D4 Geophysical Potential

Topic

Potential-field geophysics encompasses methods that measure two fields derived from an underlying potential: the gravitational field and the magnetic field. Outside of their respective sources, both fields obey the same mathematical relationship governing their smooth spatial variation; this relationship allows for surface measurements of either field to be used to infer the subsurface distribution of density and magnetic susceptibility. These are passive methods—requiring no artificial source to operate—with a scope ranging from the centimeter scale of a borehole to the planetary scale of an orbiting satellite.

Gravimetry measures gravitational acceleration at a specific point, or the variation of that acceleration between points. A gravity anomaly—once corrected for latitude, elevation, and the effect of the rock layer between the measurement station and the geoid reference level—directly reflects subsurface density variations. A negative anomaly appears over exceptionally thick continental crust, as the thick crust sinks deeper into the mantle to maintain equilibrium, creating a mass deficit directly beneath it; conversely, a positive anomaly appears over a particularly dense body, such as ultramafic rock or metallic mineralization.

Magnetotellurics utilizes natural variations in the Earth's electromagnetic field—whether caused by geomagnetic storms or atmospheric electrical activity—to determine the electrical conductivity of the crust and mantle. This electrical conductivity is highly sensitive to the presence of partially molten fluids or graphite within the rock; consequently, a zone containing such fluids or graphite conducts electricity far better than the surrounding dry rock. Spontaneous potential measures natural electric potential differences directly at the surface—differences that reflect subsurface fluid flow, the presence of sulfide mineralization acting as a natural galvanic cell, or a subsurface thermal gradient. Spontaneous potential is particularly useful in hydrothermal zones and active volcanoes, where the flow of hot fluids is intense.

The behavior of all these potential-field geophysical methods shares a fundamental limitation: different subsurface source distributions can produce exactly the same field measured at the surface. Consequently, no single potential-field method can uniquely determine the source distribution that produced the signal; this limitation necessitates the integration of independent constraints from other methods to resolve the ambiguity, thereby linking the gravitational and magnetic fields—with which this description began—to the uncertainty inherent in interpreting both.

ID:2187