Successful application of ground-penetrating radar in the exploration of gem tourmaline pegmatites of southern California

Application of ground-penetrating radar has been successful in delineating gem-bearing zones in the Himalaya pegmatite mine of the Mesa Grande district of southern California. The high frequency of the electromagnetic signal allows features as small as a few centimetres to be resolved within 1-2 m of the surface of a mine wall. Careful initial set-up consisted of: (i) selection of antennae with sufficiently high central frequencies; (ii) recording with a short time of scan to reduce end-of-scan noise levels; (iii) choosing appropriate colour schemes to highlight extreme amplitude variations. Operation during data collection consisted of prepainting marking points on the mine face and air launching the signal to reduce false anomalies caused by rocking of the antenna on the rough surfaces. Data processing using the Hilbert transform provided images of the cavity geometry that were then used by the blasting captain for accurate placement of explosives. The instantaneous frequency plot was found to be effective in distinguishing air-filled from clay-filled pockets, and the instantaneous phase plot was helpful in selecting potential targets where the amplitude was less than the maximum range. When carefully used in conjunction with good knowledge of the geological conditions, the method promises to provide an important tool for mapping internal structures of pegmatites, thus assisting future mining activities.