Exploration Geophysics - Volume 49, Issue 1, 2018

We quantified effective VP, VS and the ratio of VP to VS of the 3D digital rock models with different crack-filling minerals. By comparing the elastic properties derived from the digital rock models with the seismic velocities around the Nankai seismogenic fault, we characterised the evolution process of the seismogenic fault.

To understand the characteristics of the Nankai seismogenic fault in the plate convergent margin, we calculated the P- and S-wave velocities (VP and VS) of digital rock models constructed from core samples of an ancient plate boundary fault at Nobeoka, Kyushu Island, Japan. We first constructed 3D digital rock models from microcomputed tomography images and identified their heterogeneous textures such as cracks and veins. We replaced the cracks and veins with air, water, quartz, calcite and other materials with different bulk and shear moduli. Using the Rotated Staggered Grid Finite-Difference Method, we performed dynamic wave propagation simulations and quantified the effective VP, VS and the ratio of VP to VS (VP/VS) of the 3D digital rock models with different crack-filling minerals. Our results demonstrate that the water-saturated cracks considerably decreased the seismic velocity and increased VP/VS. The VP/VS of the quartz-filled rock model was lower than that in the water-saturated case and in the calcite-filled rock model. By comparing the elastic properties derived from the digital rock models with the seismic velocities (e.g. VP and VP/VS) around the seismogenic fault estimated from field seismic data, we characterised the evolution process of the deep seismogenic fault. The high VP/VS and low VP observed at the transition from aseismic to coseismic regimes in the Nankai Trough can be explained by open cracks (or fractures), while the low VP/VS and high VP observed at the deeper coseismic fault zone suggests quartz-filled cracks. The quartz-rich fault zone characterised as low VP/VS and high VP in this study could partially relate to the coseismic behaviour as suggested by previous studies, because quartz exhibits slip-weakening behaviour (i.e. unstable coseismic slip).

Data processing techniques are often used to estimate the noise-free response of marine controlled-source electromagnetic (CSEM) data and magnetotelluric transfer functions. We have implemented a new CSEM data processing scheme that uses a robust method based on independent component analysis (ICA) to extract interpretable datasets from noisy marine CSEM data. We applied the data processing scheme to signals from a new CSEM observation system comprising a remotely operated vehicle (ROV) and an ocean bottom electromagnetometer (OBEM). These datasets were obtained around the Iheya North hydrothermal field, Okinawa Trough, Japan. The observation system allows a small-scale CSEM survey to be conducted in areas of steep topography, such as hydrothermal fields, because the ROV can deploy the OBEM at the exact observation site. The results show that the coherent and environment noise that exists in the raw time series is reduced sufficiently by ICA processing. It makes interpretation of the resulting electric field data possible. The results also show that the processed data has a higher signal-to-noise ratio in the middle-to-high-frequency band than the data without ICA. The normalised spectrum, obtained by normalising the observed data from the hydrothermal area, indicates that a conductive anomaly exists in the near-offset area around the OBEM. We apply 2D inversion to the electric field data and find that a low resistivity body exists beneath the OBEM and 50 m offset from the OBEM. This resistivity structure is consistent with images taken by the ROV that show characteristic organisms in hydrothermal seepage around the OBEM site.

We have developed a new controlled-source electromagnetic (CSEM) data processing scheme that uses a robust method based on independent component analysis to extract interpretable datasets from noisy marine CSEM data. The results show that the coherent and environment noise is reduced sufficiently. It makes interpretation of the resulting data possible.

This study uses ultrasonic physical modelling to verify that Fermat’s minimum-time principle is better than the anisotropic non-hyperbolic moveout and conversion-point (CP) equations for calculating the traveltime and CP position of a P-SV wave reflected from a strong vertical transversely isotropic medium.

