Measurement Strategies for Geoelectrical Tomography: Specialized Techniques for Multichannel Instruments
David Belfrage, presenterar sitt examensarbete
How to select which measurements to make during a geoelectrical resistivity survey has been an active field of research for decades. Despite this, techniques for optimizing the measurement sequences so as to take full advantage of the multichannel capabilities of modern state of the art equipment are scarce. In this thesis special attention is directed towards full usage of the measuring system's multichannel capabilities. The thesis also aims to be an introduction to the field of geoelectrical tomography for the unfamiliar reader, in particular the concept of the sensitivity of a measurement is fully derived. The methods presented are all relying on sampling the sensitivity function and from these samples design a set of measurements. The methods discussed will largely rely on selecting measurements that are as 'different' from each other as possible, where the difference is measured in either the correlation of the sensitivities or the distance of the sensitivities' mass centers. These new methods are later adapted for multichannel use, after a presentation of how the multichannel system works.
The resulting sets of measurements are tested on a simple model consisting of resistive prisms buried in otherwise homogeneous ground. The simulated measurements and the inversion of these measurements are done with the third party programs res2dmod and res2dinv respectively. From these simulation it is concluded that the methods optimized for multichannel successfully schedules close to the maximum number of measurements per current injection without losing much performance compared to their single channel counterpart. Field measurements with a multichannel-optimized protocol are also presented and compared with a reference protocol.