| Summary: | Depth Image Processing (D IP) of transient electromagnetic data collected over layered structures such as sedimentary basins has proven to be an effective and cost efficient method of imaging the electrical conductivity variation in two dimensions. Data collected over the Kapuskasing Structural Zone (KSZ) of the Ardieun Superior Province in central Canada were D IP processed in an attempt to determine the structure associated with the resistive upper crust in the region. Two conductive horizons beneath the overburden are imaged a t 2 and 5 km depth with conductance of approximately 0.05 S and 0.15 S respectively. An increase in conductivity at mid-crustal depths is also observed. The shallower horizon is truncated in the vicinity of the Ivanhoe Lake Cataclastic Zone (ILCZ). Both conductive horizons are interpreted as saline fluids in a connected porosity. The saline waters are most likely a mixture of fluids with m any origins. The fluids m ay be genetically related to such phenomena as prograde metamorphic reactions during the creation of the extensive gneiss domains in the region as observed in the deep well of the Kola Peninsula, the uplift of the KSZ, infiltration from nearby sedimentary basins or meteoric influx. The conductive zones show some correlation with seismic reflections interpreted as “ramp and flat” style thrust planes. This correlation suggests the fluid-filled porosity m ay be controlled by structures associated with the uplift of the KSZ. The interruption in the continuity of the 5 km deep layer is possibly due to a high-angle fault, unseen by seismic methods, penetrating to m id-crustal depths in the region of the ILCZ. The data collected across the KSZ were strongly contaminated by local galvanic effects associated with an inhomogeneous overburden covering the region. To approximate the contaminated data, synthetic channeling responses, generated using inhomogeneous thin sheet algorithm s, were super imposed on a layered earth response. The layered earth response used was that indicated by the Depth Imaging and an inversion of the data to a layered earth structure. The synthetic responses were Depth Image Processed using single- and multi-fold geometries to investigate the reliability of the conductivity estimation from data strongly distorted by galvanic effects. The results of the modelling show the image of the layered earth structure may be distorted due to lateral variations in the conductance within the overburden that are com parable to cumulative conductance of the layered earth model. Although the drastic change in conductivity between the overburden and the gneiss (10-2 to 10-,6 S /m ) cause a certain amount of ringing in the numerical differentiation to derive the apparent conductivities, significant artifacts are not introduced into the imaged conductivities. These results demonstrate the imaged conductive horizons under the KSZ are the result of inductive effects in conductive layers at 2 and 5 km depth. Ph.D.
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