximum kurtosis operator and the data and design parameters.The scheme put forward by Fourmann in 1984, whereby the data are corrected by the phase rotation that maximizes their kurtosis, is a more practical method. This preserves the main attraction of MED, its potential for phase control, and leaves trace whitening and noise control to proven conventional methods. The correction can be determined without actually applying a whole series of phase shifts to the data. The application of the method is illustrated by means of practical and synthetic examples, and summarized by rules derived from theory. In particular, the signal-dominated bandwidth must exceed a threshold for the method to work at all and estimation of the phase correction requires a considerable amount of data.Kurtosis can estimate phase better than other norms that are misleadingly declared to be more efficient by theory based on full-band, noise-free data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1988.tb02155.x
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  • 3
    Electronic Resource
    Electronic Resource
    THE PERFORMANCE OF OPTIMUM STACKING FILTERS IN SUPPRESSING UNCORRELATED NOISE (1977)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 25 (1977), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Optimum stacking filters based on estimates of trace signal-to-uncorrelated noise ratios are assessed and compared in performance with conventional straight stacking. It is shown that for the trace durations and signal bandwidths normally encountered in seismic reflection data the errors in estimating signal/noise ratios largely counteract the theoretical advantages of the optimum filter. The more specific the filter (e.g. the more frequency components included in its design) the more this is true. Even for a simple weighted stack independent of frequency, the performance is likely to be better than a straight (equal weights) stack only for relatively high signal/noise ratios, when the performance is not critical anyway.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1977.tb01159.x
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  • 4
    Electronic Resource
    Electronic Resource
    Multichannel estimation of frequency-department Q from VSP data (1997)
    Oxford, U.K. and Cambridge, USA : Blackwell Science Ltd
    Geophysical prospecting 45 (1997), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A Method of estimating attenauation from the first arrivals of VSP data is presented. The motivation is the desire to investigate the effects of scattering on wave propagation, and particularly the apparent attenuation and associated phase delay due to fine layering (the O'Doherty-Anstey effect).In order to take account of the frequency dependence of the predicted scattering attenuation, and to provide robust statistics for the estimates, a beam-forming method is used to measure the attenuation. This simularaneously estimates the slowness and polarization angle of the different wave modes, and results in attenuation measurements which are largely free of interference from reflected and mode-converted energy. By working in the frequency domain and measuring amplitude decay with depth, the frequency dependence of the attenuation is also accounted for. The beam-forming algorithm works in two passes, the first of which estimates slownesses and polarization angles over a small depth range, while the second uses the information from the first pass over a larger depth range to estimate attenuation.An approximate error analysis of the method shows that the standard variance of the estimated Q values is proportional to Q2 and the data quality (measured by its spectral coherence), and inversely proportional to the square of the analysis depth range and the square of the frequency. Hence the depth resolution is traded against the stability of the results.The method is applied to a zero-offset three-component VSP. The data are of good quality, with a bandwidth ranging from 180 Hz in the shallow part to 100 Hz in the deepest part. Stable results were obtained using a 450 m depth range. Above about 50 Hz, there is little evidence of frequency dependence in the attenuation. There is a clear division in depth into layers of higher and lower attenuation, with values of Q typically between 50 and 200. Below 50 Hz, however, attenuation increases rapidly with decreasing frequency throughout the depth range, with values of Q of less than 10 at 15 Hz. This behaviour appears anomalous since on physical grounds we expect very high values of Q at low frequency, and we have no explanation for these observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1997.tb02272.x
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  • 5
    Electronic Resource
    Electronic Resource
