Exploration Geophysics - Volume 15, Issue 4, 1984

In the search for hydrocarbons a technique is now available that will not only just define favourable structure or stratigraphy, but that also appears to be responsive to a manifestation of the presence of the hydrocarbons. It has the added advantage of being usable from an airborne platform, thus making it a rapid coverage technique especially suitable for reconnaissance of large sedimentary basins. It is based on the hypothesis that leakage of hydrocarbons, which always occurs to some extent, causes a reducing zone to form vertically above the hydrocarbons. One of the several effects of this zone is the formation of diagenetic magnetite from the postulated reduction of any hematite present. The latter acquires a chemical remanent magnetization and is located in depths at or near the water table. The effect is observed as relatively low amplitude, high wavenumber magnetic anomalies. Such anomalies have been observed over the Cement field, Oklahoma, where anomalous concentrations of magnetite were also measured at shallow depths in the oil wells. Confusing effects such as variations in the diurnal field or micropulsations can best be discriminated by the use of horizontal gradiometer measurements. The gradiometer cancels all common mode variations and is also more sensitive to anomalies with shallow sources, as in the case with diagenetic magnetite occurrences. Geometries have investigated the usefulness of this technique by examining results of previous flying at 120 m altitude over petroleum basins in the USA for the US Department of Energy. In several disparate areas the correspondence of oil fields and the characteristic anomaly pattern due to this effect is remarkably good, with no similar patterns occurring where there are no oil fields. A theoretical model based on the magnitude of susceptibility obtained from the magnetite content in the Cement field, Oklahoma, confirms the order of magnitude of the amplitude and the wavenumber as consistent with those observed.

High resolution seismic reflection methods have been applied in Australian coalfields since 1978, with the aim of locating small faults and other geological features which may be hazardous to coal mining. BHP conducted a seismic reflection campaign over the Cook Colliery during the period from 1978 to 1980 when a total of approximately 90 line km were completed. The seismic survey detected a number of faults with complex geometry and made it possible to construct a more detailed map of colliery structure than was previously possible. In addition, by comparison with borehole data, it was possible to correlate reflections with particular coal seams and identify areas of seam variation and splitting. The Cook Colliery case history clearly demonstrates a role for seismic reflection techniques in colliery exploration and development.

Seismic reflection, hitherto neglected as a prospecting technique for base metal exploration, is worthy of consideration now that the targets sought are deeper and more elusive than in the past. As a supplement to drilling, or in its own right, it has more potential than any other geophysical method to delineate subsurface stratigraphy in detail. Model studies of two-dimensional situations have suggested that orebodies themselves may give characteristic signatures, a factor which may find application in direct detection: The physical validity for supposing the method workable rests in the fact that velocity is not a single-valued function of density alone, as commonly and tacitly accepted in petroleum exploration. Rather it is a function of density and mean atomic weight (defined as the formula weight of a mineral divided by the number of particles in its molecular formula). The difficulties of obtaining velocity measurements in basement rocks under in situ conditions suggests that it is appropriate to consider density changes as a measure of acoustic impedance contrast. There is more merit in this approach than might be expected, for it appears that in ore-bearing rocks density variations are of more consequence than velocity variations. For example, increasing substitution of pyrrotitic ore into a country rock consisting of siltstone does not alter a basic average velocity of 5.5 km sr¹, but the density changes from 2.7 to 4.51 nr³. Experimental studies of the seismic response of basement rocks indicate that reflected events can be recorded and interpreted from zones within basement, provided that seismic data have a white spectrum whose bandwidth exceeds 2 octaves, and with upper limiting frequencies in excess of 200 Hz. Detector spacing should be in accordance with the maximum dip in an area, but 5-10 m is recommended.

A test survey was carried out in the Sydney Basin using a wire source, loop receiver, deep transient electromagnetic sounding system. A field system was designed and built, the data acquisition system automated to simplify field operation, and current US processing software adapted to the hardware. A test survey was conducted in the Sydney Basin over horizontally layered strata with resistivity contrasts. Three hundred transients were recorded at eight stations within two days. After processing and interpretation, the geoelectric models obtained from the inversions qualitatively correlate with the basin geology, to a depth of 2 km. These measurements suggest that the deep transient electromagnetic sounding method could be successfully used for deep mineral and oil and gas exploration problems in Australia.

Five concrete pads containing elevated concentrations of the three radioelements, potassium, uranium and thorium, have been constructed at the CSIRO Institute of Energy and Earth Resources, North Ryde. Analyses of samples taken during construction showed that the pads are homogeneous, and radiation monitoring with a calibrated instrument showed that radioelement concentrations are in agreement with those assigned to similar facilities in Canada, Finland, Sweden and the USA. The North Ryde facility is available to users of gamma-ray spectrometers wishing to calibrate their instruments.

