Exploration Geophysics - Volume 51, Issue 4, 2020

If a magnetic dipole is placed above the surface of the earth, the electromagnetic induction (EMI) effect, encoded in Maxwell's equations, causes eddy currents in the soil which, on their turn, induce response electromagnetic fields. The magnetic field can be measured in geophysical surveys to determine the conductivity profile of the ground in a non-destructive manner. The forward model used in the inversion of experimental data usually consists of a set of horizontal homogeneous layers. A frequently used analytical model, proposed by McNeill, does not include the interaction between the eddy currents and therefore fails for larger conductivities. In this paper, we construct a new forward, analytical, model to estimate the magnetic field caused by a horizontally stratified earth which approximates the interaction between eddy currents. This makes it valid for a broader range of parameters than the current state of the art. Furthermore, the error with the (numerically obtainable) exact result is substantially decreased. We also calculate the vertical sensitivity (“depth of exploration”) of the model and observe that it is in good agreement with the values obtained from the exact model.

In rock physics one of the important purposes of the determination of P- and S-wave velocities is to obtain the elastic constants and anisotropy parameters. This method is standardised in American Society for Testing and Materials (ASTM) by a standard D2845-95. The most challenging yet important part of these experiments is picking the first arrivals correctly and consistently. A sharp, clean first arrival of a wave is not always possible, especially when testing dry, porous sandstones at low pressures. The signal transmitted through such samples is weak due to a high damping effect, and picking the first break introduces uncertainties. Harvey sandstone is a sample type with high porosity and, therefore, some degree of anisotropy is expected. In order to study the anisotropy parameters of the Harvey 3 sandstone which belongs to Lesuer- Wonnerup Member of the Yalgorup Member, three core plugs in directions of horizontal, vertical and diagonal, were experimentally investigated. The laboratory measurements on three different samples which are cut at three different directions with regards to the base deposition system enabled us to calculate the anisotropy parameters using a standard acoustic equipment while applying stress. We concluded that Harvey 3 poses a more complex symmetry axis of anisotropy, and no sign of low-grade anisotropy was observed through the experimental data. To study the anisotropy degree of the Harvey 3 sandstone, a pressure cell, pore fluid injection pump, and ultrasonic system (consisting of P- and S-wave transducers, oscilloscope and pulser/receiver) were used to record the elastic waves passing through the samples. The specimens were fully saturated inside the pressure cell using vacuum injection for the best possible comparison of dry and saturated status. The results confirmed that the sample is far from weak anisotropy and possesses more axes of symmetries than simple layering anisotropy due to the pore orientation and distribution in the matrix of the background rock. The study also confirms that the anisotropy degree decreases under full compaction and full saturation, and the Harvey 3 anisotropy becomes elliptical when it’s fully saturated. Large, stress-dependent changes in the ultrasonic velocities for porous media were observed in this experimental study, which confirms the usefulness of such studies in examining the inner structure change process.

The wavelet stretch effects in pre-stack migration, which is similar to that of NMO stretch, distort the frequency spectrums and taper the high-frequency components at far offsets. The long-offset migrated results in conventional migration methods are muted, resulting in a loss of effective information about the subsurface investigation. Only short-offset data are used to obtain a high-resolution stacked image. However, long-offsets that correspond to high-incident-angle data are critical to velocity inversion and amplitude versus offset (AVO) analysis when estimating lithology and fluid product. Furthermore, attenuation of seismic waves due to the anelasticity of the subsurface medium will cause dissipation of seismic energy and loss of high frequencies, thus broadening the propagating wavelet and degrading the resolution of imaging. Both the wavelet stretch and the absorption will taper the high-frequency components. We developed a wavelet stretch correction and absorption compensation scheme during the migration process by shrinking the wavelet when applying the imaging condition, which matches the stretch effect caused by migration. The correction factor is derived based on the incidence angle associated with many factors. Moreover, the de-absorption pre-stack time migration (QPSTM) is implemented on the effective Q model that makes the Q modelling become easier. Both the application of synthetic and real data examples demonstrate that the proposed anti-stretch QPSTM can be particularly useful recovering the high-frequency components caused by anelasticity and wavelet stretch, and obtain higher-resolution images.

Elastic wave modelling in the Laplace domain is the foundation of Laplace-domain elastic wave full waveform inversion. In order to use Laplace-domain numerical modelling schemes efficiently, appropriate grid intervals must be chosen correctly. The determination of grid intervals is based on numerical dispersion analysis of Laplace-domain elastic wave equation. A new method of numerical dispersion analysis is developed for Laplace-domain 2-D elastic wave equation. The novelty of the method lies in two aspects: (1) Based on the concept of the pseudo-wavelength for both P- and S-waves, I introduce a general quantity which is a combination of the Laplace constant, velocity and grid interval. Therefore, the dispersion relations no longer directly depend on the concrete Laplace constant, velocity and grid interval. Just like the frequency-domain dispersion analysis, a general conclusion with regard to the general quantity can be made; (2) The ratio of numerical eigenvalue to analytical eigenvalue can be interpreted as the normalised attenuation propagation velocity. The number of grid points per S-wave pseudo-wavelength is determined by the accuracy of normalised P- and S-wave attenuation propagation velocities. Based on a commonly used finite-element scheme, the numbers of grid points per S-wave pseudo-wavelength are determined for different Poisson ratios and for different ratios of directional grid intervals. These results are important in applying the finite-element scheme to Laplace-domain elastic wave modelling and full waveform inversion. Comparisons with the analytical solution validate the criterion on the numbers of grid points per S-wave pseudo-wavelength.

