We recently came across account of an amateur (at least we presume so) geophysical surveyor writing software to remove the requirement that the separation of the local (A & M) and remote probes (B & N) of a twin probe array has to be about 30x the local probe spacing. This doesn’t work!
Why? Well, in simple terms, the twin probe array is defined by having one each of a current and potential probe at a fixed separation, themselves separated by a much greater distance, e.g. 30x as guidance. Any other configuration of probes is a different array and ignoring the 30x rule produces an evil (and variable!) compromise between a Wenner and square array that brings no benefit to anyone… In case anyone else feels like re-inventing the twin probe, here is some basic theory:
The shape of the current flow in the ground between two probes could be described as occupying half a rugby ball, the section through it being the ground’s surface. The angle this shape makes with the vertical is generally dependent upon the spacing of the current probes. As you traverse the ground with the local probes the potential probe is sampling the potential at the surface due to the current flow at a particular depth beneath the array, close to the local current probe. While the 30x separation is maintained, the relative positions of the local potential and the two current probes has little effect upon the measurement, however, for smaller separations this is not the case. Point-to-point array geometry becomes increasingly significant and what is more, the depth of investigation associated with the potential measurement becomes increasingly erratic.
If you ignore the 30x rule two adjacent grids of data collected using small and perhaps even different separations of remote and local probes will be measurements of different things. They won’t match and cannot be compared, even if you think they look similar. BAD idea! Always maintain a separation of 30x the local probe spacing, don’t re-invent the twin probe and best of all, if you don’t know what you’re doing, get help!