A ‘natural field’ seismic technique is possible to attain by observing microseisms with a suitably designed array and by digitally processing the data to obtain estimates of the phase velocities of Rayleigh waves. Wavelengths of interest in detecting depth to the basement of sedimentary basins are in the range 2 to 20 km, and correspond to wave periods from 1 to 7 s. An array of five or seven seismometers deployed as an expanding cross configuration simplifies field procedures and is adequate for phase velocity measurements of Rayleigh waves in the required wavelength range, provided high-resolution frequency-wavenumber spectral analysis is used. This analysis can be implemented on a minicomputer in the field. Results obtained from observation in a sedimentary basin of known structure show predominantly fundamental-mode Rayleigh wave propagation. The scatter of velocity estimates is small enough to allow inversion by curve matching, and depth to the basement can be computed to an accuracy of ±30 per cent without requiring restrictive assumptions of a seismic velocity structure.

The outputs of geophone array elements are conventionally summed into a single output trace. This summation attenuates incoherent noise, horizontally propagating surface waves, and obliquely incident events. The geophone subarray beam-steering process is a plane-wave stacking technique which removes the differential moveout and improves the resolution of seismic data by directing the subarray peak gain to the incident angle of the seismic wavefront. The plane-wave stacking process transforms the data from the offset domain to the ray parameter (p) domain, and restricts the range of p as a function of time. Studies of synthetic and marine field data show that the beam-steering process improves the signal-to-noise ratio of obliquely incident events as compared to conventional subarray summing operations. The beam-steering process, compressing high-density data while preserving the high-frequency content of the seismic signal, is a cost-effective technique to process large quantities of closely spaced seismic data for stratigraphic exploration.

The ratio of the velocity of compressional waves, Vp, to the velocity of shear waves, Vs, is an important parameter for interpreting geophysical field data. Recent studies have emphasized the role played by pore geometry in controlling Vp/Vs in homegeneous rocks. We measured the carbonate content of a set of siliceous limestones of varying proportions of carbonate and silica and observed the pore structures of these samples using a scanning electron microscope. The range of Vp/Vs of individual samples during increasing confining pressure is consistent with crack-closure theory. However, the value of Vp/Vs within the sample set as a whole is dominated by its carbonate content. Variations in Vp/Vs due to total porosity and pore geometry are around 0.1, whereas the change due to composition is 0.4. Values of pore aspect ratios gained from comparison of the velocity-porosity-composition data with theory are in good agreement with the electron microscope observations.

Six ‘optimum’ estimators for the root-mean-square (rms) seismic velocity are given and analyzed by simulation for rms error. Two of the estimators are used to test use of a priori velocity information in a Kalman-type improvement on the time measurements. Parameters varied include centre-point depth (time), a priori velocity variance, and interdelay-estimate correlation. The maximum likelihood estimator is shown to be best when a priori information is relatively good, but a least-mean-square estimator is equally good otherwise.

A signal/noise separation must recognize the lateral coherence of geologic events and their statistical predictability before extracting those components most useful for a particular process, such as velocity analysis. Events with recognizable coherence we call signal; the rest we term noise. Let us define ‘focusing’ as increasing the statistical independence of samples with some invertible, linear transform L. By the central limit theorem, focused signal must become more non-Gaussian. A measure F defined from cross entropy measures non-Gaussianity from local histograms of an array, and thereby measures focusing. Local histograms of the transformed data and of transformed, artificially incoherent data provide enough information to estimate the amplitude distributions of transformed signal and noise; errors only increase the estimate of noise. These distributions allow the recognition and extraction of samples containing the highest percentage of signal. Estimating signal and noise iteratively improves the extractions of each. After the removal of bed reflections and noise, F will determine the best migration velocity for the remaining diffractions. Slant stacks map lines to points, greatly concentrating continuous reflections. We extract samples containing the highest concentration of this signal, invert, and subtract from the data, leaving diffractions and noise. Next, we migrate with many velocities, extract focused events, and invert. Then we find the least-squares sum of these events best resembling the diffractions in the original data. Migration of these diffractions maximizes F at the best velocity. We successfully extract diffractions and estimate velocities for a window of data containing a growth fault. A spatially variable least-squares superposition allows spatially variable velocity estimates. Local slant stacks allow a laterally adaptable extraction of locally linear events. For a stacked section we successfully extract weak signal with highly variable coherency from behind strong Gaussian noise. Unlike normal moveout (NMO), wave-equation migration of a few common-midpoint (CMP) gathers can image the skewed hyperbolas of dipping reflectors correctly. Short local slant stacks along midpoint will extract reflections with different dips. A simple Stolt (1978) (f-k) type algorithm migrates these dipping events with appropriate dispersion relations. This migration may then be used to extract events containing velocity information over offset. Offset truncations become another removable form of noise. One may remove non-Gaussian noise from shot gathers by first removing the most identifiable signal, then estimating the samples containing the highest percentage of noise. Those samples containing a significant percentage of signal may be zeroed; what remains represents the most identifiable noise and may be subtracted from the original data. With this procedure we successfully remove ground roll and other noise from a shot (field) gather.

