Exploration Geophysics - Volume 53, Issue 1, 2022

When the fracture normal forms an angle relative to the borehole axis, the induction logging response to the fractured formation consists of two contributions: one is the volume contribution caused by the eddy current, and the other is the charge contribution caused by the cumulative charges on the upper and lower boundaries of the fracture; both contributions are nonlinear. Researching the distribution of surface charges and eddy current in inclined fractured formations is of great significance for logging analysts and geologists to fully understand induction logging laws in such setting. However, there is insufficient research on this aspect. In this work, first, an electromagnetic field calculation method under the condition of existing cumulative charges at the interfaces was developed. Second, an induction logging numerical simulation model of inclined fractures based on the three-dimensional (3D) finite element method (FEM) was established, and the distribution laws of cumulative charges and eddy current in fractures under different dip angles were summarised. Third, we used the annular conductive region in the fracture as the object of our research, and analysed the influence law of different conductive regions distribution on induction logging. Finally, we carried out physical experiments to verify the numerical results and achieved good agreement between the two results. Hence, it proves the validity of the methods and conclusions.

The presence of noise degrades the quality of seismic data and makes the subsequent processing tasks and interpretation more challenging. Therefore, seismic noise attenuation is a key step in the processing of seismic data. We propose a novel convolutional neural network (CNN) framework with learning noise prior. Unlike conventional CNN-based seismic denoising methods, this new network is composed of a noise extractor and a denoiser. The noise extractor extracts noise from the original data to provide a high-precision noise prior to the denoising process. The denoiser uses the noise prior for denoising of the seismic data. This method is superior to the presently used networks in terms of the denoising effect. Additionally, the proposed network can be applied for random noise suppression as well as coherent noise attenuation. Synthetic and field tests illustrated the superiority of the proposed approach over the traditional denoising methods in suppressing noise and improving the signal-to-noise ratio (SNR) of seismic data.

The present work demonstrates a case study in the Mahanadi offshore basin in the eastern margin of India, where gas hydrates have been delineated by seismic experiment, mainly by identifying a bottom simulating reflector or BSR on seismic section, based on its characteristic features. The BSR represents an acoustic impedance contrast between the high-velocity gas hydrates bearing sediments above and low-velocity gas-bearing or water-saturated sediments below. At some places along the continental margins of the world, the BSR has not been identified but gas hydrates were recovered by drilling, whereas at some other places BSR was identified but no gas hydrates were found. Thus, we need to find out an attribute that can be used to characterise gas hydrates and free gas reservoirs. The purpose of this work is to demonstrate this through spectral decomposition in the time–frequency domain. The study based on continuous wavelet transform (CWT) approach, which uses a mathematical mother wavelet, shows the illuminated zone with higher energy low-frequency anomaly, associated with free gas below the BSR. The study demonstrates that the CWT technique provides higher spectral-spatial resolution than that obtained by the short-term Fourier transform technique.

Low-frequency signal and noise are the main components of seismic data in Northwest China desert area. To remove noise and preserve data details, we introduce a method of nonlinear autoregressive with external input neural network (NARX) which is based on recent developments in nonlinear time series prediction and improve its performance according to signal detection theory. The input of NARX is a noisy seismic signal, the output is original predicted background noise, and then a threshold is set for the output to eliminate the waveform distortions. The residual error of the input and the output is filtered signal. We test the proposed method on synthetic and field seismic data and compare it with some conventional filtering methods (wavelet denoising method, f–x filter and complex diffusion filtering). The results prove that the proposed method can greatly attenuate low-frequency noise, and preserve data details as completely as possible.

Full-waveform inversion (FWI), which is among the most powerful seismic data processing techniques for imaging subsurface geological structures, has a huge computational cost in proportion to the number of sources. To increase the speed of FWI, we explored the use of common-receiver gathers (CRGs) as an alternative to common-shot gathers (CSGs) as observed data. This approach has the potential to reduce the computational cost of FWI significantly, particularly for ocean-bottom cable (OBC) acquisition. As we match modelled and observed CSGs in CSG-based FWI, we match observed CRGs with modelled CSGs of switched source–receiver geometry in CRG-based FWI. According to the reciprocity principle, CRG FWI based on the steepest descent or Gauss–Newton method yields the same inverted velocity models as CSG FWI for an identical and known source signature applied over whole source positions. However, when the source wavelet is unknown, and changes from one source position to another, each trace of a CRG contains a different source signature, making it difficult to apply conventional source estimation or source-independent methods to CRG FWI. Therefore, in this study, we proposed inversion strategies for CRG FWI in both the time and frequency domains to deal with errors arising from different source wavelets between CRG traces. Then, we used a synthetic example for the Marmousi-II model and a real data example from the North Sea to demonstrate that our strategies for CRG FWI provide very similar inverted velocity models to those obtained from CSG FWI, at a lower computational cost.

Gas hydrate exploration was conducted using logging-while-drilling (LWD) and wireline logging (WLL) at two sites (UBGH2-6 and UBGH2-10) in the Ulleung Basin, East Sea (Sea of Japan). Coring for gas hydrate sampling was also conducted. Seismic profiles revealed that the upper layers at the logged sites (∼180 m at UBGH2-6 and ∼150 m at UBGH2-10) are dominated by turbidite/hemipelagic sediments, whereas the lower-lying sediments are characterised by thick mass transport deposits. Except in the gas hydrate-bearing interval (140–160 metres below the seafloor) of UBGH2-6, the WLL velocity and resistivity data are a little higher (∼100 m/s and ∼0.4 ohm-m, respectively) than those indicated by LWD. Conversely, the density and porosity do not show a consistent pattern for the two methods. Discrepancies between LWD and WLL can be caused by the methodology, borehole condition, and gas hydrate occurrence type. The high resistivity values (∼100 ohm-m) in the gas hydrate-bearing zone of UBGH2-6 are typical of inclined fracture-filling gas hydrate. In this case, the LWD resistivity may be overestimated as a result of the data acquisition principle. The relationships between the physical properties are significantly controlled by the presence of gas hydrate. Therefore, in fracture-filling gas hydrate-bearing sediments, the WLL resistivity data are more reasonable than the LWD resistivity data. Both methods have advantages and disadvantages in terms of data quality, efficiency, and resolution. Therefore, it is necessary to compare and interpret data measured using both methods to identify the physical properties of gas hydrate-bearing sediments.

Eikonal solvers in the Cartesian coordinates often suffer from source singularity due to the plane-wave assumption for the wavefront near the source point. Traveltime errors induced by the source singularity near the source point will spread to the whole domain and reduce the accuracy of subsequent traveltimes. The source singularity can be avoided if the eikonal equation is formulated and solved in the polar/spherical coordinates. However, the grid in the polar/spherical coordinates will cause sparse non-uniform resampling in the region far away from the source point after the traveltime is transformed back to the Cartesian mesh, hence reducing the uniform accuracy in this region. To deal with the source singularity and maintain uniform accuracy, we introduce a hybrid fast sweeping method that computes the traveltime near the source in the polar/spherical coordinates and computes the traveltime far away from the source in the Cartesian coordinates. The source singularity near the source point is resolved in the polar/spherical coordinates, and the uniform accuracy is achieved by switching to the Cartesian coordinates away from the source point. Numerical examples are presented to demonstrate the method.