Given the degree of complexity of modern magnetotelluric (MT) instrumentation, comparison of the total performance for two or more systems is an important verification test. This paper compares the processed data from five MT systems which were designed and constructed separately, and which employ different electrode types, electrode separations, magnetometers, and methods of signal processing. The comparison shows that there is a high degree of agreement among the data from the different systems. The study also demonstrates the compatibility and reliability of the MT systems employed as part of EMSLAB Juan de Fuca (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate). This project, proposed by a consortium of institutions, involves not only magnetotellurics studies but also studies of magnetic variation, on land and on the sea bottom. The project calls for the real‐time MT systems to occupy stations along segments of a profile in Oregon. A composite profile will be created from the segments. Prior to commencing the main MT profiling phase, one week was set aside in August, 1984, for all groups to record and process MT data sequentially at six sites in diverse geologic terrains; this experiment was called mini‐EMSLAB.
We conducted a time-domain airborne electromagnetic (AEM) survey of part of the semiarid Pajarito Plateau of northern New Mexico to determine depths and lateral extent of perched aquifers in the vadose zone and depths and pathways of infiltration to the regional aquifer. The electrical resistivity of the plateau ranged over three orders of magnitude ([Formula: see text] to [Formula: see text]) to a depth of at least [Formula: see text]. Borehole and surface-derived data allow the correlation of resistivity images with the hydrogeology of the plateau. As expected, water exerts a significant control on resistivity. However, the presence of large amounts (up to 90%) of clay in some units, in conjunction with water, also has a significant effect, lowering resistivity (to [Formula: see text]) more than the presence of clay-free saturated zones alone. Because of the resulting low resistivity, we are able to better delineate a large,known volume of clay-altered volcaniclastic rock and postulate the presence of another. Resistivity values of [Formula: see text] cor-relate with depths to saturated zones where no clay is present, but they do not allow us to distinguish between one large or several smaller perched groundwater zones and the underlying regional zone of saturation. We imaged a region of significant infiltration related to a sewage treatment plant and to near-surface hydrogeo-logic conditions conducive to infiltration and correlated with a region of preferential transport of anthropogenic chemicals through the vadose zone. AEM data provide an important synop-tic view of the shallow (few hundred meters) resistivity structure of the plateau. Although interpretation of the data is not unique, when combined with borehole geologic, hydrologic, and geo-chemical data, it can provide relative depths to saturated zones, delineate regions of high clay content (zones of alteration), and image regions of recharge to the regional aquifer.
AbstractStructurally-controlled, meso- and epithermal gold deposition in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and thermochemical gradients. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward (“U”-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island. The deep crustal conductor suggests a volume of fluids arising from prograde metamorphism and radiogenesis within a thickening crust of paleo deep-water clastic rocks. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. Near the brittle-ductile pressure breakthrough are deposited modern hydrothermal veining, gold mineralization, and graphite of deep crustal provenance, subsequently exposed by erosion. In Nevada, Carlin Trend deposits appear to overlie central intrusives of late Eocene age which occupy the transition from conductive paleo-abyssal pelitic sediments (potential gold source rocks) eastward to more resistive shelf carbonate/quartzite sequences along an ancient continental margin normal fault. The intrusive is flanked by conductive, apparent accommodation fault zones which may possess higher porosity as well as possible graphite flushed from sediments near the high-T system core and redeposited in the periphery. Exposed deposits are modeled to form at fluid pressure breakthroughs across permeability barriers such as organic shales or thrust planes, producing strong and favorable gradients in temperature, pressure and fluid oxidation.KeywordsGold depositsore fluidscrustal controlselectrical resistivitymagnetotellurics
A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.
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