lley, Mustang Island, Texas

Magnetotelluric Investigation of a Pleistocene Nueces River Valley,
Mustang Island, Texas


Egon T. Weber, Richard Hay, and James R. Garrison, Jr.

Center for Water Supply Studies, Texas A&M University Corpus Christi,
6300 Ocean Dr., Corpus Christi, Texas 78412




ABSTRACT


Magnetotellurics is a geophysical survey technique that can measure the subsurface
electrical properties by measuring simultaneously the orthogonal electrical and magnetic
variations on the surface. At a particular frequency, the relationship between the surfi-
cial electrical fields and magnetic fields provides a measurement of the electrical imped-
ance tensor of a volume of the subsurface. Higher frequencies represent a shallower
integrated volume. Thus, by measuring the magnetotelluric response at a range of fre-
quencies, a resistivity sounding can be determined. However, the electromagnetic re-
sponse of the subsurface is a complex function of the lithology, porosity and porewater
salinity, thus such soundings do not provide a unique characterization of the subsurface.
Ideally, a properly interpreted magnetotelluric depends on other data. This study
provides such an interpretation. A geoelectromagnetic survey was conducted parallel to
two miles of shoreline on Mustang Island, Texas. This barrier island prograded across
the mouth of Corpus Christi Bay during the last two to three thousand years. Underly-
ing the fine sands of the barrier island lies a valley-fill sequence consisting of fluvial, bay
head delta and estuarine deposits that filled in a Pleistocene erosional surface at a depth
of approximately 150 to 200 ft. The survey consisted of eight magnetotelluric soundings
with resistivity measurements down to approximately 300 ft below the surface. These
soundings were used to construct a resistivity cross section with resistivities in the range
of 0.3 to 20 ohm-meters and then correlated with a nearby core and a logged well. Five
zones were identified and correlated with the lithostratigraphic facies and water quality
expectations derived from a nearby core and logged well.