Electrical anisotropy from electromagnetic array data: implications for the conduction mechanism and for distortion at long periods

Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

“…7). This is basically consistent with the reported values of *3 to [20 in the lower continental crust, especially in recent years (Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002), considering the uncertainty in MT and GDS field surveys and the technical problems in resolving electrical conductivity and layer thickness of conductive zones as pointed out above. In one of the earliest studies by Cull (1985), an anisotropy factor of 400 was reported for the lower crust (at depths of 15-35 km) beneath the Australia shield, which was probably caused by serious technical problems in both the measurement and inversion modeling involved in this very early work.…”

“…1; Hyndman and Shearer 1989); (5) although the spatial resolution of electrical conductivity, especially the vertical depth mapping, needs improving, some scholars have suggested an apparent correlation between high electrical conductivity and seismic reflectivity in some regions (Jones 1987;Hyndman and Shearer 1989;Marquis and Hyndman 1992;Hyndman et al 1993;Warner 2004); and (6) electrical anisotropy along horizontal directions has been reported in many regions, with anisotropy factors ranging from *3 to more than 20 in some areas (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002).…”

“…Technically, it is much more difficult to resolve electrical anisotropy than electrical depth profile in the lower continental crust with MT, GDS, or other electromagnetic techniques (Bahr et al 2000). Probably, this is one of the main reasons that relatively fewer field surveys for electrical anisotropy than for electrical depth structure have been carried out.…”

“…Probably, this is one of the main reasons that relatively fewer field surveys for electrical anisotropy than for electrical depth structure have been carried out. It is not known whether electrical anisotropy in the lower continental crust appears on a global scale; however, the few reports available do show horizontal electrical anisotropy beneath several places in conjunction with high electrical conductivity up to 0.01-0.1 S/m along certain directions, e.g., at Australia shield, Fennoscandian (Baltic) shield, Saxothuringia in central Germany, and Rhenish shield in western Germany (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002).…”

“…However, there are still some questions that remain poorly constrained, and one of the most significant concerns electrical properties. It has been recognized for more than *30 years that the lower continental crust is globally characterized by a remarkably high electrical conductivity, e.g., up to 0.01-0.1 S/m, with a magnitude that can vary dramatically from one region to another (Shankland and Ander 1983;Haak and Hutton 1986;Jones 1992;Korja and Hjelt 1993) and whose distribution is probably anisotropic (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002). Nevertheless, the possible origin of these electrical anomalies is a long-standing and controversial issue (e.g., Gough 1986;Jones 1992;Hyndman et al 1993;Glover 1996;Yardley and Valley 1997).…”