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Self potentials in cave detection

Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.

Corresponding author: yerv@erl.mit.edu

The first 20% of the full text of this article appears below.

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 paths increases water flow velocity and intensifies streaming potentials caused by filtration. In the case we are studying, the drainage paths are defined by flow into cavities. SP anomalies can be from tens to hundreds of millivolts depending on the pressure drop, lithology, and chemical composition of the water.

On the boundary of solid and liquid phases, an electrically charged electrochemical layer is formed between the mineral and pore moisture. Streaming potentials are directly proportional to the potential difference between the immobile part of the electrochemical layer and the free solution. Streaming potentials decrease as the electrolyte concentration increases. Most mineral surfaces are negatively charged, hence an excess of positive ions is drawn to the surface to form the electrochemical boundary layer. The positive ions are carried in the direction of the water flow—hence areas of water inflow are characterized by positive anomalies and those of flow sources by negative ones. In a permeable and homogeneous medium, the streaming potentials show the contours of groundwater flow. The slope from a line plot of SP data is proportional to the hydraulic gradient in the direction of the survey line.

The theoretical basis of the streaming potential was first worked out by . . . [Full Text of this Article]