Monitoring of tidal influences on the saline interface using resistivity traversing and cross-borehole resistivity tomography

Tidal influences on the saline interface of a shallow unconfined aquifer have been investigated by using both dc resistivity traversing and cross-borehole resistivity tomography (CRT). Variations in bulk resistivity structure obtained through resistivity traversing provide a clear general picture of the structure of the saline interface and its mixing zone. On all three resistivity traverses there are also indications that a small degree of mixing of saline and fresh water is occurring ahead of the main saline boundary. For one of the traverses much more detail of variations in resistivity structure is provided by CRT utilising three 10 m long electrode strings installed at 10 m separation. CRT images of the saline mixing zone do not reveal any significant difference between low and high tide for tidal ranges of about 2 m, but for a tidal range of approximately twice this clearly show effects due to both increased wave run-up and tidal head. Correlation of the bulk resistivity with fluid resistivity values measured in parallel bores suggests that although there is no apparent widening of the mixing zone during the tidal cycle significant variations in salinity do occur which can be envisaged in terms of the horizontal movement of near vertical salinity contours. (C) 2010 Elsevier B.V. All rights reserved.