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Journal of the Australian Society of Exploration Geophysicists
RESEARCH ARTICLE

Robust 1D inversion of large towed geo-electric array datasets used for hydrogeological studies*

David Allen 1 3 Noel Merrick 2
+ Author Affiliations
- Author Affiliations

1 Groundwater Imaging Pty. Ltd., 279 Fitzroy St, Dubbo NSW 2830, Australia.

2 National Centre for Groundwater Management, University of Technology Sydney, Sydney NSW 2007, Australia.

3 Corresponding author. Email: David@GroundwaterImaging.com

Exploration Geophysics 38(1) 50-59 https://doi.org/10.1071/EG07003
Submitted: 14 July 2006  Accepted: 23 December 2006   Published: 5 April 2007

Abstract

The advent of towed geo-electrical array surveying on water and land has resulted in datasets of magnitude approaching that of airborne electromagnetic surveying and most suited to 1D inversion. Robustness and complete automation is essential if processing and reliable interpretation of such data is to be viable. Sharp boundaries such as river beds and the top of saline aquifers must be resolved so use of smoothness constraints must be minimised. Suitable inversion algorithms must intelligently handle low signal-to-noise ratio data if conductive basement, that attenuates signal, is not to be misrepresented. A noise-level aware inversion algorithm that operates with one elastic thickness layer per electrode configuration has been coded. The noise-level aware inversion identifies if conductive basement has attenuated signal levels so that they are below noise level, and models conductive basement where appropriate. Layers in the initial models are distributed to span the effective depths of each of the geo-electric array quadrupoles. The algorithm works optimally on data collected using geo-electric arrays with an approximately exponential distribution of quadrupole effective depths. Inversion of data from arrays with linear electrodes, used to reduce contact resistance, and capacitive-line antennae is plausible. This paper demonstrates the effectiveness of the algorithm using theoretical examples and an example from a salt interception scheme on the Murray River, Australia.

Key words: geo-electric, inversion, resistivity, hydrogeology, rivers.


Acknowledgments

The Co-operative Research Centre for Sustainable Rice Production provided a research stipend which made this research and development possible. The NSW Department of Infrastructure Planning and Natural Resources provided funding for data acquisition at Mildura. Niels Christensen of EMmodel (nbc@geo.au.dk) and Scott MacInnes of Zonge Engineering provided useful advice.


References

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by generalising the derivations of the same formulae, derived for specific cases, by Roy and Apparao (1971).