oa Crosshole electrical imaging of aquifer properties and preferential flow paths at the Bolivar ASR site

A time-lapse crosshole resistivity tomography trial was conducted at the Bolivar Aquifer Storage and Recovery (ASR) trial site, north of Adelaide between 1999 and 2001, to image aquifer properties and preferential flow paths. A bipole-bipole electrode configuration was used in order to reduce the effect of the top layers in the experiment. Five monitor wells were drilled on a circle of radius 75m for acquiring the crosshole resistivity data.

In total, seven time-lapse crosshole resistivity surveys were carried at different stages of fresh water injection. All survey data were processed, inverted and interpreted with the aid of numerical resistivity modelling and inversion code. It appears that the injected water flows in all directions, but mainly flows towards the south and the north; the injected water reached wells in the north and south but did not reach wells in the east and west by the end of the water injection. In general, the resistivity distribution in the region decreases with depth in the aquifer. A thin high resistivity layer at a depth of 130m was detected, which separates the T2 aquifer into two parts.

From this project, we learnt that resistivity inversion is a very necessary tool for crosshole resistivity data interpretation. There is no better way to analyse and interpret the data properly and accurately. However to obtain a good inversion result, adequate data and the right survey configuration are crucial.