Capacitive Resistivity Imaging With Towed Arrays

Abstract

Capacitive resistivity (CR) is an emerging technique which can extend the scope of DC<br>resistivity to environments where galvanic measurements tend to fail, for example on engineered<br>structures (roads, pavements), dry soil or frozen ground. Low frequencies (say 1 to 25 kHz) allow for<br>capacitive coupling between sensors and the ground, but theoretical equivalence with the DC case is<br>only achieved in a low-induction number mode of operation associated with quasi-stationary fields.<br>Under such conditions, conventional DC interpretation schemes are applicable to CR data.<br>In general, the transfer impedance across a four-point capacitive array is a complex function<br>of frequency and geometry. For standard geometries with plate-wire sensors on the surface, typical<br>survey parameters and moderate electrical properties of the subsurface, responses are strongly inphase.<br>However, there are circumstances under which the quadrature component is non-negligible so<br>that significant phase-shifts are observed and a phase-sensitive expression for apparent resistivity is<br>more adequate. We have validated these fundamental properties of CR by measuring the fully<br>complex voltage (magnitude and phase) with a prototype CR instrument developed by BGS.<br>The nature of the CR technique facilitates dynamic measurements with towed multi-offset<br>arrays, which we have employed for the collection of low-noise, highly repeatable apparent<br>resistivity data at centimeter intervals. Such data allow near-surface electrical tomographic imaging<br>at resolutions that would be impractical to achieve with conventional DC resistivity. We conclude<br>that capacitive resistivity imaging (CRI) has the potential to become a key tool for environmental<br>and engineering site investigation surveys in the built environment.