Exploration of oil and gas resources in complex areas has always been a key issue in seismic exploration. The Tarim Basin, covered by a large area of desert, contains abundant oil-gas reservoir resources. However, due to the complex exploration environment, the signal-to-noise ratio of seismic data collected in desert area is low and the resolution is poor. The frequency band overlaps between effective reflected signal and low-frequency desert noise. Aiming at the specific characteristics, a desert noise suppression method based on signal oscillation component decomposition had been proposed for desert seismic data. According to the oscillation characteristics of desert seismic data, tunable-Q wavelet transform was introduced to achieve oscillation component decomposition. We put forward the multi-Q (multi-quality factors) model to determine basis function, so as to achieve the better matching of signals and noise. It can better adapt to the decomposition of desert seismic data, and complete the separation of reflected events and low-frequency desert noise with overlapping frequency band under the condition of low signal-to-noise ratio. It can be seen from the processing results of synthetic records and real data that the proposed method has better advantages in desert noise suppression compared with the Shearlet transform, empirical mode decomposition and F-x deconvolution methods.

Body waves may affect phase velocity obtained from microtremor array surveys in some rare cases. Fitting theoretical phase velocities based on a surface-wave theory to observed phase velocities affected by body waves would therefore result in distorted images of subsurface S-wave velocity structure. In this study, we present a method for the theoretical calculation of phase velocities in which the full-wave field (i.e. a wavefield including not only surface waves but also body waves) is taken into account. In numerical experiments conducted in this study, in which we considered the full-wave field, we generated synthetic microtremors by randomly distributing point vibration sources on the surface of a horizontally stratified velocity model. We then determined the phase velocities by applying the spatial autocorrelation (SPAC) method to the synthetic vertical-component wave data. The phase-velocity dispersion curve thus obtained exhibited a shape with a clear peak, with a peak value (peak phase velocity) exceeding the S-wave velocity of a bedrock in the model, which was not explainable with a surface-wave (Rayleigh-wave) theory.

We conducted systematic numerical experiments and clarified the following two features of the peak phase velocity: (1) the peak phase velocity becomes large as the contrast of the S-wave velocities between the surface layer and the bedrock, or the P-to-S-wave velocity ratio (related to the Poisson’s ratio) in the surface layer gets large, and (2) the frequency at which peak phase velocity occurs (peak frequency) lies in the vicinity of the S-wave resonance frequency of the ground. Both the peak phase velocity and the peak frequency were theoretically reproduced by the calculation method that we propose in this study, based on a SPAC method modified to consider the full-wave field. These results imply the possible improvement in the accuracy of microtremor array survey analysis for velocity-structure inference, by applying a full-wave theory to the peak phase velocity.

Development of an accurate reservoir model and prediction of reservoir behaviour associated with geothermal exploitation using that model are fundamentally important for sustainable geothermal utilisation. Gravity observation for a few tens of days after a field-wide shut-in of geothermal wells potentially provides valuable constraints to improve reservoir models. We investigated the applicability of superconducting gravimeters (SGs) to short-term observations after shut-ins, as a first step towards the development of a method for reservoir model improvement using short-term gravity data. Results from a reservoir simulation of a shut-in at a geothermal field in Japan showed that gravity change at 20 days after the shut-in was 10 nm/s². To detect such changes with SGs, locations of SG stations should be chosen to minimise noise sufficient to resolve the signal. Considering that field operations and traffic in geothermal fields can make SG data noisy, laboratory gravity measurements were carried out with a SG to investigate the influence of noise level on signal detection. Test masses, which produced a gravity change (7.7–9.0 nm/s²) comparable to the simulation result, were repeatedly placed and removed above the SG during both day and night times with different ambient noise levels. Test results showed that noise level greatly affected signal detection. The signal was only detected twice, under low-noise conditions with the standard deviations of 1.1 and 5.3 nm/s². In addition, tests in which the SG was operated without a cryogenic refrigerator revealed that the absence of the refrigerator did not increase the noise level. Enhancing the portability of a SG by reducing its weight can make multi-location noise surveys feasible. Our method, of combining reservoir simulations and preliminary noise surveys, would enable successful SG observations to be made in relatively short periods after shut-ins, which would possibly contribute to the development of a method to obtain additional constraints to be used for reservoir model calibration.