Complex geological models typical of hard rock mineral exploration in the Yilgarn Craton of Western Australia have been created. Through full waveform synthetic modeling, borehole reflection seismology has been tested in these environments with borehole geometries typical of hard rock exploration techniques. One such example is presented here. Synthetic testing has shown that borehole reflection seismic sections suffer from lack of aperture in the down-dip direction. Thus Large offsets and higher shot density is required on the down-dip side of the borehole to compensate for this. However at large offsets wavefield identification is complex and correct separation of wavefields for imaging is difficult. These limitations and acquisition specific geometries and processing are discussed here. Initial field studies conducted during a pilot study show potential for seismic imaging from angled boreholes.
Recent 2D and a pilot 3D seismic reflection studies in the Yilgarn Craton of Western Australia have successfully produced high resolution images delineating nickel bearing rock units (Urosevic et al., 2007). These successes have led to a number of mining companies conducting larger scale 3D seismic surveys in the Kambalda and Agnew – Wiluna regions of the Yilgarn. Hard Rock (HR) environments are geological highly complex and produce many seismic paterns which are difficult to interpret (Urosevic et al., 2005). With this complexity arises many interpretation questions and a need to accurately calibrate seismic data by combining borehole and geological information. As such seismic “borehole ties” and targets are now being drilled. These should include the use of seismic velocity check-shot and Full Waveform Sonic (FWS) surveys to improve images and interpretation through; accurate depth to time conversions, improving migration velocities, determining wavelet phase shifts, identifying target and rock unit attributes. Due to the typical angled borehole and steeply dipping target geometries found in HR exploration, check shot surveys offer the possibility to seismically image around the hole when combined with walk-away and offset shot locations. Inherently borehole seismic imaging is of higher resolution than surface seismic due to the receivers being within the host formations, preserving frequency content from the destructive scattering effects of irregular thick overburdens (regolith). Also reflection events can be directly correlated to core logs or wire-line data. In particular borehole reflection seismology can offer an increased understanding of the origin of the events and structural complexity around the borehole. To test the viability of borehole reflection seismology in HR environments, we modeled a complex geological environment found in the Yilgarn Craton. The objectives of the study were to; investigate the applicability of borehole reflection seismology, determine major limitations, and develop new field surveying techniques and approaches to data processing.
The initial cross section (model) was taken from a nickel deposit in the south of the Yilgarn Craton. This is shown in Figure 1. The nickel deposit lies on an upturned interface between sediments’ and ultramafics. Two boreholes X and Y approximately 400m apart were populated with receiver stations every 5m.
