An evaluation of Bristow’s method for the detection of subsurface cavities

Abstract

The Bristow method, an electrical resistivity technique employing a pole‐dipole measurement array in conjunction with a simple graphical method of interpretation, has proven an effective means of locating subsurface cavities. There have been questions, however, regarding the limits of the method and whether the Bristow method is indeed the most suitable of the various electrical resistivity techniques for cavity detection. In hopes of resolving some of the controversy surrounding Bristow’s method, resistivity traverses are numerically modeled over spherical and cylindrical cavities given a variety of circumstances. Using a slight variation of Bristow’s original interpretive technique on modeled data, the size and location of subsurface cavities can be determined with surprising accuracy. However, when the simulation is altered to incorporate geologic noise, the maximum depth at which a cavity can be detected is found to be far less than has been reported in field investigations. In this instance the presence of a cylindrical cavity cannot be discerned beyond a depth to the top approximately equal to the diameter of the cavity, and spherical cavities are indistinguishable at depths much greater than the radius. One should note that the noise field generated for this model may not be representative of what would normally be found in the real earth. In the field, the maximum achievable depth of detection will vary depending on the actual geologic conditions and whether some technique is employed to reduce the effects of noise. In any case, a comparison of traverses using various electrode arrays confirms that the Bristow method is the most satisfactory of the applicable electrical resistivity techniques.

Evaluation of Pole-dipole technique (Bristow's method) to detect the dimension of K-3 cave in Haditha area -west Iraq- Case study

“…[7] were able to detect both air-filled and mud-filled cavities, and [8] successfully delineated air-filled cavities, all of which are confirmed by drilling. [9], [10], [11], [12], and [13] concluded that the Bristow method is a powerful tool not just for detection, but also for delineation of cavities, and it is probably the most sensitive electrical resistivity technique advanced for those purposes. There are few previous studies in Iraq that used resistivity method for detecting cavities; for example [14] used Wenner array to detect the cavities in Hamam Al-Aleel, north Iraq.…”

“…The potential difference is measured between two potential electrodes, by moving current electrode (C1) incrementally with (n=1, 2 ...) for a distance (na) equal approximately to (10a) on either side of the local current electrode (C1), and along the traverse. The measured resistivity profiles will overlap, and the voltage measurements are then expressed as apparent resistivity values.These resistivity values are indicated by the measured voltage given the relative positions of the electrodes, and assuming the ground has invariant electrical properties throughout[12].Therefore, the apparent resistivity values are plotted against the potential electrode at midpoint position, as shown inFigure (4)to detect the probality location of a cavity.…”

Bayesian Inversion for Cavity Investigation

“…However, it is very difficult to characterize or infer the cavity structure by indirect way like geophysical exploration, because cavities might be made from the many origins (Lowry and Shive 1990), and geophysical anomaly due to a cavity is in general very small. Therefore, it may be within the range of ambient noise.…”