This study uses ultrasonic physical modelling to test the accuracies of numerical calculations of traveltimes and conversion-point (CP) positions for P-SV wave propagation in a horizontal transversely isotropic (TI) medium. Study results show that the traveltimes and CP positions for P-SV wave propagation on the isotropic plane of a TI medium computed using Fermat’s minimum-time principle are the same as those of using the isotropic non-hyperbolic moveout equation and the isotropic CP equation. However, for P-SV wave propagation on the symmetry-axis plane of a TI medium, the arrival times and CP positions of SV-waves are difficult to determine by any ray methods when the propagation directions of SV-waves are within the cuspoidal SV-wave group velocities zone. But the first arrival times and the propagation of the dominant energy of P-SV waves can still be analysed by ray methods. Based on the calculation of Fermat’s minimum-time principle, if the source-receiver offset is greater than a critical distance, the reflection angles of the converted SV-waves are fixed at a specific angle with a local maximum SV-wave group velocity of the neighbourhood area. This is because the converted SV-waves prefer to propagate along the cuspoidal directions with larger amplitude and higher velocity. Verified by the physical modelling, the Fermat’s minimum-time principle used to calculate traveltimes of P-SV waves is better than the anisotropic non-hyperbolic moveout equation. The physical modelling for the CP position experiment can give a clearer visualisation of the variations of CP positions in the profile, and the feasibility of using Fermat’s minimum-time principle to determine CP positions is also better than that of the anisotropic CP equations. Therefore, in the seismic data processing, Fermat’s minimum-time method is recommended to accurately determine the arrival times and CP positions of P-SV wave propagation in TI media.

Common midpoint (CMP) stacking is one of the essential stages in seismic data processing. Conventional straight mean stacking is based on the assumption that all signals are coherent while all noises are random, which is not always valid in practice. Consequently, many alternative stacking techniques have been introduced in the field of seismic data processing during the past five decades. In this study, a new alternative stacking procedure, called outliers-out (OlO) stack, is proposed and tested using both synthetic and field seismic data. The OlO stacking method is based on analysing the statistical spread of each time sample and excluding a distinctive range of outliers from each time sample. Outliers are calculated independently for each time sample based on the amplitudes distribution within the sample. The outliers’ exclusion process is automatic but it is also adjustable by the user to suite the data under processing. The experimental results of applying the proposed stacking method to both synthetic and field seismic data clearly show that the OlO stacking technique outperforms conventional stacking techniques in terms of the temporal resolution and lateral coherency of the output seismic reflections. Results also show that the tuning function in the OlO stacking algorithm is efficient in dealing with different types of seismic data.

The outliers-out (OlO) stack is a novel seismic data stacking technique that is based on excluding a specific number of outliers from each time sample before stacking. The OlO stacking parameters are automatically obtained from the data according to the statistical distribution of amplitudes. The superiority of the OlO stack has been verified by experiments.

We study seismic data decomposition and reconstruction problems in this paper. Seismic data representation using sparse Gaussian beams is proposed. We formulate this problem as an l0-norm constrained minimisation problem. In solving the l0-norm minimisation, dip-angle scanning is performed and a quasi-Newton method is utilised to calculate the waveform function. Numerical experiments on synthetic data and real data indicate that the seismic data can be properly represented using sparse Gaussian beams with the sparse optimisation method. Moreover, the method has the ability to remove random noise and recover missing data. Finally, the waveform function obtained by sparse decomposition can be used for Gaussian beam migration. The approach described here can obtain a higher signal-to-noise ratio image than the traditional poststack Gaussian beam migration, and the overall runtime is comparable.

Sparse Gaussian beam decomposition and reconstruction is an ill-posed inverse problem. We construct a zero-norm constrained minimisation model, develop a dip-angle scanning strategy and adopt a quasi-Newton method to find the solution. This method can recover full wave field data and can be used for sparse Gaussian beam migration.

