Two and three-dimensional resistivity imaging in a shallow coastal sand aquifer

Abstract

Surface resistivity imaging was proposed to map the saline interface produced by the interaction of a tidal creek and a shallow fresh water aquifer. Forward modelling was used to determine the relative merits of using two and three (2D and 3D) dimensional electrode arrays. The results of the forward modelling indicate that under ideal circumstances the surface techniques can resolve the conceptual saline interface near the tidal creek. One 3D and four 2D surveys were completed on the bank of Korogoro Creek. The 2D surveys were orientated parallel (3) and perpendicular (1) to the creek. The apparent resistivity data was inverted using RES2DINV and RES3DINV. The resulting 2D images demonstrate that the conceptual model used in the forward modelling oversimplified the distribution of saline water in the subsurface. The 2D images indicate that freshwater is discharging into the creek via preferred pathways at several locations along the creek bank. The 3D image shows that the distribution of saline water is constrained to an area close to the creek. Neither the 2D or 3D images were able to clearly identify the thin saline wedge observed using downhole techniques (EM39). The results of the resistivity surveys indicate that the techniques are useful in identifying relatively large-scale saline patterns but are unable to identify relatively small anomalies. Most importantly the results highlight the critical need to control interpretation of high resolution surface resistivity data with downhole techniques.