Seismic reflection and mineral prospecting

Abstract

Seismic reflection, hitherto neglected as a prospecting technique for base metal exploration, is worthy of consideration now that the targets sought are deeper and more elusive than in the past. As a supplement to drilling, or in its own right, it has more potential than any other geophysical method to delineate subsurface stratigraphy in detail. Model studies of two-dimensional situations have suggested that orebodies themselves may give characteristic signatures, a factor which may find application in direct detection. The physical validity for supposing the method workable rests in the fact that velocity is not a singlevalued function of density alone, as commonly and tacitly accepted in petroleum exploration. Rather it is a function of density and mean atomic weight (defined as the formula weight of a mineral divided by the number of particles in its molecular formula). The difficulties of obtaining velocity measurements in basement rocks under in situ conditions suggests that it is appropriate to consider density changes as a measure of acoustic impedance contrast. There is more merit in this approach than might be expected, for it appears that in orebearing rocks density variations are of more consequence than velocity variations. For example, increasing substitution of pyrrotitic ore into a country rock consisting of siltstone does not alter a basic average velocity of 5.5 km s-1, but the density changes from 2.7 to 4.5 t m-3. Experimental studies of the seismic response of basement rocks indicate that reflected events can be recorded and interpreted from zones within basement, provided that seismic data have a white spectrum whose bandwidth exceeds 2 octaves, and with upper limiting frequencies in excess of 200 Hz. Detector spacing should be in accordance with the maximum dip in an area, but 5?10 m is recommended.