Borehole radar - Why it is different from lower frequency DHEM techniques

Abstract

Borehole radar is a geophysical technique which uses high to ultra-high frequency radio waves generated and detected in boreholes to produce a high resolution image of the subsurface.The physics of the propagation of radio frequency electromagnetic signals in the earth is quite different to the physics of audio frequency electromagnetic signals used in most other drillhole electromagnetic techniques. The difference is due to the difference in value of the loss tangents of geological materials at these two frequencies. The loss tangent is equal to the ratio of conductivity to the angular frequency multiplied by real permittivity. At audio frequencies the loss tangent of geological materials is much greater than 1. Consequently, the full damped wave equation that describes the propagation of electromagnetic reduces to a minor variation on the diffusion equation and electromagnetic signals travel predominantly by diffusion. At radio frequencies the loss tangent is much less than 1 and the damped wave equation reduces to a minor variation on the standard undamped wave equation and electromagnetic signals travel predominantly as waves. As a result of the wave behaviour of electromagnetic signals at radio frequencies, borehole radar can be used to produce high resolution images of the subsurface, albeit at smaller ranges. Sample measurements of the electrical properties of the host rock from two large Australian base metals mines (Mt Isa, Broken Hill) suggest that borehole radar can be used to obtain reflections from interfaces up to 25 m away at 50 MHz and up to 70 m away at 20 MHz. The wave behaviour of electromagnetic signals at radio frequencies makes it possible to obtain very detailed images of rock structure with borehole radar. At 20 MHz, structures as small as 2.5 metres in size may be detected, and at 50 MHz, structures may be detected down to a size of about 1 m.