Detection and characterization of fracture zones in bedrock in marine environment: possibilities and limitations
G.A. Tassis, P.I. Tsourlos and J.S. Rønning
Journal name: Near Surface Geophysics
Issue: Vol 18, No 1, February 2020 pp. 91 - 103
Info: Article, PDF ( 3.63Mb )
Price: € 30
Geoelectrical measurements have so far been tested in marine environments worldwide in order to detect subsea fracture zones. However, many of these datasets are processed without considering the extremely high electrical conductivity of seawater and its implications. This study summarizes our efforts to establish the basic rules as to whether marine electrical resistivity tomography can detect weak zones inside a resistive bedrock, a problem which the engineers in Norway usually encounter in tunnel construction sites. This study examines the theoretical response of electrical resistivity tomography in a classic Nordic environment where a highly resistive bedrock is located below the highly conductive seawater, and the capability of electrical resistivity tomography to detect fractured zones, as relevant in a geotechnical study. We performed a large number of synthetic modelling tests examining several factors that marine geoelectrical surveys are particularly sensitive to, such as the depth of the seabed, the seawater conductivity and the bedrock variation, and the survey layout and the inversion scheme. Our results indicate that electrical resistivity tomography surveys for fracture zone detection in geoelectrically demanding marine environments can be promising in case of a limited water depth, and with the use of either dipole–dipole or multiple gradient array and availability of a detailed knowledge of the conductivity distribution in water. However, results of electrical resistivity tomography surveys in such circumstances can be ambiguous since they potentially suffer from reduced resolution and due to the loss of electrical current in water and other artificial effects. Based on the results of modelling, we were able to improve interpretations of electrical resistivity tomography data from a field survey, where marine acquisition was carried at a strait in Kvitsøy, southern Norway.