The non-destructive measurement of the radio frequency properties of hard rock borehole cores

[摘要] ENGLISH ABSTRACT:The effective use of borehole radar in geophysical exploration requires accurate knowledge of the dielectricproperties of the geological system in which it is employed. The attenuation and propagation velocityof pulses through rock must be known in order to plan and interpret experimental data. Conventionaldestructive methods for the measurement of hard rock cores require the careful preparation of samples.This firstly necessitates the selection of sampling position and the resulting estimation of rock propertiesbased on sparse measurements, and secondly results in the loss of material and thus data.The ready availability of cylindrical borehole core samples invites the use of a nondestructive meansof measuring their properties. A novel design for a flexible guarded capacitor which conforms to acore's cylindrical surface is presented here. T he proposed device has numerous advantages over previousmethods. No material is lost to sample preparation and a detailed characterisation of the entire core,including inclusions and transitions between rock types, may be performed.A detailed methodology for the rapid construction of a robust capacitor is given. Guidelines for itsoperation to achieve repeatable and accurate measurements of the complex dielectric constant of samplesof varying homogeneity in the 1- 25 MHz frequency range are presented.The increased amount of data collected from complete core samples is analysed statistically, andamongst other things allows the estimation of the rock's homogeneity. Comparisons of the dielectricproperties measured in the laboratory to propagation velocity estimates obtained from crosshole boreholeshoots show that a more homogeneous sample is a better predictor of bulk propagating conditions.Detailed studies of the dielectric properties of economically important diamondiferous and platiniferousgeological systems show that borehole radar is a feasible tool for the high resolution delineation of orebodies and other geological targets.

[发布机构] Stellenbosch University