We have developed a new interpolation process for insufficient 3D field data that applies 2D curvelet transform-based projection onto convex sets (POCS) to the kx-ky transformed data of each time slice of 3D data. Additionally, to acquire accurate interpolation results, we have designed the preparation process to render the input data irregularly distributed with small-sized gaps.

Complete three-dimensional (3D) land seismic data are often difficult to acquire due to physical or financial limitations. Therefore, in recent years, interpolation techniques with 3D field data have played an important role in seismic data processing. To improve the efficiency of interpolation for insufficient 3D field data, we developed a new interpolation process that applies two-dimensional curvelet transform-based projection onto convex sets (POCS) to the kx-ky transformed data of each time slice of 3D data. Additionally, to acquire accurate interpolation results with the proposed new method, we designed the preparation process to render the input data irregularly distributed with small-sized gaps. We found that the interpolation results of our proposed new method were similar to those using a 3D curvelet transform-based POCS method, with reduced computational cost. The quality of the stacked images of sparsely sampled 3D field data was significantly enhanced by applying our interpolation approach.

For land exploration areas with irregular surface topography, there are many challenges and problems for full waveform inversion (FWI); for example, which type of wave equation should be used to calculate high-accuracy seismic wavefields, how to deal with diffraction of irregular surface topography, what initial velocity model should be utilised to improve the inversion accuracy and how to enhance the computational efficiency of iterative FWI. Aiming at these difficulties, we first simulate the seismic waves with the first-order acoustic wave equation in an auxiliary coordinate system, which easily describes irregular surface topography. Then, we apply this wavefield simulation frame to FWI to improve inversion quality of near-surface regions with strong elevation and velocity variation. Furthermore, to enhance the robustness and computational efficiency, a time-domain multi-scale decomposition method based on the Wiener filter and an optimised encoding strategy are introduced to the proposed inversion frame, and are critical to promoting the practical application of our method. Typical numerical tests prove that the proposed method can obtain more accurate inversion results than the traditional time-domain FWI.

We implement full waveform inversion in an auxiliary coordinate system to improve inversion quality of near-surface regions with strong elevation and velocity variation. Furthermore, a time-domain multi-scale decomposition method and an optimised encoding strategy are introduced to the inversion frame to promote the practical application of our method.

This paper describes a method to generate dataset on stepped frequency continuous wave (SFCW) ground penetrating radar (GPR) for land mine detection. Probability of target detection as well as accuracy of GPR is hindered by ground undulations and GPR mounted vehicle velocity variation. This paper proposes a novel method of filtering out ground undulation effect by ground bounce removal filter and also mitigating GPR mounted vehicle velocity variations. This work also focuses on migration of simulated B-scan and C-scan data using Kirchhoff and F-K migration algorithms. The irregular surface condition of the ground or ground undulation is modelled and a ground bounce removal filter is developed to eliminate the effects of ground undulation. Non-uniform sampling of B-scan replicates the scenario of variation in velocity of the GPR mounted vehicle. The Kirchhoff and F-K migration algorithms applied to the outcome of ground bounce removal filter dataset results in no/less error with respect to true depth and position of the landmine in all possible scenarios. An interactive graphical user interface (GUI) for generating and testing the SFCW GPR data is also discussed in this paper.

This paper describes a method to generate dataset on stepped frequency continuous wave ground penetrating radar for land mine detection. A novel ground undulation and ground bounce removal filter has been proposed which mitigates the effects of vehicle velocity variations. Kirchhoff and F-K migration algorithms are applied to the filtered output.

Airborne multi-wavelength, multi-polarimetric and interferometric radar data (NASA’s AIRSAR/TOPSAR) was processed and applied for geological mapping within densely forested north-western Tasmania. The results successfully compared with geophysical, topographic and vegetation classification information and showed the potential of such data to assist in the upgrading of existing published geological mapping.

