Controlled-source audiomagnetotellurics in geothermal exploration. Topical report

Technical Report · Mon Sep 01 04:00:00 EDT 1980

DOI:https://doi.org/10.2172/6749290· OSTI ID:6749290

Sandberg, S K; Hohmann, G W

Theoretical and field tests indicate that the controlled-source audiomagnetotelluric (CSAMT) method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system. Utilizing a transmitter overcomes the main limitation of conventional AMT: variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. Calculations for a half-space show that the plane wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration. Three dimensional numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane wave source, showing that the method is valid. Comparisons between 2D and 3D model results show that a 2D MT program can be used to interpret CSAMT data. A CMAST survey at the Roosevelt Hot Springs KGRA produced apparent resistivity contour maps at four frequecies: 32, 98, 977, and 5208 Hz. These maps show the same features as a first-separation dipole-dipole resistivity map. Detailed CSAMT data was also collected at 10 frequencies on two profiles. Two-dimensional MT modeling (TM mode) of the resulting pseudosections yields models similar to those derived by modeling the dipole-dipole resistivity data. However, CSAMT resolved details not shown by the resistivity modeling.

• Research Organization: Utah Univ., Salt Lake City (USA). Dept. of Geology and Geophysics
• DOE Contract Number: AC07-79ID12079
• OSTI ID: 6749290
• Report Number(s): DOE/ID/12079-5
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
ELECTRICAL SURVEYS
ELECTROMAGNETIC SURVEYS
FIELD TESTS
GEOPHYSICAL SURVEYS
GEOTHERMAL SYSTEMS
Geothermal Legacy
HYDROTHERMAL SYSTEMS
KGRA
MAGNETOTELLURIC SURVEYS
MAPS
MATHEMATICAL MODELS
MEASURING METHODS
NORTH AMERICA
ROCKY MOUNTAIN REGION
ROOSEVELT HOT SPRINGS
SURVEYS
TESTING
THREE-DIMENSIONAL CALCULATIONS
TWO-DIMENSIONAL CALCULATIONS
USA
UTAH
VALIDATION