    SOME FADS AND FALLACIES IN SEISMIC DATA ANALYSIS (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 40 (1992), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: In many branches of science, techniques designed for use in one context are used in other contexts, often with the belief that results which hold in the former will also hold or be relevant in the latter. Practical limitations are frequently overlooked or ignored. Three techniques used in seismic data analysis are often misused or their limitations poorly understood: (1) maximum entropy spectral analysis; (2) the role of goodness-of-fit and the real meaning of a wavelet estimate; (3) the use of multiple confidence intervals.It is demonstrated that in practice maximum entropy spectral estimates depend on a data-dependent smoothing window with unpleasant properties, which can result in poor spectral estimates for seismic data.Secondly, it is pointed out that the level of smoothing needed to give least errors in a wavelet estimate will not give rise to the best goodness-of-fit between the seismic trace and the wavelet estimate convolved with the broadband synthetic. Even if the smoothing used corresponds to near-minimum errors in the wavelet, the actual noise realization on the seismic data can cause important perturbations in residual wavelets following wavelet deconvolution.Finally the computation of multiple confidence intervals (e.g. at several spatial positions) is considered. Suppose a nominal, say 90%, confidence interval is calculated at each location. The confidence attaching to the simultaneous use of the confidence intervals is not then 90%. Methods do exist for working out suitable confidence levels. This is illustrated using porosity maps computed using conditional simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1992.tb00376.x
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  • 6
    Electronic Resource
    Electronic Resource
    SEISMIC CHARACTER MAPPING OVER RESERVOIR INTERVALS (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 38 (1990), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: This paper presents some results from an investigation into the utility of pattern recognition methods in seismic interpretation. The seismic instantaneous attributes of amplitude, phase and frequency provide a way of quantifying the character of a simple reflection. Measures of character can be developed from cross-plots and cluster analysis of these attributes. It is demonstrated that such seismic character can produce better-defined maps than a single attribute. These procedures can be extended to attributes derived from seismic trace segments, such as trace energy and centre frequency, and to multitrace attributes, but more effort is then needed to analyse the attributes and search out useful ones.An introduction is given to projection pursuit which has proved a useful exploratory tool for the anlysis of attribute relationships.It is important to stress that pattern recognition techniques simply help bring relationships and patterns in the data to the attention of the interpreter and the most persistent problem in applying these techniques is the evaluation of potentially interesting patterns. The decision on what use can be made of them is highly interpretive and their calibration is difficult. Well control is vital but it normally allows only very limited supervision of a seismic classifier. An example is presented to illustrate these problems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1990.tb01884.x
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  • 7
    Electronic Resource
    Electronic Resource
    TWO METHODS FOR CONTINUOUS MONITORING OF HARMONIC DISTORTION IN VIBROSEIS SIGNALS (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The quality of Vibroseis survey data can be improved by continuously monitoring the vibrator's baseplate and reaction mass accelerations. Equipment failures can be detected as they occur, rather than relying on similarity trials at the beginning and end of the day's production. Equipment faults can then be corrected as they happen and thus would not have a detrimental effect on the quality of the survey data. Source efficiency can be optimized by monitoring the amount of harmonic distortion generated by the vibrator at different drive levels on the different surfaces which may be encountered during a survey. Phase problems introduced by poor coupling of the baseplate to the ground can also be identified and addressed in the field.Rapid analysis of vibrator signals is required if continuous monitoring is to be useful. Frequency-time (f-t) analyses of vibrator signals are often used in processing centres, but are slow and require a large storage capacity which makes the technique unsuitable for a field analysis system.The two methods proposed to analyse vibrator signals entail the use of hodograms and time-varying notch filters. Hodograms provide a qualitative analysis of harmonic distortion and vibrator performance. A fast, time-varying notch filter gives quantitative and qualitative information about the harmonic distortion present in the signal and can be used to identify problems with vibrator behaviour. Both the hodogram and fast, time-varying notch filter methods can analyse the vibrator's reaction mass and baseplate accelerations as it progresses through its sweep and can present automatically interpreted results to the operator before moving to the next vibrator point.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1989.tb02237.x
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  • 8
    Electronic Resource
    Electronic Resource
