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DEVELOPMENT OF ELECTRICAL TOMOGRAPHIC IMAGING AND INVERSION TECHNIQUES FOR MINERAL EXPLORATION

Objective


A prototype resistivity system has been designed and developed for the acquisition of high-density tomographic datasets. PC-control permits the addressing of smart electrodes and automated data-capture from a very large number of electrodes, both on the surface and in deep mineral drillholes. The system may be programmed for any electrode configuration, including pole-pole, pole-dipole and dipole-dipole arrays, and for single hole, surface-to-hole, and cross-hole scanning. A smart electrode design, housed in a semi-rigid sonde has been successfully tested at minimal prospects in France and Greece to depths of 400m and between drillholes spaced 100m apart.

Advanced 2D and 3D numerical inversion schemes have been developed to provide reliable image reconstructions which more accurately reflect the true size, shape, location and geoelectric properties of subsurface targets. These algorithms were developed using a boundary integral equation technique to generate the forward model and a non-linear least squares optimisation procedure to achieve convergence between the theoretical and observed data. A smoothness constrained formulation is used to improve the stability of the iterative process. The results show that electrical tomographic imaging, based on the acquisition of very large bivariate datasets and optimal inversion schemes, can give realistic resistivity images which are visually self-evident and which could significantly improve the targeting of exploratory drillholes.
It is proposed that independent research currently being undertaken by (UK), BRGM (France) and IGME (Greece) be combined and jointly directed towards the development of an innovative Electrical Tomographic Imaging system for mineral exploration.

Specifically the R & D will include the design, development and testing of a unified cross-borehole and surface-to-borehole resistivity system (both hardware and software) which, under microcomputer-control, will facilitate the rapid geoelectric scanning of intersected or off-hole mineralisation.

Conventional resistivity systems cannot provide the high sampling densities needed for 3D tomography and current interpretative aids are inadequate for tomographic inversion. A major objective is the development of optimum algorithms for reconstruction of resistivity image sections which are visually self-evident to the geologist and which closely reflect the true size, shape and deposition of the target mineralisation.

Advanced 2D, and 3D numerical modelling schemes (forward and inverse solutions) will be further developed to specifically exploit the unprecedented data density acquired with the proposed system.

The system, once developed, will be tested at mineral prospects in Greece, France and Ireland in co-operation with IGME, TCM-F and RTZ.

Successful completion should significant improve the cost-effectiveness of exploratory drilling, and thereby reduce the number of drillholes needed for prospect evaluation.

Funding Scheme

CSC - Cost-sharing contracts
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Coordinator

NERC British Geological Survey
Address
Keyworth
NG12 5GG Nottingham
United Kingdom

Participants (2)

Bureau de Recherches Géologiques et Minières (BRGM)
France
Address
Avenue De Concyr
45060 Orléans
IGME
Greece
Address
Messoghion Street,70
11527 Athens