An environment application of self-potential geophysics

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1997.Includes bibliographical references (leaves 84-85).by Yervant Vichabian.M.Eng

Deep Resistivity Tomographic Imaging of The Qualibou Caldera, Saint Lucia

The Qualibou Caldera has been studied since the 1970’s for possible development of geothermal power generation. In 1974 dipole-dipole resistivity measurements were performed in the area. The apparent resistivity data was plotted as contours and a single line running through Sulphur Springs was interpreted by using forward models to generate a best fit model. The data is reanalyzed using a robust 2D inversion method. The result shows a resistive body beneath Sulphur Springs, the presence of which has been debated for nearly thirty years. The data from all 2D tomograms is interpolated into 3D, which generates images showing conductive features reminiscent of hydrothermal convection plumes

Self potentials in cave detection

The major application of the self potential (SP) method has been in mineral exploration and in recent years increasingly in environmental and engineering investigations. The SP method simply measures a naturally occurring potential between electrodes on the surface or in boreholes. There are three mechanisms that generate self potentials: streaming potentials due to fluid flow, electrochemical potentials generated by concentration differences of electrolytes, and thermoelectric potentials from temperature gradients. Self potential, SP, and streaming potentials will be used interchangeably as we are only interested in potentials generated from fluid flow in this study. Deformation of groundwater flow by preferential drainage..

Minimization of self-potential survey mis-ties acquired with multiple reference locations

Self-potential (SP) surveys often involve many interconnected lines of data along available roads or trails, with the ultimate goal of producing a unique map of electric potentials at each station relative to a single reference point. Multiple survey lines can be tied together by collecting data along intersecting transects and enforcing Kirchhoff's voltage law, which requires that the total potential drop around any closed loop equals zero. In practice, however, there is often a nonzero loop-closure error caused by noisy data; traditional SP processing methods redistribute this error evenly over the measurements that form each loop. The task of distributing errors and tying lines together becomes nontrivial when many lines of data form multiple interconnected loops because the loop-closure errors are not independent, and a unique potential field cannot be determined by processing lines sequentially. We present a survey-consistent processing method that produces a unique potential field by minimizing the loop-closure errors over all lines of data simultaneously. When there are no interconnected survey loops, the method is equivalent to traditional processing schemes. The task of computing the potential field is posed as a linear inverse problem, which easily incorporates prior information about measurement errors and model constraints. We investigate the use of both l2 and l1 measures of data misfit, the latter requiring an iterative-solution method with increased computational cost. The l1 method produces more reliable results when outliers are present in the data, and is similar to the l2 result when only Gaussian noise is present. Two synthetic examples are used to illustrate this methodology, which is subsequently applied to a field data set collected as part of a geothermal exploration campaign in Nevis, West Indies