This paper discusses the close relationship between seismic migration and multidimensional inversion according to the linearized inverse scattering theory. The linearized inverse scattering approach represents a mixed modeling-inversion procedure. Unlike seismic migration, the actual inversion process is carried out on the difference between a modeled reference response and the actually measured data. The output is generally presented in terms of the elastic parameters of the medium. Siesmic migration represents a direct inversion method: the downward extrapolation process is carried out directly on the measured data. Output is presented in terms of reflectivity. If the reference medium has been chosen in such a way that (1) the total wave field in the reference medium can be split into a downward traveling source wave field and an upward traveling response (the propagation of both wave fields being defined by the one-way wave equation) and that, (2) the upward traveling response in the reference medium can be neglected with respect to the upward traveling response in the actual medium, then seismic migration and linearized inversion define identical inversion processes. Typically, the above conditions are fulfilled in a homogeneous reference medium. In iterative multidimensional inversion, the full inverse scattering problem is approached by a number of linearized inversion steps. I show that each linear step consists of a prestack migration process and a prestack modeling process, the modeling output being used to remove the contribution of multiple scattering. Finally, I argue that for a proper inversion process, information on the elastic parameters outside the seismic frequency bandwidth (temporarily and spatially) should be accounted for in the reference medium.

A comparison of common-midpoint (CMP), single-shot, and plane-wave migration was made for simple two-dimensional structures such as a syncline and a horizontal reflector with a laterally variable reflection coefficient by using synthetic seismograms. The seismograms were calculated employing the finite-difference technique. CMP sections were simulated by 18-fold stacking and plane-wave sections by slant stacking. By applying a finite-difference scheme, the synthetic wave field was continued downward. The usual imaging condition of CMP migration was extended in order to carry out migration of single-shot and plane-wave sections. The reflection coefficient was reconstructed by comparing the migrated wave field with the incident wave field at the reflector. The results are: (1) all three migration techniques succeeded in reconstructing the reflector position; (2) as a consequence of the finite aperture of the geophone spread, only segments of the reflector could be reconstructed by single-shot and plane-wave migration; (3) for single-shot and plane-wave migration the reflection coefficient could be obtained; and (4) CMP migration may lead to incorrect conclusions regarding the reflection coefficient.

One of the most important problems in exploration seismology is to relate the surface seismic measurements with the subsurface geologic parameters. The concept of wavefront curvature has been in extensive use for this purpose. Byun (1982) developed relationships between several measurable seismic parameters (e.g. geometrical spreading and normal moveout velocity) and parameters of the media with elliptical velocity dependencies. This paper extends the wavefront curvature concept to more general, transversely isotropic media. After a brief discussion on ray tracing, a procedure is developed to describe the local properties of the ray based on an elliptical surface fit to the actual wave surface. The apparent velocities of the elliptical fit are then used to generalize the seismic parameters developed in Byun (1982). Simple numerical experiments are given to demonstrate the explorational significance of the theory. It is shown that the measurements of the normal moveout velocity are not sufficient to estimate the velocity structure of the transversely isotropic medium. The ‘side-slip’ effect can lead to significant errors in depth-mapping dipping reflectors.

Plane-wave decomposition of the vertical displacement component of a spherical-wave field corresponding to a compres-sional point source is solved as a set of inverse problems. The solution method utilizes the power and stability of Backus and Gilbert (smallest and flattest) model-construction techniques, and achieves computational efficiency through the use of analytical solutions to the involved integrals. The theory and algorithms developed in this work allow stable and efficient reconstruction of spherical-wave fields from a relatively sparse set of their plane-wave components. Comparison of the algorithms with discrete integration of the Hankel transform shows very little or no advantage for the transformation from the time-distance (t-x) domain to the intercept time angle of emergence (T-y) domain if the seismograms are equisampled spatially. However, when the observed seismograms are not equally spaced or the transformation r-y to t-x is performed, the proposed schemes are superior to the discrete integration of the Hankel transform. Applicability of the algorithms to reflection seismology is demonstrated by means of the solution of the problem of trace interpolation, and also that of the separation of converted S modes from other modes presented in common-source gathers. In both cases the application of the algorithms to a set of synthetic reflection seismograms yields satisfactory results.

