The inversion of 2-D and 3-D resistivity data from surveys in aquatic areas
M.H. Loke, T. Dahlin and D. Rucker
Event name: EAGE-GSM 2nd Asia Pacific Meeting on Near Surface Geoscience and Engineering
Session: Session 9 : Electric, EM, MT, MRS and NMR Methods - Resistivity
Publication date: 24 April 2019
Info: Extended abstract, PDF ( 824.12Kb )
Price: € 20
Resistivity surveys are now also carried in areas covered by water. The surveys involve electrodes planted on the water bottom or on a streamer towed by a boat. As the water layer has a large effect on resistivity measurements, its effect must be accurately modelled. The water resistivity and depth to the bottom are usually independently measured with a conductivity meter and depth sounder. The upper part of a finite-element grid is used to model the water layer, including possible variations in the water resistivity with depth. We show the results from a 2-D survey in Stockholm with electrodes planted on the sea bottom. The sediment thickness from the inverse model agrees well with drilling results and a possible weak zone in the bedrock was detected. Surveys with floating electrodes do not follow a straight line due to water currents and a 3-D inversion approach is required. An example is shown from a survey in the Panama Canal where the data from 19 sub-parallel lines are collated into a 3-D data set. The inverse model shows a conductive bottom with weathered marine sedimentary rocks and a remnant of an old river channel filled with more resistive sands and gravels.