Binzhong Zhou

A staggered-grid convolutional differentiator for elastic wave modelling

The computation of derivatives in governing partial differential equations is one of the most investigated subjects in the numerical simulation of physical wave propagation. An analytical staggered-grid convolutional differentiator (CD) for first-order velocity-stress elastic wave equations is derived in this paper by inverse Fourier transformation of the band-limited spectrum of a first derivative operator. A taper window function is used to truncate the infinite staggered-grid CD stencil. The truncated CD operator is almost as accurate as the analytical solution, and as efficient as the finite-difference (FD) method. The selection of window functions will influence the accuracy of the CD operator in wave simulation. We search for the optimal Gaussian windows for different order CDs by minimizing the spectral error of the derivative and comparing the windows with the normal Hanning window function for tapering the CD operators. It is found that the optimal Gaussian window appears to be similar to the Hanning window function for tapering the same CD operator. We investigate the accuracy of the windowed CD operator and the staggered-grid FD method with different orders. Compared to the conventional staggered-grid FD method, a short staggered-grid CD operator achieves an accuracy equivalent to that of a long FD operator, with lower computational costs. For example, an 8th order staggered-grid CD operator can achieve the same accuracy of a 16th order staggered-grid FD algorithm but with half of the computational resources and time required. Numerical examples from a homogeneous model and a crustal waveguide model are used to illustrate the superiority of the CD operators over the conventional staggered-grid FD operators for the simulation of wave propagations.

Fault and dyke detectability in high resolution seismic surveys for coal: a view from numerical modelling*

Modern underground coal mining requires certainty about geological faults, dykes and other structural features. Faults with throws of even just a few metres can create safety issues and lead to costly delays in mine production. In this paper, we use numerical modelling in an ideal, noise-free environment with homogeneous layering to investigate the detectability of small faults by seismic reflection surveying. If the layering is horizontal, faults with throws of 1/8 of the wavelength should be detectable in a 2D survey. In a coal mining setting where the seismic velocity of the overburden ranges from 3000 m/s to 4000 m/s and the dominant seismic frequency is ~100 Hz, this corresponds to a fault with a throw of 4–5 m. However, if the layers are dipping or folded, the faults may be more difficult to detect, especially when their throws oppose the trend of the background structure. In the case of 3D seismic surveying we suggest that faults with throws as small as 1/16 of wavelength (2–2.5 m) can be detectable because of the benefits offered by computer-aided horizon identification and the improved spatial coherence in 3D seismic surveys. With dykes, we find that Berkhout’s definition of the Fresnel zone is more consistent with actual experience. At a depth of 500 m, which is typically encountered in coal mining, and a 100 Hz dominant seismic frequency, dykes less than 8 m in width are undetectable, even after migration. We use numerical modelling in an ideal noise-free environment and homogeneous layering to investigate the detectability of small faults by seismic reflection surveying. It is shown that the detection of faults relates not only to the seismic wave frequencies and wavelengths, but also to the presence of background structures.

Toward improved coal density estimation from geophysical logs

Density plays an important role in coal resource estimation and reconciliation, as well as in defining coal quality. Current practice employs direct density measurements on widely spaced core samples, rather than utilising abundant geophysical logging data. This is mostly due to the perception that the precision and accuracy of density estimation from geophysical logs is unsatisfactory. This paper demonstrates that the density wireline log, supported by other geophysical logs, provides a reliable direct measurement of in-situ coal density. We have produced a consistent and reliable correlation of geophysical log density with a laboratory-derived density to within an accuracy of ±3%. This is achieved through careful constraints such as compensating for lost pore spaces and moisture to bring the laboratory relative density closer to in-situ environmental conditions, matching the laboratory sample depths with geophysical logs, excluding thin, boundary, and stone-band samples from the dataset, and calibrating the geophysical density with laboratory testing data and other geophysical logs by linear regression or Radial Basis Function and Self-Organised Mapping techniques. In addition, we also illustrate that the improved geophysical log density can be used for coal quality estimation.

Subsidence Assessment Using 3-D Seismic Data at Collingwood Park, Brisbane

A 3-D reflection seismic trial survey was conducted at Collingwood Park, on the outskirts of Brisbane, Australia, where coal mining activity ceased in the 1980’s and two subsidence events occurred in 1988 and 2008. The objective of the survey was to demonstrate the feasibility of using seismic methods to locate subsurface structures and features such as faults and old mining workings. In spite of the strong contamination of refractions and surface waves, the survey was able to confirm the location of the previously known Waterline fault, as well as identify another fault. It accurately delineated the subsidence zone based on differences in the amplitude of seismic reflections from the surveyed area, and mapped the surface subsidence boundary, which correlated well to existing data from observations in the field. In addition, the seismic data were used to map the failure boundary at the seam level in relation to the subsidence zone. It was found that the mapped area at the seam level was larger than the surface subsidence boundary, with an estimated angle of ,21u between the subsidence failure surface and the vertical depth axis, depending on location. Post 3-D seismic drilling also confirmed that pillar failure extended beyond the Waterline fault. However, the survey failed to image the mine workings because of insufficient seismic resolution. To our knowledge, this is the first report on the use of 3-D seismic surveying to map deeply buried mining-related geotechnical failure boundaries.

