Exploration Geophysics - Volume 17, Issue 2, 1986

The seismic time expression of structures in the Gippsland Basin are severely distorted due to Miocene channelling. Studies in the Kingfish-Gurnard area show that the linear velocity function method offers significant advantages over traditional depth conversion processes and allows the separation of regional velocity trends from both the lateral or raypath-induced residual fields.

A simple physical model for ionic conduction in shaly sands is modified and adapted to unconsolidated clayey sands. A brief review of the effects of porosity and cation exchange in clays on formation resistivity is made, assuming a linear relationship between clay content and porosity. The formation factor can be expressed in terms of the modified porosity and used to calculate the formation resistivity. The formation resistivity is adjusted for the effects of clay conductance under different saline conditions. Characteristic curves are constructed for particular environments, showing the effects of variations in clay content and porosity on formation resistivity. The use of characteristic curves and equations derived from the model is recommended as a semiquantitative aid to interpreting lithology from apparent resistivity data.

Migration of VSP data can be effectively accomplished using a focused-lag sum imaging technique, similar to the Kirchoff ‘diffraction stack’ method. The imaging procedure is sufficiently flexible to handle the irregular source-receiver geometry associated with walkaway-source and deviated hole VSP. Signal dispersion is readily accommodated by admitting both group and phase velocity parameters. The migration can be performed on transmission data (downgoing wave) to yield a velocity map or on reflection data (upgoing wave) to produce a reflector map. The accuracy of both reflector mapping and velocity mapping depends on the nature and form of the signal. Pulse compression and multiple suppression are desirable pre-processing operations. The technique has been tested on synthetic VSP data and successfully applied to shallow VSP data collected in the Sydney Basin coalfields. For the real data, average velocities of the coal measure sequence and blurred images of the main seams were obtained. An equivalent section migration technique, based on least-squares theory, is formulated in the Appendix.

Considering a homogeneous half-space to consist of an infinite number of sheets, infinitesimally thin, each sheet being semi-infinite in the vertical direction and extending from minus infinity to plus infinity, perpendicular to the collinear electrodes in a resistivity survey, the individual response from each sheet has been calculated from the solution of a boundary value problem. The lateral response functions for a general array, two-electrode and three-electrode arrays have been computed and presented. The results are somewhat astonishing as it can be seen that some portion of the ground does not contribute at all to the total response while some part has a negative contribution.

For determination of depth of investigation characteristics in direct-current resistivity methods, individual contribution from an infinite horizontal sheet of infinitesimal thickness within a homogeneous half-space has been obtained through the solution of boundary value problems. In this study, no assumption is made of electrostatic equivalence or dipole polarization, which has been utilized by earlier workers.

Detailed magnetic surveys were conducted in the Chikativari-palle-GolIapalle area of the Cuddapah Basin, AP, India, where the geological mapping was carried out. Since the magnetic anomalies of this area are of very low amplitude, a new filter was generated, using the Hilbert transform, in order to enhance the signal from noisy data. Filter Mj is the summation of the multiplied values of the horizontal derivative of the anomaly and its corresponding Hilbert transform (or vertical gradient), three values each at a time (three point data window), from one end of the profile. The process was repeated by sliding the data window along the entire profile. Each filter coefficient was attributed at the middle point of the data window. The filter Mj was convolved with the first horizontal derivative (M XA) and with its Hilbert transform (M X H). The resolved magnetic signals were identified with the contacts and faults. Also these were identified with the mineralized zone on the eastern part of the profiles; the central and western part of the profiles were identified along the axes of synclinal and anticlinal structures respectively. It was correlated with borehole data and confirmed the existence of structures and mineralized zones. These studies are immensely helpful for qualitative interpretation of weak magnetic anomalies.

One-dimensional spectral analysis has been applied to two long aeromagnetic profiles over Deccan Flood Basalts of India, one from Goa to Parbhani and the other from Hyderabad to Surat, to determine the thickness of the basaltic layer. The estimated thickness along the Goa-Parbhani profile varies from 220 to 1155m and that along the Hyderabad-Surat profile from 200 to 800 m. The Goa-Parbhani profile indicates three subsurface faults at a distance of 98, 183 and 310 km from Goa.

The Bureau of Mineral Resources (BMR) has begun producing and releasing a series of standard 1 : 1 000 000 sheet pixel maps of gridded airborne magnetic data for various parts of Australia. The new series is based on a standard mesh size of 0.25 min. The first of these maps are being produced for those areas where digital data exists; other areas will follow as analogue data are converted to digital form.

Maps produced on high resolution pixel plotters are superior to contour maps and are the only practical means of displaying gridded data over large areas. Over small areas, machine contouring may be viable because of the ease with which a hard copy with desired projection and scale can be produced.

Grids of magnetic data are warped to accommodate projection, annotated, and displayed on a pixel plotting device, which can accommodate up to 4000 X 4000 pixels on a photographic negative in black and white or colour. The negative is then enlarged linearly by photographic means to produce a hard copy with the required scale.

Examples are shown here of magnetic pixel maps of the Roper River 1 :1 000 000 sheet. They include total intensity in black and white, colour, and two gradient presentations. These maps reveal new information in the form of magnetic dykes, volcanic extrusions, and deep-seated, possibly granitic, bodies.

The automatic recording of geophysical field information, and the automatic unloading of the information into a computer for processing, can speed up the entire task of geophysical field surveying. Unfortunately, undetected spurious values (glitches) can contaminate the data and have deleterious effects on the final calculated results. The non-linear median-value window-filter method of conditioning data is highly efficacious for the removal of glitches. The advantage of the method is in not degrading significantly the signal compared with other filtering methods. An undesirable feature of the median-filter is the clipping of each maximum and minimum. However, the clipping has been overcome by the addition of a rounding process to the program, thereby improving the appearance of the conditioned waveform. A computer program, written in BASIC, is provided.