Underwater resistivity measurements

Motivation

Whereas the sediment distribution onshore and for the deep offshore is relatively well known by boreholes, the coast-near regions of up to 10m water depth is still a relatively unknown region, which is interesting regarding development of the seas.
For a routine geophysical investigation of the sea bottom seismic and electric measurements are well suited. The first give very detailed structural information, but cannot give hints to the sediment types without boreholes. On the other hand, resistivity measurements are able to delineate sediments with different porosities, clay and humus content due to their different electric conductivity.

Challenges

1. high measuring progress: fast continuous measurements
2. low voltages due to high conductivity: special processing
3. additional sensors: GPS, electrode depth; water depth, conductivity and temperature
4. automated 1D-interpretation in situ
5. efficient and accurate 2D-analysis

We favour a measuring scheme using a bottom-towed electrode cable. Fig. 1 shows the measureing scheme. See details here.

Data processing

Due to the poor data quality the registrations for each channel a processed using a harmonic least squares fit. The measured errors are used as weighting factors. Result is a low-passed signal on a coarse and spatially equidistant grid. Fig. shows the raw and processed data for an example profile.

Inversion

1D-Inversion

A fast method that is already applied in the field is a 1d inversion of the individual soundings. This is accomplished by the program EM1dInv (Auken et al., 2005). Conducitivity and thickness of the first layer, the sea water, are held constant. Fig. 3 shows the result of such a 1d inversion.

2d Roll-along inversion

In case of lateral inhomogeneities a 1D inversion is not sufficient. In order to process the very long profiles we developed the so-called Roll-along inversion (Günther, 2007). The inversion is done piecewise and consistency is achieved by global smoothness constraints and fixation. Fig. 4 shows the resistivity distribution for a profile in the Greifswald bay.

Detailed inversion using BERT

Another possibility is given by the  Finite Elemente package BERT (Günther et al., 2006). By using unstructured triangles an arbitrary shaped sea bottom, but also structural information from seismics and boreholes can be incorporated. Since there is no roll-along method so far, this is only efficient for detailed investigation of interesting profile parts. Fig. 5 shows such a result.

References

Auken, E., Christiansen, A. V., Jacobsen, B. H., Foged, N., and Sorensen, K. I. (2005). Piecewise 1d laterally constrained inversion of resistivity data. Geophysical Prospecting, 53(4):497-506.

Günther, T. (2002-2007a). DC2dInvRes - Direct Current 2d Inversion and Resolution. resistivity.net productions, http://dc2dinvres.resistivity.net

Günther, T. (2007): Roll-Along Inversion – A New Approach for Very Long DC Resistivity Profiles. Ext. abstract, 13th European Meeting of Environmental and Engineering Geophysics, 03.–05.09.2007; Istanbul, Türkei.

Günther, T., Rücker, C., and Spitzer, K. (2006). 3-d modeling and inversion of dc resistivity data incorporating topography - part II: Inversion. Geophys. J. Int., 166(2):506-517.