HR: 0830h
AN: T41D-0258    [PDF]
TI: Anisotropic schist foliation orientation determined using time domain electromagnetics
AU: * Collins, J L
EM: jamiec@tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics Texas A&M University, College Station, TX 77843-3115 United States
AB: Remote detection of metamorphic rock formations beneath sedimentary cover using geophysical methods would greatly assist efforts of geologists to study regional tectonics. Metamorphic rocks are often characterized by foliations oriented in a preferred direction. The foliations may generate anisotropic physical properties. Electromagnetic methods can be used to detect anisotropy in electrical conductivity at depth within the earth. Around 1.1 billion years ago the Llano uplift exposed Precambrian igneous and metamorphic rocks in central Texas. In centrally located Mason, Texas the uncovered Packsaddle Schist maintains the characteristic preferred foliation orientation of metamorphic rocks, providing an ideal survey region. Time domain electromagnetic surveys with a 20 - 40 meter offset loop configuration were conducted azimuthally in the Packsaddle Schist. A transmitter loop with a five meter radius was centrally located. Voltage induced within a receiver loop was recorded over time and plotted on 360 degree polar azimuthal graphs. The graphs consistently show elliptical voltage responses over time with larger voltage readings trending northwest-southeast at early time. Larger voltage readings within the elliptical response correspond to slower decay of the induced voltage and lower apparent conductivity. Further interpretation of electromagnetic responses in anisotropic media involves consideration of the paradox of anisotropy. A paradox of anisotropy occurs when apparent conductivity is larger across strike than along strike due to local induced electric current flow control of the response. Accordingly, lower apparent conductivity values correlate with the along strike direction. Therefore, the graphs generated by electromagnetics show the orientation of the Packsaddle Schist foliation trends northwest-southeast. Additional DC resistivity surveys of the same region show higher apparent resistivity oriented more subtly northwest-southeast. Under the paradox of anisotropy, higher apparent resistivity, or lower apparent conductivity, correlates with the along strike direction of the foliation. DC resistivity supports the same preferred foliation orientation found based on time domain electromagnetics. Utilizing time domain electromagnetics with an understanding of the paradox of anisotropy provides an effective and noninvasive means to detect the foliations characteristic of metamorphic rocks and determine the preferred orientation of the foliations.
DE: 0600 ELECTROMAGNETICS
DE: 0684 Transient and time domain
DE: 0905 Continental structures (8109, 8110)
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2003 Fall Meeting