RESISTIVITY & CHARGEABILITY IMAGING
Chargeability (IP) surveys have had a similar history as resistivity surveys. For the most part it has been restricted to mining applications. In recent years it has found a niche within the environmental and engineering fields. It became apparent to some that more understanding and perspective was given to interpreting resistivity surveys.
Equipment developers have made it possible to conduct both types of surveys with the same apparatus (properly adapted) concurrently. The same software mentioned in the resistivity imaging section is also applicable. By blending the two techniques we have discovered that far more information of the subsurface than heretofore. The value of information content relative to the dollar expended has been enormous.
There follows below an example of results obtained from a joint resistivity and IP survey over an area which was examined for its water supply potential. The site consisted of sands and gravels (with intermixed clay) and some peat. The water table was situated at approximately 2-3 meters in depth.
The resistivity image in the top half of the illustration below confirmed the distribution of the suspected material. The chargeability image in the bottom half of the same picture complemented the resistivity by showing a top layer of impervious sands/gravels displaying pathways "leaks" between the surface and the water table immediately below. A high chargeability region around the 55 meter station which was interpreted as a suspected low concentration (between 100 and 1000 parts per billion) of a certain contaminant known to exist in the area. The site was rejected as a water source.
Immediately below is another sample of results taken from another site similar to that mentioned above. Again the objective of the survey was the same.
The resistivity image shows similar results as that described above. On the other hand the chargeability image in the lower half of the same illustration points to a different subsurface situation. The horizontal half of this site has an impervious layer (approximately 5 meters thick) overlying the groundwater, which appears to be charged from a stream and a permeable in the sloping half of the site. A potable water source was developed at between the 70 and 75 meter station.the