Exploration Geophysics - Volume 46, Issue 4, 2015

This paper presents the experimental design and modelling of self-potential anomalies produced by a sphere-shaped copper–zinc source using a water tank and polarising electrodes. Self-potentials were measured by placing the sphere at a given depth in a rectangular glass tank filled with water.

The relationship between the self-potential (SP) produced by a polarised sphere and its depth was studied in a laboratory experiment. This was carried out by using a sphere made up of two hemispheres of different materials: one copper and the other zinc. Self-potentials were measured by placing the sphere at a given depth in a rectangular glass tank filled with water. The surface of the water was covered by a sheet with 684 brass electrodes. A sensitive, high impedance digital voltmeter was used to measure potentials from each electrode to a ‘base’. We have measured the SP response of the metallic body and our work shows that SP signals of several millivolts are generated due to the sphere placed within water. The gridded SP data show a clear anomaly over the sphere at shallow depths, and as the depth of the sphere increases, the measured SP signal due to the sphere decreases. An analytical formula is given to determine the maximum depth of the sphere at which the presence of the anomaly can be detected. Responses from other geometries are examined as well.

Deblending is a currently popular method for dealing with simultaneous-source seismic data. Removing blending noise while preserving as much useful signal as possible is the key to the deblending process. In this paper, I propose to use a space-varying median filter (SVMF) to remove blending noise. I demonstrate that this filtering method preserves more useful seismic reflection than does the conventional version of a median filter (MF). In SVMF, I use signal reliability (SR) as a reference to pick up the blending spikes and increase the window length in order to attenuate the spikes. When useful signals are identified, the window length is decreased in order to preserve more energy. The SR is defined as the local similarity between the data initially filtered using MF and the original noisy data. In this way, SVMF can be regionally adaptive, instead of rigidly using a constant window length through the whole profile for MF. Synthetic and field-data examples demonstrate excellent performance for my proposed method.

In this paper, I propose to use a space-varying median filter (SVMF) to remove blending noise. I demonstrate that this filtering method preserves more useful seismic reflection than does the conventional version of a median filter (MF). SVMF can be regionally adaptive, instead of rigidly using a constant window length through the whole profile for filtering.

This paper examines anisotropic prestack Kirchhoff migration. We used pseudo-acoustic wave equations in the complex frequency domain to describe the wave propagation in a vertical transversely isotropic (VTI) medium. Both amplitudes and traveltimes were calculated efficiently using the suppressed wave equation estimation of traveltime (SWEET) algorithm. The accuracy of the traveltimes obtained with the SWEET algorithm was verified by comparing the traveltime contours simulated with the anisotropic elastic wave equation using a staggered-grid method. Finally, we tested our migration algorithm using the two-dimensional HESS VTI model. We correctly imaged the shape of both the salt and background layer structures. We also reduced the numerical artefacts compared to the isotropic technique.

This paper examines anisotropic prestack Kirchhoff migration using the pseudo-acoustic wave equation in a vertical transversely isotropic (VTI) medium. Both amplitudes and traveltimes were calculated efficiently for migration using the suppressed wave equation estimation of traveltime (SWEET) algorithm.

One-way wave equation migration is computationally efficient compared with reverse time migration, and it provides a better subsurface image than ray-based migration algorithms when imaging complex structures. Among many one-way wave-based migration algorithms, we adopted the generalised screen propagator (GSP) to build the migration algorithm. When the wavefield propagates through the large velocity variation in lateral or steeply dipping structures, GSP increases the accuracy of the wavefield in wide angle by adopting higher-order terms induced from expansion of the vertical slowness in Taylor series with each perturbation term. To apply the migration algorithm to a more realistic geological structure, we considered tilted transversely isotropic (TTI) media. The new GSP, which contains the tilting angle as a symmetric axis of the anisotropic media, was derived by modifying the GSP designed for vertical transversely isotropic (VTI) media. To verify the developed TTI-GSP, we analysed the accuracy of wave propagation, especially for the new perturbation parameters and the tilting angle; the results clearly showed that the perturbation term of the tilting angle in TTI media has considerable effects on proper propagation. In addition, through numerical tests, we demonstrated that the developed TTI-GS migration algorithm could successfully image a steeply dipping salt flank with high velocity variation around anisotropic layers.

We expand the vertical transversely isotropic generalised screen propagator into a tilted transversely isotropic generalised screen propagator (TTI-GSP) by adopting tilted coordinates. The validity of the developed algorithm has been tested by increasing the accuracy of wide-angle propagation in TTI-GSP with high-order expansion of the velocity perturbation.

In wave-equation-based migration, the imaging condition is an important factor that impacts migration accuracy and efficiency. Among the commonly used imaging conditions, the excitation amplitude imaging condition has high resolution, accuracy and low storage and input/output burden when compared with others. However, the excitation amplitude extracted by this imaging condition in its current form will produce a distorted migration image for certain scenarios. In this paper, a modified excitation amplitude imaging condition is proposed that addresses the above problem and produces migrated images free from distortion for complicated geologic models. In this paper, we propose a method to effectively use the modified shortest path method (MSPM) for extracting the maximum amplitude around the first-arrival events. Then, the excitation amplitude imaging condition is applied to obtain a continuous and clear migration image. This process can, to some extent, improve the distorted migration image produced by the traditional excitation amplitude imaging condition. Some numerical tests with synthetic data of Sigsbee2a and Marmousi-II models show that the improvement is feasible and effective in complex-structure media.

