Exploration Geophysics - Volume 50, Issue 1, 2019

This study aims to assess the ability of shallow distributed acoustic sensing (DAS) to serve as a cost-effective seismic sensor array for permanent monitoring applications. To this end, as part of the CO2CRC seismic monitoring program, a fibre-optic DAS array was deployed alongside a permanently buried geophone array at the Otway Project site (Victoria, Australia). The DAS array consisted of a standard commercially available tactical fibre-optic cable, which was deployed in 0.8 m deep trenches. A custom-designed helically wound (HW) cable was also deployed in one of the DAS trenches for comparison of the cable designs. Simultaneous acquisition of the seismic data was carried out using ∼ 3000 vibroseis source points and geophones, DAS standard and HW cables. For initial assessment of the seismic images acquired with DAS and to compare different cable designs, preliminary 2D seismic reflection processing is conducted on both DAS cables and geophone data along a single 2D line. The geophone data processing guided processing of the DAS data. Several shallow structures (100–450 ms) and some important reflectors at 450–600 ms are observed on the final DAS images. Comparison of the two different DAS cable types demonstrated that seismic imaging would benefit DAS technology. However, the benefit of utilising HW cable is modest compared with the standard cable. The workflows and results of this study pave the way for processing of the 3D seismic data set acquired with the DAS array, as well as further detailed analysis of the DAS cables and the system itself.

The Lujiang–Zongyang (Luzong) ore district is well known for porphyry iron deposits that are mainly located in the Luzong volcanic basin, an important iron ore area in eastern China. Since the discovery of copper deposits in the ore district, regional tectonic evolution and geodynamic processes have become important issues for further mineral exploration. However, most prior research focused on geology and metallogeny, and little attention has been paid to the deep structure and its relationship to mineralisation. To understand this problem further, data were collected at 76 broadband magnetotelluric stations in the central Luzong basin, and then inverted in 3D using the nonlinear conjugate gradient method. Based on the inverted 3D electrical model, we consider that hot material upwelled beneath the lower crust in the centre of this region, where the crustal resistor is shallow and thin compared with surrounding resistors. We infer that one local uplift belt existed upon the upwelling material. The regional high aeromagnetic anomalies and low residual gravity anomalies hint at the presence of a shallow volcanic basin and upper mantle material. Because of the copper metallogeny in the adjacent district, we suggest that the local uplift belt in the centre basin has great potential for copper prospecting.

It is important to study the responses of the magnetotelluric (MT) method in anisotropic media. However, MT anisotropy research has focused mainly on one-dimensional (1D) and two-dimensional (2D) solutions. Therefore, we developed a three-dimensional (3D) finite element (FE) algorithm for MT modelling in anisotropic media. This approach is based on the weak formulation of the governing Maxwell equations using Coulomb-gauged potentials. The node-based FE method is adopted here, and the values of the coefficient matrixes are obtained with hexahedral meshes. To validate the correctness and accuracy of this method, its results are compared with previous solutions for a 2D anisotropic model and a 3D arbitrary anisotropic model, respectively. Different solvers with different preconditioners are tested, and the results show that the quasi-minimum residual method with the incomplete LU preconditioner is more stable and faster compared with the other schemes. We then studied a 3D anisotropic model in three different conditions, and analysed the results in detail. Finally, three main conclusions are obtained: the xy- and yy-mode apparent resistivities remain almost unchanged if a principal conductivity is in the x-direction; the yx- and xx-mode apparent resistivities remain almost the same if a principal conductivity is in the y-direction; a principal conductivity in the z-direction has almost no influence on apparent resistivities.

Controlled-source audio-frequency magnetotellurics (CSAMT) is an important geophysical technique. Numerous studies have shown that the electrical anisotropy in the Earth cannot be ignored because it would probably lead to misinterpretations of electromagnetic data. It is necessary and meaningful to study CSAMT responses in three-dimensional (3D) electrical anisotropic media; therefore, we have developed an edge-based finite element method for 3D CSAMT forward modelling in generalised anisotropic media. The total electric field in this approach is decomposed into a primary electric field and a secondary electric field, and the Galerkin weighted residuals method is adopted to obtain the variational equation. The accuracy of this algorithm was initially validated by comparing solutions with those obtained in previous work on a 3D arbitrary anisotropic model. We then studied the responses of an oblique source as well as equatorial and axial configurations and the tensor source for anomalies with different Euler’s angles. Several meaningful conclusions can be derived from this work and a synthetic model is developed; the results also confirm the validity of previous conclusions. This study shows that a tensor source is necessary for a CSAMT survey in 3D anisotropic media, and the anisotropic parameters of anomalies have complex and significant influences on CSAMT responses.

