Inversion of data from electrical imaging surveys in water-covered areas

Abstract

Electrical imaging surveys that are now widely used in many environmental and engineering studies have also been carried out in water-covered areas. These surveys include conventional surveys with multi-electrode resistivity meter systems where part of the survey line crosses a river. In some surveys that are located entirely within a water-covered environment, the electrodes are mounted on a streamer that is towed behind a boat. The streamer is dragged along the river/sea bottom, or floats on the water surface. The smoothness-constrained least-squares inversion method that is used to interpret the data from land surveys is adapted for underwater surveys. To accommodate the underwater topography, a distorted finite-element grid is used to calculate the apparent resistivity values for the inversion model. The first few rows of elements are used to model the water layer, while the lower part of the grid is used for the sub-bottom resistivity distribution. For an accurate inversion, the water resistivity as well as the depth to the bottom surface must be accurately known since a large proportion of the current flows through the water layer. The section of the Earth below the bottom surface is subdivided into a large number of rectangular cells. The water resistivity in the model is fixed, and the inversion program attempts to determine the resistivity of the cells that would most accurately reproduced the observed measurements. Examples from surveys in rivers, marine environments and across rivers are shown.