Exploration Geophysics - Volume 54, Issue 4, 2023

Estimating the electromagnetic response of a conductive sphere in a layered earth is of great interest in terms of both modelling and interpretating data acquired via geophysical electromagnetic methods where the target is at some distance from the source and receiver. This is particularly the case when using Airborne Electromagnetic Method (AEM) where the source and receiver are located at some height above the subsurface. This problem can be solved by utilising field expansions representing derivatives of cylindrical functions, which describe the fields propagating in the layered earth, and spherical functions, which describe the fields reflected by the sphere. Furthermore, these representations allow the development of relationships between cylindrical and spherical functions. These functions and subsequent relationships have been used to develop an algorithm for estimating the electromagnetic response of a conductive sphere in an isotropic layered earth. Software based on this algorithm has been tested on both synthetic and field data. The field data presented were collected with the AEM AirTEM system over the Reid-Mahaffy test site, Ontario, Canada. Results from these tests prove the importance and utility of integrating the sphere in a layered earth model in the AEM interpretation toolbox.

The pressure dependence of elastic parameters of rocks is mainly controlled by the geometry of the pore space. In general, the compliant-stiff pore structure model can be used to reasonably describe this pressure dependence. However, our experiment measurements revealed that for tight sandstone rock with complex pore structures, the contribution of the compressibility of the stiff pores to the elastic modulus is significant. The dual porosity is not sufficient to explain the variation of ultrasonic velocity with pressure. For this reason, we adopted a triple pore structure to divide the rock pore space into equant pores, intermediate pores and compliant pores. Our laboratory measurement and model results show that this pore space division can better describe the pressure dependence of the elastic moduli of rocks. The low-frequency stress–strain measurements show that the fluid-saturated tight sandstone has obvious dispersion in the seismic frequency band, which is primarily attributed to the squirt flow effect. In order to study the pressure and frequency dependence of the elastic moduli of tight sandstone, we retrieved the geometric parameters of the pore structure from the pressure variation of the ultrasonic velocities under dry conditions. Based on this complex pore structure and the extension of the squirt flow model, we constructed an elaborate rock physics model to explain the pressure and frequency dependence of velocity. The model does not require adjustable parameters, and all parameters are measured and calculated by the laboratory, which improves the accuracy of theoretical modelling. The modified squirt flow model can be used to describe dispersion and attenuation in a wide frequency band, and fit well with the velocity measurements in both the low-frequency range and the ultrasonic frequency range under different pressures. Therefore, this rock physics model could be applied in the extraction of pore microstructure and fluid properties provided elastic moduli or velocities can be estimated accurately.

This study improves a collection of attributes to detect subtle faults in three dimensional data obtained from the Krishna-Godavari (KG) basin, with results displayed on synthetic and real datasets. Seismic attributes, for instance, curvature and coherence, are often used to delineate discontinuities, such as faults and fractures where hydrocarbons may have been trapped. These attributes have their advantages subjective to the seismic data. In this paper, we propose a multi-attribute framework for identifying subtle faults inside seismic volumes. Curvature attribute is a powerful and popular technique to deal with these faults. The faulted horizon is fitted on the quadratic surface using the least-square method, and the most positive and most-negative curvature attributes are calculated, which are further used in saliency map calculations. Several signal processing techniques, such as Hough transform and ant tracking, have been used to delineate faults. Here, we have proposed a novel signal processing approach based on energy variations known as top-down saliency on the curvature attributes using 3D-FFT local spectra and multi-dimensional plane projections. To analyze the directional nature of seismic data, the directional center-surround technique is employed for visual attention. Furthermore, the log-Gabor filter and image erosion are applied to the saliency-rendered seismic volume to highlight the oriented amplitude discontinuities at faults. Most of the time, these discontinuities may not be very prominent to find the subtle faults and other trace-to-trace hidden geological features in three-dimensional seismic data. In our work, calculated attributes assist us in mapping these changes, because they are all differently sensitive to the faults and fractures in unique ways. Experimental results on real field seismic data from the Krishna-Godavari basin prove that the proposed algorithm is effective and efficient in tracking subtle and minor faults, better than previous works.