Tube wave suppression and the separation of elastic up-going and down-going waves are the most important steps in vertical seismic profile (VSP) data processing. Different methods have been developed to suppress tube waves and separate wave fields. In this work, we use a pyramidal dual-tree directional filter bank as a multi-directional and multi-scale method. We then design two different adaptive algorithms to suppress tube waves and aliased tube waves, and to separate elastic up-going and down-going waves. In the first proposed algorithm, subscales containing tube waves and aliased tube waves were detected adaptively through energy calculation in the 2D Fourier domain, through which the filtered VSP were achieved by zeroing these subscales. When applied to field VSP data with strong tube waves, the proposed filter can attenuate both the tube wave and aliased tube wave successfully, with results outperforming those of f–k filtering, singular value decomposition and median filtering. Similarly, following tube wave and aliased tube wave suppression in the second designed algorithm, subscales with elastic up-going waves were detected fully. By reconstructing these subscales and subtracting them from the original VSP data, the wave fields were separated successfully. The performance of both algorithms in the presence of random noise was investigated and none of the algorithms were found to be highly sensitive.

Specifying survey parameters is more challenging when dealing with complex structures such as faults due to the complicated wavefield behaviour involved in these structures. Seismic modelling enables geophysicists to have a better understanding of the subsurface image before seismic acquisition, processing and interpretation. In this regard, seismic survey modelling is employed to construct a model close to the real structure, and to obtain very realistic synthetic seismic data. Therefore, a survey designer can investigate the effects of various survey parameters such as shot and receiver positions, and receiver line bearing on the resulting image using illumination and resolution analyses. The primary objective of this study is to apply seismic modelling to designing seismic survey parameters such as receiver line bearing, wavelets and their frequencies to analyse the resolution and illumination of the faulted target. A ray-based seismic model was built using 2D seismic data, geological reports and well logs. A pre-stack depth migration simulator was used to evaluate the effect of source wavelets with different peak frequencies (20, 60 and 80 Hz) on the resulting seismic image. This simulation indicated that a source wavelet with a peak frequency of 20 Hz could not image the target fault plane, but higher peak frequencies (60 and 80 Hz) show higher resolution and better recognition of the fault plane. Therefore, for any wavelet frequency, it is necessary to assess the ability of the fault illumination. The results proved that a survey designer could enhance the image of fault planes in a seismic section by applying ray-based analyses, concerning illumination and resolution of the fault image.

Seismic reservoir characterisation includes seismic inversion and property prediction which provides valuable information from seismic data. A good reservoir characterisation workflow requires a reliable seismic inversion method. The most common and reliable approach for reservoir property prediction from surface seismic data is rock physics in which a physical or empirical model is used to describe the properties. However, these models require a huge number of predefined constant parameters, which makes their use complicated in practice. To overcome this, we propose a simple regression-based methodology to estimate reservoir properties (including porosity, water saturation and shale volume) from the elastic properties. The proposed methodology is a modified version of Lambda-Mu-Rho analysis for regression of water saturation and shale volume estimations. To find the elastic properties, we use Bayesian linearised amplitude versus offset inversion. Both the inversion method and the proposed methodology for seismic reservoir characterisation provide reliable information that correlates very well with the well data. Compared with the rock physics method, which is complicated and requires a number of parameters, the proposed methodology is straightforward and easy to apply.

Current staggered-grid finite-difference (SFD) schemes for anisotropic wave equations can ensure only high-order spatial modelling accuracy, whereas the time discretisation accuracy is still of second-order. To improve temporal modelling accuracy, we develop a rectangular SFD scheme with fourth-order temporal accuracy to solve the first-order pseudo-acoustic wave equation in vertical transversely isotropic media. High-order temporal accuracy is achieved by applying the high-order temporal derivatives, which can be replaced with the high-order spatial derivatives to reduce the computational cost. The corresponding high-order finite-difference (FD) coefficients are generated by using Taylor-series expansion and least-squares. Dispersion and stability analyses indicate that our proposed method has higher accuracy and better stability than the conventional method. Several modelling examples confirm that our proposed scheme exhibits low temporal dispersion even for a large time step. Because of the high-order accuracy in time and space domains, our new SFD scheme allows for a larger time step and shorter operator length, which is more efficient.