We propose a process to effectively use the modified shortest path method for extracting the maximum amplitude around the first-arrival events. Then, the excitation amplitude imaging condition is applied to obtain a continuous and clear migration image. Numerical tests show that the improvement is feasible and effective in complex-structure media.

To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo method is modified by implanting a focal mechanism algorithm. The modified method shows more reliable results compared to those of the original one, for events with more than 12 recorded stations.

To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo (DSMC) method is modified by implanting a focal mechanism algorithm. We compare the results of the modified DSMC method (DSMC-2) with those of the original DSMC method (DSMC-1). DSMC-2 shows more or similarly reliable results compared to those of DSMC-1, for events with 12 or more recorded stations, by weighting twice for hypocentral distance of less than 80 km. Not only the number of stations, but also other factors such as rough topography, magnitude of event, and the analysis method influence the reliability of DSMC-2. The most reliable result by DSMC-2 is obtained by the best azimuthal coverage by the largest number of stations. The DSMC-2 method requires shorter time steps and a larger number of particles than those of DSMC-1 to capture a sufficient number of arrived particles in the small-sized receiver.

We developed a spherical harmonic series that represents the gravitational potential and its gravity field due to a buried right vertical cylinder. This series can be used at far- and intermediate-regions, and is fast and accurate, using only a few terms. We compared the values of the fields acquired by this new spherical harmonic series, with ones computed by direct numerical integrations, using a fine-mesh structure for a vertical cylinder. Results of the calculations are shown and performances of the two different methods are compared. Faithfulness of the spherical harmonic series is tested with an inversion example.

In this paper, the spherical harmonic series of the gravitational potential and its gravity field due to a right vertical cylinder was developed. This series can be used at far and intermediate regions, and it is fast and accurate, using only a few terms.

A small-loop EM survey and electrical resistivity survey were applied to a carcass disposal site to delineate the burial shape and extent of leachate arising from the burial. The small-loop EM survey was adequate for a reconnaissance survey of the burial zone and the resistivity survey was useful for quantitatively interpreting the pathway of leachate flows in the site.

Leachate from livestock mortality burial is harmful to the soil and groundwater environment and adequate assessment approaches are necessary to manage burial sites. Among the methods used to detect leachate, geophysical surveys, including electrical resistivity and electromagnetic (EM) techniques, are used in many engineering approaches to environmental problems, such as identifying contaminant plumes and evaluating hydrogeological conditions. Electrical resistivity, with a small-loop EM survey, was used in this study as a reconnaissance technique to identify the burial shape and distribution of leachate from livestock mortality burial in five small separate zones. We conducted a multi-frequency small-loop EM survey using lattice nets and acquired apparent conductivity values along several parallel and perpendicular lines over a burial site. We also compared geophysical results to the geochemical analysis of samples from both a leachate collection well and a downstream observation well within the study area. Depth slices of apparent conductivities at each frequency (obtained from the small-loop EM survey data) clearly identified the subsurface structure of the burial shape and the extent of leachate transport. Low-resistivity zones, identified from two-dimensional (2D) electrical resistivity imaging results, were matched to the five burial zones (within a depth of 5 m), as well as high electrical conductivity of the leachate obtained from leachate collection wells, and depth slices of the apparent conductivity distribution obtained from the small-loop EM survey. A three-dimensional (3D) inversion of resistivity data provided a detailed 3D structure of the overall burial site and leachate pathways. Moreover, these zones were widely spread over the burial site, indicating that leachate potentially extended through damaged regions of the composite liner to a depth of 10 m along the downstream groundwater flow. Both the small-loop EM method and the electrical resistivity method were considered suitable for identifying the shape of the livestock mortality burial and the extent of leachate.

The application of both geophysical imaging and geotechnical testing in road foundation design is a cost-effective enhancement for site characterisation of soft soils and for risk assessment of potentially unstable embankments. The incorporation of geophysics allows concentration on fewer but higher quality soil probings and geotechnical boreholes.

Population growth along the coast of eastern Australia has increased demand for new and upgraded transport infrastructure within intervening coastal floodplains and steeper hinterland areas. This has created additional challenges for road foundation design.

The floodplain areas in this region are underlain by considerable thicknesses of recently deposited alluvial and clayey marine sediments. If characterisation of these deposits is inadequate they can increase road construction costs and affect long-term road stability and serviceability. Case studies from a major coastal highway upgrade demonstrate how combining surface wave seismic and electrical geophysical imaging with conventional geotechnical testing enhances characterisation of these very soft and soft soils. The geophysical results also provide initial foundation design parameters such as void ratio and pre-consolidation pressure.

A further significant risk issue for roads is potential embankment instability. This can occur during new road construction or when upgrades of existing embankments are required. Assessing the causes of instability of existing steeper embankments with drilling and probing is often difficult and costly due to access and safety problems. In these situations combinations of electrical, ground penetrating radar and P-wave seismic imaging technologies can rapidly provide information on the likely conditions below both the roadway and embankment. Case studies show the application of these technologies on two unstable road embankments.

It is concluded that the application of both geophysical imaging and geotechnical testing is a cost-effective enhancement for site characterisation of soft soils and for risk assessment of potentially unstable embankments. This approach overcomes many of the current limitations of conventional methods of site investigation that provide point location data only. The incorporation of geophysics into a well crafted site investigation allows concentration on fewer but higher quality soil probings and geotechnical boreholes.