Iain Mason

The Effect of Conduction on VHF Radar Images Shot in Water-Filled Boreholes

A downhole digital memory-logging pulsed borehole radar transceiver operating in the 10-125-MHz band was run repeatedly down two 1.25-km-deep, uncased, water-filled 60-mm boreholes in the Northern Limb of the Bushveld Complex, South Africa. Suspended on an insulating cord, it mapped a steep fault in the Main Zone from a range of 75 m down through its intersection with the borehole. Lowered on a wire rope, the transceiver launched guided ~75 m/mus first-order transverse magnetic pulses which shuttled axially between the radar and the bedding planes. Decoupled from the wire by 2 m of insulating cord, it yielded a profile in which radar reflections and guided bedding plane echoes superimposed. As the radar descended through the mineralized, stratified Platreef, traces were found to be imprinted with voltage level shifts that showed, with Laterolog-comparable resolution, the conductivity profile of the Platreef.

In‐mine seismic delineation of mineralization and rock structure

Significant progress has been made towards the goal of generating detailed seismic images as an aid to mine planning and exploration at the Kambalda nickel mines of Western Australia. Crosshole and vertical‐seismic‐profiling instrumentation, including a slimline multi‐element hydrophone array, three‐component geophone sensors, and a multishot detonator sound source, have been developed along with special seismic imaging software to map rock structure. Seismic trials at the Hunt underground mine established that high frequency (> 1 kHz) signals can be propagated over distances of tens of meters. Tomographic as well as novel 3-D multicomponent reflection imaging procedures have been applied to the data to produce useful pictures of the ore‐stope geometry and host rock. Tomogram interpretation remains problematic because velocity changes not only relate to differing rock types and/or the presence of mineralisation, but can also be caused by alteration/weathering and other rock condition variations. Ultrasoni...

Borehole radar surveying for orebody delineation

The Arcolabs borehole radar system was developed to assist detailed orebody delineation particularly for mines extracting thin vein ore deposits. The probes are 32 mm and therefore fit in most common exploration and mining drillholes. The system has been successfully used at a number of mine sites to assist with ore delineation at a range of scales. Images from holes drilled alongside the extension of a massive sulphide orebody downdip from current development clearly showed the extent of the orebody and changes in thickness and dip of the orebody. The costs of these surveys were considerably less than the cost of delineation by drilling alone and the resulting image provided more detailed data which could be used to optimize the position of development. Estimates of potential savings from using the system to define the bottom of the orebody were of the order of hundreds of thousands of dollars. Images from short holes drilled at short intervals beneath the footwall of another orebody enabled a detailed wireframe of the orebody contact to be constructed. Better knowledge of the contact location helped to maximize ore extraction and minimize dilution.

Tuning seismic resolution by heterodyning

This paper presents a new seismic data analysis and interpretation tool—the heterodyning technique of radio engineering. When applied to seismic data, it enables us to tune up seismic apparent resolution by shifting the dominant frequency of the signal with a given frequency bandwidth. Heterodyning provides a trade-off between detection precision and the signal-to-noise structure of the seismic data field. This is realized by tuning the amount of sidelobe energy and the phase characteristics of the seismic signals. Moderate phase rolls introduced by heterodyning do not affect the interpretation process because the subtlety in the behaviour of individual peaks and troughs is preserved and the eye tolerates gradual changes in phase characteristics. For steeply dipping beds, horizon flattening can be used to minimize the phase roll. In this paper, we introduce the principles of heterodyning and illustrate the process using both synthetic and real data.

Remotely sensing the thickness of the Bushveld Complex UG2 platinum reef using borehole radar

The planar, 80 cm thick, lossy dielectric reefs of the Bushveld are embedded in rocks that are almost transparent at ground penetrating radar frequencies of 10–125 MHz. Pothole sensing practices are based largely on using borehole radars to observe departures of the reefs from planarity. Surveys are run in ∼200 m near-horizontal boreholes that are drilled into the footwalls of the reef. Careful laboratory measurements of the Jonscher dielectric parameters of the stratigraphic column through the UG2 reef are translated by electro-dynamic modelling into a prediction that platinum reef thinning can be sensed remotely by footwall borehole radars. This proposition sheds light on the results of a recent borehole radar survey that was shot in ∼180 m long AXT (48 mm diameter) boreholes. Areas of sub-economical UG2 thickness (typically less than ∼50 cm) were mapped by studying the relative amplitudes of echoes from the reef and a pyroxenite–anorthosite interface in its hanging wall, with the radar deployed beneath the UG2 in its footwall.