The Temblador Field is located on the southern flank of the Eastern Venezuela Basin. The Oficina Formation, present in the field, represents one of the most important producing Formations in the country. In this project, a detailed characterisation and delimitation of the sands of the Jobo and Morichal Members of the Oficina Formation. The work was outlined in three stages. The first stage consisted of studying the elastic properties of the reservoirs through unconsolidated rock physics modelling through the elaboration of the “Rock Physics Template” (RPT). Likewise, during this phase, different crossed graphs were prepared that allowed lithological discrimination. In the second stage, cubes of elastic properties such as P-wave impedance, S-wave impedance, Mhu-Rho (μρ) and Lambda-Rho (λρ) were obtained through the inversion of the Extended Elastic Impedance. Finally, the third stage consists of the classification of the lithofacies, of the cubes resulting from the inversion, through a Bayesian classification. These results allowed the construction of sand probability maps that allow delimiting the best quality sands within the Jobo and Morichal Members. In this way, the present work provides an additional tool when carrying out reservoir exploration studies, reducing uncertainty when locating a new prospect.

The Permian coal seams in eastern Yunnan and western Guizhou are thin, numerous, and staggered with other thin coal seams. Depicting the fine characteristics of coal reservoirs is pivotal for the safe and efficient exploitation of coal and coalbed methane (CBM), and is important for transparent mining. To improve the inverse resolution and accuracy of predicting reservoir thickness, this study used the model-based acoustic impedance (AI) inversion method that utilizes seismic and logging data. This method changes seismic data, reflecting stratigraphic interfaces, into AI data, reflecting lithologic structures. Moreover, it avoids the relevant assumptions of wavelets and reflection coefficients. Compared with other inversion methods, model-based AI inversion strengthens the description of thin reservoir horizontal and vertical changes. The results showed that comprehensively using intermediate-frequency seismic information and high-low-frequency logging data greatly broadens the seismic data frequency band and improves the dominant frequency of the reflected wave. Furthermore, the AI profile resolution and the prediction accuracy of the physical parameters for the target geological body can be improved. A cross-validation comparing the inverted thickness and measured thickness of borehole cores was applied to achieve fine prediction (error of appropriately 0.02–0.4 m), providing a basis for CBM development.

Rayleigh and body waves are both solutions of the same propagation equation, but correspond to different wavenumber regions and boundary conditions, so their interaction with the elastic parameters (Vp, Vs and density) provides independent constraints during inversion. We develop and illustrate concurrent, elastic, full-waveform inversion of P and S body- and Rayleigh-waves using interleaved envelope- and waveform-based misfit functions, in a gradually-increasing frequency, multi-scale, inversion strategy. A wavelet and its envelope have different effective bandwidths, spectral shapes, and provide complementary frequency and wavenumber weighting in concurrent inversion. Because of the greater depth extent sampled by the exponentially decaying tail of a Rayleigh wave, compared to a body waveform, the depth extent of the model required to support both body and surface waves in concurrent inversion is defined by the Rayleigh waves. Correlation coefficients provide quantitative measures of the contributions of the data subsets to the fits of the solutions. For both smooth and constant starting models, concurrent interleaved inversion gives smaller data misfits than the envelope-only and waveform-only solutions. Treating the whole wavefield as a single data set means that it is not necessary to separate, or even to identify, different types of body and surface waves.

In the paper, we describe an original 3D travel-time tomography approach. It is based on the new realization of the bending method which to some extent takes into account the band-limited nature of real seismic signals propagation. As a result, two-point ray tracing provides more reliable ray trajectories and travel times in complex media. Another original feature of the proposed tomography is that the model is represented using the Chebyshev polynomials. Such parameterization allows analytical calculation of travel times and their derivatives with respect to model parameters and significantly reduces the number of parameters to be recovered during inversion compared to more common grid tomography. In certain situations, the proposed approach provides significant computational advantages. Numerical examples prove its efficiency.