Geological mapping within densely forested north-western Tasmania presents problems with access that requires the assistance of remote sensing geophysical technologies. Airborne multi-wavelength, multi-polarimetric and interferometric radar data (NASA’s topographic synthetic aperture radar (TOPSAR)) was applied and compared with geophysical, topographic and vegetation classification information to evaluate its potential to assist in the upgrading of existing published geological mapping. Detailed statistical analysis assessing the accuracy of TOPSAR interferometric imagery showed its reliability with only a small dependence on geological lithological units apparent. The finer 5 m resolution TOPSAR data (C-VV InSAR digital elevation model (DEM)) also reveals information about dykes, lineaments and possible structural trends, not seen in the coarser 30 m resolution shuttle radar topographic mission (SRTM) data. Other sources of DEMs are available, such as LiDAR (light detection and ranging), but at a much higher cost. The longer wavelength P band radar with a horizontal–vertical polarisation (P-HV), also shows improvement over shorter wavelength C and L band radar for mapping lineaments and ground cover related to geological units and/or their geobotanical associations. The use of detailed vegetation classification information is recommended as part of any geological interpretation of such multi-wavelength and multi-polarimetric radar data.

The Tertiary magmatic belts host the porphyry copper deposits in Iran. The combination of upward continuation and edge enhancement filters were used to find and predict the pattern of distribution of these deposits. The results prove that the deposits locate on regional magnetic lithologies and structures.

Tertiary magmatic activities in Iran are the main magmatic events responsible for porphyry copper deposit occurrences. The Urumieh-Dokhtar magmatic belt (UDMB), the Ahar-Arasbaran, the Taleghan-Tarom, the Kavir-Sabzevar zones, and the Lut Block contain the greatest volume of the Tertiary magmatic rocks in Iran. Many porphyry copper deposit indications have been found, especially in the south-east and north-west of the UDMB in the Kerman and the Ahar-Arasbaran zones, respectively. New porphyry copper mineralisation found in central parts of the UDMB and the East Iran magmatic belt (the Lut Block), in Tertiary rocks, have made these areas more interesting for porphyry copper prospecting, where before that they were not noteworthy. In this paper, we used the combination of upward continuation and edge enhancement filters to find and predict the pattern of distribution of porphyry copper deposits in the Tertiary magmatic belts. The results show the high correlation between magnetic lithologies/structures and porphyry copper deposits at a regional scale and new attractive areas have been proposed for prospecting based on deep geophysical features.

The majority of published in-situ stress information in the Australian continent is confined to petroleum provinces where industry technologies facilitate the capture of contemporary crustal stress information. In contrast, the stress pattern of non-petroleum regions such as the Flinders and Mount Lofty Ranges in South Australia, where intraplate deformation is localised, has not been investigated comprehensively so far. The ongoing activities of the mining industry in South Australia has enabled us to access recently drilled boreholes for image logging techniques that have typically been under-utilised by the mineral sector. Herein, we conduct stress analysis by analysing borehole image logs from 16 boreholes in the basement rocks of South Australia as well as two geothermal wells and one coal seam gas well in South Australia’s northern Flinders Ranges, the Gawler Craton and the Eyre Peninsula. The resulting dataset of stress orientations is further accomplished by including recent seismological observations, which provide crustal stress information derived from focal mechanism solutions. The results of this study suggest a regional E–W orientation of the maximum horizontal compressive stress that is consistent with numerous observed neotectonic structures in this region. The focal mechanism solutions in this study suggest that the majority of events occur in a thrust faulting stress regime, which is consistent with the observed Quaternary fault scarps. However, our data compilation also indicates the presence of strike-slip and normal faulting stress regimes in the region, which has not been suggested extensively before this study.

The results of this study suggest a regional E–W orientation of the maximum horizontal stress in the Flinders and Mount Lofty Ranges. Focal mechanism solutions of earthquakes suggest that the majority of events occur in a thrust faulting stress regime. However, our data also indicate the presence of strike-slip and normal faulting stress regimes in the region, which has not been suggested extensively before this study.