WELLOG SPONTANEOUS POTENTIAL (SP)
AND SURFACE SP
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All Rights Reserved
From: Applied Geophysics, W.M. Telford, et. al.
Natural currents flowing within the earth cause a voltage to be produced. Sato and Mooney in 1960 postulated a theory that two electrochemical half-cell reactions of opposite sign take place, one cathodic above the water table, the other anodic at depth. In the cathode half-cell there is chemical reduction of the substances in solution – that is – they gain electrons while in the anode cell an oxidation reaction takes place and electrons are lost. The mineral zone functions only to transport electrons from anode to cathode. The overall SP effect is determined by the difference in oxidation potential between the solutions at the two half cells.
HOW IS SPONTANEOUS POTENTIAL MEASURED?
Spontaneous potential is a DC voltage. Equipment designed to measure small DC voltages and having the ability to filter low frequency noise are used to measure SP. Specialized probes are used to connect the instrumentation to the surface of the earth. Probes must be non-polarizing, otherwise they may produce a “contact” voltage thru electrochemical action of the probe itself. Non-polarizing probes using a “porous pot” containing metal salt (copper sulfate) solution are used for surface SP. In borehole environments, inert metal electrodes are used.
WHERE IS SPONTANEOUS POTENTIAL USED?
SP is used in applications that require definition of massive sulfide deposits. Other applications are related to underground water seepage from containment areas. Subsurface water movement may be detected. A potential called “streaming potential” may be observed due to water movement.
SP may be used as a surface geophysical technique or subsurface (borehole) technique.
Combination of surface and subsurface SP profiles provide 2D and 3D images of mineralized zones.
WHAT CAN SPONTANEOUS POTENTIAL REVEAL?
SP will reveal the presence of mineralized bodies that may or may not be visible at the surface of the earth. Because the currents produced radiate outward around areas containing mineralization, the voltage potential may be measured at a great distance from it. Surface and borehole SP profiles may be combined to reveal the true extent of a buried massive sulfide or other anomalies.
Spontaneous Potential can be measured in exploratory or other open-hole boreholes. The method usually requires a water/mud filled borehole in order to make electrical contact with surrounding formations. Tools having a spring caliper with an electrical contact that remains in direct contact with the borehole wall can be used in dry (non water filled) boreholes. The method provides vertical SP definition of sulfide targets/mineralized contacts and non-mineral rock interfaces. A reference electrode is placed in a shallow water filled depression on the surface. Voltage is measured between the reference electrode and the down-hole electrode located in the borehole. Voltages are recorded using a data acquisition system at specific intervals from the bottom of the hole to the surface or other predetermined interval to be logged. An SP log is measured in millivolts.
When used in lithology identification, SP voltage is greater in shale when compared to sandstone. In the case where NaCl is the primary source of ions and cations, the Na (+ ions) are able to permeate the shale and the Cl (- cations) cannot. The resulting voltage is measured as a potential referred to as spontaneous potential. Only a very small amount of permeability is required to produce an SP voltage in water filled holes. SP alone, however, should not be used as a reliable indication of porosity or permeability. It is an invaluable tool used in definition of lithology sequence and thickness.
WELLOG will provide a surface SP contour map with GPS UTM coordinates. If borehole SP is logged, a color print (log) of depth versus SP is provided to the customer for each borehole.
Applied Geophysics, W.M Telford, L.P. Geldart, R. E. Sheriff, D.A. Keys,
Basic Exploration Geophysics, Edwin S. Robinson, John Wiley and Sons, 1988
Principles of applied Geophysics, D.S. Parasnis, Chapman and Hall, Ltd, 1972
Mining Geophysics, D.S. Parasnis, Elsevier Scientific Publishing Company, 1973