INITIAL GEOPHYSICAL EVALUATION BY THE SAGE PROGRAM OF A GEOTHERMAL PROSPECT IN THE CERROS DEL RIO VOLCANIC FIELD NEAR SANTA FE, NEW MEXICO

High temperature gradients (up to 58^oC/km) in water wells in the Caja del Rio (CDR) area of the Cerros del Rio volcanic field about 25 km northwest of Santa Fe, New Mexico motivated the SAGE (Summer of Applied Geophysical Experience) program to conduct field geophysical evaluations in 2011. Over 50 exposed vents, numerous rift-related faults, and high 3He/4He ratios in groundwater are consistent with high-temperature fluids of magmatic/mantle origin ascending along permeable pathways. Alternatively, elevated groundwater temperatures may result from warm, deep-basin circulation forced upward over a basement constricting horst block.

SAGE 2011 gravity and magnetotelluric (MT) measurements in the CDR were focused in a large (over 50 sq. km) area that lacked previous gravity and MT data. Inverse modeling of new, although sparse, SAGE 2011 gravity and MT measurements in the area of the proposed horst indicate basin depths of near 2 km to high density/high resistivity basement rocks assumed to comprise Paleozoic limestone overlying Precambrian granite. Such depths agree with previous, nearby SAGE findings and industry seismics. The MT results also reveal a midcrustal conductor, ubiquitous in tectonically active regions of the western U.S., at depths of 10-15 km. The top of this conductor is considered to be a proxy for the depth to the 500^oC isotherm, therefore, it may be a valuable regional geothermal assessment tool.

Geothermal calculations assuming 500^oC at 10 km depth and estimates of thermal conductivities and heat production yielded a temperature of about 170^oC at the bottom of the basin. A temperature of 200^oC would occur at about 3 km depth within the impermeable basement rock. These estimates indicate direct-use geothermal possibilities within the basin aquifer and high-grade, enhanced geothermal systems (EGS) potential in the basement rocks.