Borehole radar imaging from deviating boreholes

We have carried out borehole radar (BHR) surveys at gold mines in the Witwatersrand Basin. South Africa in order to map the Ventersdorp Contact Reef (VCR). In one such survey 20 kW transmitter and receiver pairs, 32 mm in diameter, with a bandwidth of 10-125 Mllz were used to profile a ~300 metre long section of the reef from a borehole that intersected it at an angle of 26°. Structures on the VCR were visible to a distance of 8Om, before noise started to dominate the signal. We established that the V.C.R. is sufficiently reflective, and its host rocks are transparent enough to open not only the certainty of high resolution echo sounding along the nadir line, but also the possibility of mapping off-axis back-scatterers by applying modified SAR reconstruction techniques to VHF BHR data. One of the problems facing synthetic aperture borehole radar is that it is difficult to build thin, efficient, directional radar antennas. Thin borehole radars are cylindrically omnidirectional and cannot be used to distinguish left from right. In this paper we show that borehole curvature can be used to address the difficulty of determining on which side ofthe survey line a backscattering object might lie.

Seeing coal-seam top ahead of the drill bit through seismic-while-drilling

Blast damage to the tops of coal seams due to incorrect blast standoff distances is a serious issue, costing the industry in Australia about one open-cut mine for every ten operating mines. The current approach for mapping coal-seam tops is through drilling and pierce-point logging. To provide appropriate depth control with accuracy of ±0.2 m for blast hole drilling, it is typically necessary to drill deep reconnaissance boreholes on a 50 m x 50 m grid well in advance of overburden removal. Pierce-point mapping is expensive and can be inaccurate, particularly when the seam is disturbed by rolls, faults, and other obstacles.Numerical modelling and prototype-field testing are used in this paper to demonstrate the feasibility of two seismic-while-drillingbased approaches for predicting the approach to the top of coal during blast hole drilling: (i) reverse “walk-away” vertical seismic profiling recording, in which the drill bit vibration provides the source signal and the geophones are planted on the surface near the drill rig, and (ii) in-seam seismic recording, in which channel waves, driven by the coupling to the coal of the seismic signal emitted by the approaching drill bit, are guided by the seam to geophones located within the seam in nearby or remote boreholes.

Broadband synthetic aperture borehole radar interferometry

Three-dimensional structural maps are important in optimizing the design and safe development of stopes. As mines deepen, it becomes increasingly difficult to map potentially productive horizons by drilling or geophysical sounding from the surface. Geologists develop plausible 3D ore body images from the cores of underground boreholes drilled in vertical fans from mine galleries. However the fans usually sample the rock volume too sparsely to acquire the detail needed for accurate mining. Trials in Western Australian mines have established that wideband VHF borehole radars can be used to probe the rockmass between boreholes. There is evidence that ore bodies reflect VHF radar signals both specularly and diffusely, much as the ground-air interface does when it is overflown by synthetic aperture radar (SAR). Interferometric borehole radar (InBHR) is analogous to SAR interferometry (InSAR) but it differs in certain important respects. There is also evidence of diffuse backscattering, at least from sheet-like ore bodies. This paper will examine the differences between InSAR and InBHR, and discuss the possibility of tapping into this backscatter by using interferometric methods to map both fault-hosted and other ore bodies in three dimensions.

Resistively loaded insulated antennas for narrow borehole radar

The transmission-line model for an insulated symmetric dipole in a lossy medium has been extended for the case of an insulated asymmetric dipole with a conductive arm and a uniformly resistive arm. The model is used to improve the understanding of the characteristics of a narrow borehole impulse radar recently developed. It predicts that sufficient loading of the resistive arm will dampen the antenna suitably for pulse transmission, despite the presence of the conductive arm. The model is partly verified by laboratory experiments. Examples of pulses received in situ in marble by the radar are presented.

Sparse Borehole Radar Array Imaging of Orebodies in Three Dimensions

Broadband very high frequency (VHF) borehole radars (BHR) can be used as tactical tools to map orebodies, faults and marker horizons; to identify hazards well in advance of mining and to optimize mine development. In principle, radar profiles and cross-hole scans from fanned arrays of boreholes can be used to synthesize three-dimensional images interferometrically if: borehole trajectories are accurately known; signal-to-noise ratios are adequate; the spatial sampling rate is sufficient; target echoes are well correlated, and if the target space is sufficiently uncluttered to limit the formation of mirages during reconstruction. However, automatic methods of projecting data into three-dimensional image space make stringent demands upon rock homogeneity, translucence and the accuracy of borehole trajectories. These demands can be relaxed by reconstructing objects from parts of geologically plausible 3D primitives such as cylinders, planes and spheres. In this paper we show that a number of useful primitive...