    ON ERRORS OF FIT AND ACCURACY IN MATCHING SYNTHETIC SEISMOGRAMS AND SEISMIC TRACES (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 32 (1984), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A synthetic seismogram that closely resembles a seismic trace recorded at a well may not be at all reliable for, say, stratigraphic interpretation around the well. The most accurate synthetic seismogram is, in general, not the one that displays the smallest errors of fit to the trace but the one that best estimates the noise on the trace. If the match is confined to a short interval of interest or if the seismic reflection wavelet is allowed to be unduly long, there is considerable danger of forcing a spurious fit that treats the noise on the trace as part of the seismic reflection signal instead of making a genuine match with the signal itself. This paper outlines tests that allow an objective and quantitative evaluation of the accuracy of any match and illustrates their application with practical examples.The accuracy of estimation is summarized by the normalized mean square error (NMSE) in the estimated reflection signal, which is shown to be(/n)(PN/PS)where PS/PN is the signal-to-noise power ratio and n is the spectral smoothing factor. That is, the accuracy varies directly with the ratio of the power in the signal (taken to be the synthetic) to that in the noise on the seismic trace, and the smoothing acts to improve the accuracy of the predicted signal. The construction of confidence intervals for the NMSE is discussed. Guidelines for the choice of the spectral smoothing factor n are given.The variation of wavelet shape due to different realizations of the noise component is illustrated, and the use of confidence intervals on wavelet phase is recommended.Tests are described for examining the normality and stationarity of the errors of fit and their independence of the estimated reflection signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1984.tb00744.x
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  • 9
    Electronic Resource
    Electronic Resource
    The utility of Burns’s equation to describe solute movement through soil under various boundary and initial conditions (1999)
    Oxford, UK : Blackwell Science Ltd
    European journal of soil science 50 (1999), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Burns’s equation for describing solute movement through soil is attractive because it is simple and predicts adequately in many instances. However, the assumptions implicit in it are not inconsistent with preferential solute flow. We have explored the consequences of this by leaching initially resident chloride and surface-applied tritium and nitrate through 250-mm-long intact cores of a silt loam soil. The applied flow rates of 3 and 5 mm h−1 (realistic rainfall intensities) produced unsaturated soil conditions, except near the base where free water dripped out. Burns’s equation described the movement of the three solutes fairly successfully with the water content parameter having values between 0.29 and 0.48, similar to the actual volumetric water content of 0.47.The leaching of resident chloride to 450-mm-deep mole drains in the field was also successfully simulated using Burns’s equation. However, simulation of the leaching of bromide applied to the soil surface as a solid salt was problematic. This resulted from uncertainty as to whether to treat the application as a pulse input to the flux or resident concentration. The observed behaviour fell about midway between the simulations for these contrasting initial conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2389.1999.00262.x
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  • 10
    Electronic Resource
    Electronic Resource
    MEASUREMENTS OF EARTH ATTENUATION FROM DOWNHOLE AND SURFACE SEISMIC RECORDINGS (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 32 (1984), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Frequency-dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from〈list xml:id="l1" style="custom"〉1spectral power ratios of the coherent events in separate time gates on the seismic section2matching a broadband synthetic trace with seismic data at the well, and3determining the operator that transforms one down(up) going pulse recorded in the well into another recorded at a deeper (shallower) level.The accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals, and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence. Spectral ratios from (1) showed a smoother variation with frequency—and one more consistent with other estimates—when they were compensated for the spectra of the reflectivities over the time gates employed, but they did not provide more than a broad indication of attenuation over a substantial depth interval. Approach (2) was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched; approach (3) gave the best results. Here matching is a much more powerful tool than the spectral-ratio techniques that are commonly applied since it can yield the form of the attenuation operator, i.e., both its amplitude and phase response, together with properly defined measures of its accuracy, while at the same time it minimizes the influence of noise and local interference effects at each recording level.For seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion. Application of approach (3) to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency-dependent geophone coupling and their effect on values of Q determined from downgoing pulses only.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1984.tb00745.x
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