Seismic wave propagation through surface basalts – implications for coal seismic surveys

Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basalt flows are needed. In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that: 1) basalts with high impedance contrasts and multiple flows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.

Delineation of Geological Structures by 3D Seismic Surveys in Australian Coal Mining

Summary Successful underground coal mining requires certainty of geological conditions and knowledge of any geological structures which might disrupt the coal seams. Conventionally, such information has been sought through drilling programs. However, drilling is an expensive process and its relevance is limited to the immediate neighborhood of the borehole. The geological uncertainty remains. Seismic reflection surveying, especially 3D seismic, could provide much of the detailed information needed by coal mining, without resorting to excessive drilling. In recent years, 3D seismic reflection surveys have been undertaken at a number of coal mines in Australia in an attempt to provide a “no surprises” guarantee of seam conditions before mining. Superior seismic results were obtained. In this paper, the ability of seismic reflection methods to resolve localised geological features has been examined from a theoretical viewpoint and through using data obtained in recent 2D and 3D seismic surveys undertaken in Australia. The results show how subtle details in the coal seams are revealed. Techniques for computer aided interpretation are also demonstrated.

Common-angle image gathers for shot-profile migration: an efficient and stable strategy

Shot-profile-based prestack depth migration is quite attractive for sparse-shot wide-azimuth geometries. Meanwhile, angle-domain common image gathers (ADCIGs) have become an essential tool in seismic processing (e.g. velocity analysis). In this paper, we first revisit the popular schemes of generating ADCIGs by wavefield continuation migration. Then we present an alternative approach to produce common-angle image gathers after shot-profile migration based on a one-way wave equation. The angle information is obtained from a division of two images: the conventional image and the angle-weighted image. The effects of sparse shot sampling on extracting ADCIGs have been investigated. Numerical results demonstrate that the described scheme is efficient and stable and can handle the problem of shot-aliasing well.

Dip angle-compensated one-way wave equation migration

Conventional migration algorithms based on one-way wave equations in a Cartesian coordinate system often under estimate amplitudes, especially at large propagation or reflection angles. This has a deleterious effect on seismic images and should be corrected. We illustrate the nature of the problem by working in the more natural spherical coordinate system and offer two simple solutions to the problem: (1) a wave combination scheme where the wave extrapolation is done independently for each Cartesian coordinate and the resulting wavefields are summed and (2) a simple wave projection scheme whereby the conventional one-way propagator is corrected by means of a factor 1/cos(θ), where θ is the angle of the wave measured from the vertical axis or the reflector angle. The wave combination scheme is applicable to waves with propagation angles beyond 90°, but will roughly triple the computation time compared with conventional one-way propagators in the 3D case. The wave projection scheme is more economical and can easily be implemented in the wavenumber (or slowness) domain at no extra computational cost. These schemes are valid both in depth-dependent media and in laterally heterogeneous media. In addition, the proposed amplitude-preserving schemes can be applied to all methods based on the conventional one-way wave equation. We then develop and implement the second approach to demonstrate its validity by means of numerical examples.

Acoustic Impedance Inversion For Geotechnical Evaluation In Underground Coal Mining

In underground coal mining, knowledge of the geomechanical properties of the strata surrounding the mining horizons is essential for the prevention of unexpected rock failures that can disrupt production and jeopardize mine safety. Model based acoustic impedance inversion integrates drill hole and seismic information to allow assessment of the geomechanical environment. We demonstrate our approach using results from a 3D and a 2D seismic survey. We introduce a means of converting acoustic impedance to the Geophysical Strata Rating (GSR). The GSR is a rock mass rating scheme that is normally derived from geophysical logging data. When expressed in terms of GSR, acoustic impedances have values that are meaningful to coal mine engineers.

Feasibility analysis of drill bit tracking using seismic while drilling technique

Summary Check-shot survey measures the first arrival time with a known depth receiver in borehole to assess formation velocity. This information can be used in correlation with sonic log and surface seismic products for adjustment of interpretation. Check-shot survey can also be implemented with seismic-while-drilling using drill bit noise as the source. This differs from usual check-shot survey as source is in the borehole. It provides a real time, cost saving, and safe measurement. Check-shot survey needs a known receiver depth, thus velocity can be obtained by fixed wave travel path and the measured first arrival time. However, in seismicwhile-drilling (SWD), drill bit position can vary a lot from vertical drilling to deviated drilling. To address this issue, we present a method that finds the location of the source and estimates the velocity of the formation at the same time. Using a synthetic model, with medium receiver offsets, this method shows good estimation of the drill bit depth location and formation velocity in a layered Earth model.