Exploration Geophysics - Volume 16, Issue 4, 1985

Vertical seismic profiles (VSP) have been used extensively throughout Australia to provide improved seismic information in the vicinity of the borehole. In its simplest form, VSP can be regarded as the extension of a check shot survey. The positioning of a wall lock geophone within the borehole allows the simultaneous recording of upgoing and downgoing reflected energy. When correctly processed, a normal vertical raypath VSP provides improved time versus depth correlation of seismic events. Careful analysis of VSP data in conjunction with well logs and synthetic seismograms yields good information on seismic events close to the well. The simple concepts of VSP can be extended to non-vertical raypath or offset VSP where the energy source is positioned at one or more non negligible offsets from the well. By recording multigeophone levels in the well with varying source offsets a detailed high resolution seismic picture can be built up around the borehole to laterally extend VSP seismic coverage away from the well. The different methods for offset VSP and recommendations for offset VSP shooting are presented. Processing methods are explained and several examples from Australia are discussed.

An experimental in-seam seismic survey was recently conducted at West Wallsend No. 2 Colliery in the northern coalfield of New South Wales. The objectives were to acquire field data on scattering from a known dyke and to determine the transmission characteristics of old workings. Shot timing errors, gas bubbling in the geophone holes, and the limited dynamic range of the recording instruments all affected data quality. The Borehole Seam supports leaky P, SH and P-SV modes. These propagate in the plane of the seam, with group velocities at 75 Hz of 3.7, 2.0 and 1.6 km s¹. Attenuation rates are high. The seam is lossy (Q ≈ 10). Dispersion is influenced by the proximity of the Young Wallsend Seam. Two prominent structures were mapped between the geophone roadway and the dyke, by migrating SH arrivals. One is believed to be a feeder dyke, the other may be a sand channel. These obstructions prevented scattering analysis of the main dyke. The dyke was modelled as a specular, resonant reflector. The reflection coefliceni is low for leaky modes with appreciable energy outside the seam. The water filled heading (old workings) acted as a barrier to SH waves. It was relatively transparent to P waves at 100 Hz. The P wave shadow cast at 300 Hz was back-projected by algebraic reconstruction tomography (ART). The map showed up a north-south corridor of competent rock between the mines between 40 m and 90 m wide – compared to the actual figure of 75 m.

A drill hole magnetometer system has been developed primarily to help in the search for gold bearing magnetite bodies in the Tennant Creek area in the Northern Territory. The probe contains three fixed orthogonal fluxgate magnetometers with three corresponding orthogonal inclinometers. The inclinometers are used to remove the effects in the magnetometer’s data produced by probe rotation and drill hole inclination. Readings are then corrected for drill hole deviation and the background field is removed to produce anomalous vectors. A drill hole surveyed at the West Gibbet Prospect at Tennant Creek shows the potential of the instrument in locating and defining magnetic bodies. In addition, the magnetometer is an extremely sensitive drill hole survey tool and can be used to obtain an idea of the magnetic susceptibility of rock units intersected by the hole.

Geophysical data often comprises several variables measured over a large number of samples. Several statistical techniques, e.g. cluster analysis and principal components analysis, have been developed for the study of such data sets. As an example, these techniques are applied to the natural gamma, density and neutron-neutron logs of three boreholes in the Meandu coalfield. The cluster analysis demonstrated that there were seven significant groups in the data. These corresponded reasonably to the lithological groups: coals, ‘dirty’ coals, carbonaceous siltstones, siltstones, fine sandstones, granular sandstones and conglomerates. Principal components analysis then established criteria for distinguishing these groups from the geophysical data.

One dimensional very low frequency surface impedance measurements were made in an area with a number of mineralization bands. The results compare favourably with soil geochemistry and drill core analysis in the location of this mineralization in an environment with a relatively shallow overburden. The dependence of surface impedance on measurement direction over an anisotropic earth plane agrees well with theory. The horizontal electric field component was found to be elliptical!) polarized while the horizontal magnetic field component was linearly polarized. Curve fitting of the electric and magnetic field components independently as a function of measurement angle allowed the magnitude and direction of the maximum and minimum resistivity values to be determined.