Borehole effect in 2.5D crosshole resistivity tomography

• The resistivity contrast between the borehole fluid and the formation produces the borehole effects.
• The borehole effects arise mainly for the inline data.
• The borehole effect in crosshole ERT produces serious image artifacts.
• A new 2.5D inversion algorithm incorporating the boreholes is proposed.

It is well known that borehole fluid effects in crosshole electrical resistivity tomography (ERT) can produce serious image artifacts using conventional inversion algorithms that do not account for borehole effects. Using the analytical solution for an infinitely long borehole model in a half-space, we demonstrated that the borehole effects depend on the borehole radius and on the resistivity contrast between the borehole fluid and the surrounding rock formation. The results also show that the borehole effects arise mainly for the inline data where the current and potential electrodes are placed at the same borehole, and they are not severe for the crosshole data. To overcome the borehole effects in the crosshole ERT surveys, we developed a new 2.5D inversion algorithm incorporating the boreholes. The new inversion algorithm yielded an electrical resistivity image that was devoid of artifacts around the boreholes.