Resistivity survey maps bedrock

An underground drainage tank was going to be constructed at a site in the Lake District. The tank had to be sited in an area of homogeneous geology to ensure that it would not settle or subside and so a borehole survey had been undertaken to assess the ground conditions.

Unfortunately one borehole had identified shallow bedrock on one part of the site but less than ten metres away a second borehole identified sands and gravels to a depth of 25 m but no bedrock. The client needed to know if this was a localised variation and if so was there an area with a level bedrock profile. Carrying out further boreholes would have been costly and may not have fully identified the bedrock profile and so Phase SI recommended carrying out a resistivity survey.

Despite two days of very heavy rain prior to the survey, and several torrential downpours during the course of the survey (not unexpected for the Lake District in autumn!) the heavily saturated ground did not adversely affect the resistivity survey as there was a very good geophysical contrast between the solid bedrock and the sands and gravels.

The resistivity survey provided high quality data that confirmed the presence of significant variations in the bedrock profile across the site. The data suggested the presence of a number of old river channels which had cut into the bedrock and then been infilled with sands and gravels.

By surveying several resistivity profiles it was possible to map the variations in the bedrock profile across the majority of the site. The client now had a clearer picture of the sub-surface conditions without having to resort to additional intrusive work. This was achieved in just a couple of days of site work in difficult survey conditions.