A new finite-difference (FD) method is presented for modeling SH-wave propagation in a generally heterogeneous medium. This method uses both velocity and stress in a discrete grid. Density and shear modulus are similarly discretized, avoiding any spatial smoothing. Therefore, boundaries will be correctly modeled under an implicit formulation. Standard problems (quarter-plane propagation, sedimentary basin propagation) are studied to compare this method with other methods. Finally a more complex example (a salt dome inside a two-layered medium) shows the effect of lateral propagation on seismograms recorded at the surface. A corner wave, always in-phase with the incident wave, and a head wave will appear, which will pose severe problems of interpretation with the usual vertical migration methods.

The response of an electromagnetic induction logging tool passing through many invaded thin beds is simulated by the finite-element method. This simulation has achieved high accuracy by using a difference potential which enables the transmitter-receiver mutual coupling to be treated analytically. Consequently the removal of the mutual coupling from the induction tool response has no numerical ill effects. The finite-element model is truncated at a very large distance with a zero field outside the model. In order to achieve both accuracy and computational efficiency, the grid is projected to the truncation surface by gradually increasing its size according to an estimated error analysis of the finite-element method. The numerical results were verified against analytical solutions for limiting cases and excellent agreement was obtained. In the presence of skin effect, which is beyond Doll’s analysis by geometrical factor theory, the finite-element solution conveniently provides a way to check and improve the interpretation of induction logs. It also lends itself to future applications in tool design, signal processing, and resistivity inversion schemes.

The possibility of using various types of electrical methods to locate oil or gas fields has been proposed in recent years. In an effort to quantify the anomaly to be expected with electrical sounding methods, average geoelectrical parameters have been determined by studying electrical well logs from several oil fields characterized by different geoelectrical sections. The dc resistivity anomaly due to the presence of an oil-bearing layer at depth depends upon the sequence of resistivities above and below and the electrode array employed. The radial dipole array gives the largest anomaly values, and is followed by other arrays such as the Schlumberger and Wenner arrays. The maximum anomaly in apparent resistivity is observed when the resistivity beneath the target zone is lower than that above; the relative anomaly in apparent resistivity is almost the same as the contrast ratio of the transverse resistance of the oil-bearing layer to the overlying beds. When the radial dipole array is used, a limited areal extent of the oil-bearing layer does not cause a significant change from the anomaly value due to a layer of infinite lateral extent. In that case the least dimension is about four times the depth. Use of one buried current electrode in the vicinity of the oil-bearing layer increases the amplitude of the anomaly; the maximum anomaly appears at a separation comparable to the depth. Typical anomalies in apparent resistivity caused by these oil fields range from less than 0.1 per cent to more than 10 per cent. Such anomalies would be detectable only with an order of magnitude improvement in the capabilities of electrical sounding methods, or with considerably larger oil field targets.

Bias effects due to uncorrelated noise on electrical and magnetic channels are cast into a quantitative framework which allows for an immediate conservative judgment of impedance tensor quality from a single station set up. The horizontal magnetic field is regarded as input to a linear system with the horizontal electrical field and the vertical magnetic field as outputs. The coherence between inputs, the predicted coherence between outputs and inputs, and the polarization characteristics of the inputs determine the extremal bias effects. Exact expressions for impedance tensor elements and tipper elements are given. Moreover, rotation angles of principal impedance direction, skew, and ellipticity are calculated. Finally, we study some practical examples that show some of the characteristics predicted by our model.

The formal electromagnetic coupling solution for a dipole dipole electrode array configuration has been modified to include cultural coupling in a uniform conducting half-space. Solutions are obtained for survey lines oriented at an arbitrary position and angle with respect to a cylindrical structure. The convergence properties of the general mutual impedance solution are analyzed using a low-frequency approximation which is useful in predicting cultural anomalies in the frequency range of spectral IP surveys as long as all significant dimensions are less than one skin depth. Both interfacial polarization and induced currents in the cylindrical conductor are considered in examining the behaviour of the overall spectrum as seen by an external observer. Spectral responses for dipole-dipole arrays oriented perpendicular and parallel to the buried conductor show that the phase shift is the most diagnostic parameter for pipe depth and survey data distortion. The results also show that field survey procedures can be devised to minimize such interference effects when the pipe position is known.