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The pole-dipole (PD) electrical resistivity array is used as part of a geophysical technique, which can be used in exploration for mineral, geothermal and hydrological resources. Furthermore, it can be used in archaeological investigations. The PD array is useful in obtaining large depths of investigation, but the array requires one pole to be planted at a greater distance from the other electrodes. Placing the infinite pole is time consuming and costly, especially in rough terrains. It can also be a liability in populated areas. Previous data inversion software assumes the infinite pole to be planted at infinity. Therefore, the field data collected has to mimic the assumptions of the inversion software. Some recent inversion programs use all 4-electrode positions for data inversion. In this case, is a good approximation of infinity for the infinite pole still necessary? Is the depth of investigation still the same or relevant with a non-infinite, infinite pole? Is the resolution of the cross-sections retained? To answer the above questions a Generalized Electrode Array (GEA) 1 dimensional (1D) inversion program is used. Different sets of field data were collected with non-infinite pole (NIP) PD arrays; these arrays are set up as PD arrays with an infinite pole planted relatively close to or on the survey lines itself. A COMSOL finite element model were modelled with varying infinite pole distances to identify the distance required for the retention of the depth of investigation and resolution of the PD array. Modelling from GEA shows the NIP distances do not affect the 1D inversion accuracy of the resistivity nor the layer thickness. The field data indicates that it is necessary to have an infinite pole, but the distance to the infinite pole can be substantially less than 10 times the array length, which is the usual rule of thumb for infinite pole placement. With 3 dimensional (3D) COMSOL modelling results, it indicates a minimum pole distance to be 2.5-array length to retain the depth of investigation and precision of the inverted sections of the PD array.

Pointe du Hoc overlooking the English Channel in Normandy, France was host to one of the most important military engagements of World War II. While the site is a valuable historic cultural resource, it is vulnerable to cliff collapses that already have endangered the observation post and Rudder?s command post. The observation post has been closed to visitors for some time due to safety concerns. Geophysical techniques have been used increasingly in recent years for slope stability investigation purposes. The objective of this study is to apply advanced 3?D resistivity tomography toward a detailed site stability assessment with special attention to the two at-risk buildings. 3?D resistivity tomography datasets at Pointe du Hoc in the presence of extreme topography and dense cultural clutter have been successfully acquired, inverted, and interpreted. The cliff stability in the areas around the two at?risk buildings has been analyzed. A hazard assessment scheme has been designed in which regions of high resistivity are interpreted as zones of open, dry fractures with a moderate mass movement potential. Regions of low resistivity are zones of wet, clay?filled fractures with a high mass movement potential. The observation post tomography results indicate that the highest mass movement hazard appears to be associated with the marine caverns at the base of the cliff that are positioned at the point of strongest wave attack. These caverns likely occupy the future site of development of a sea arch which will definitely threaten the observation post building. A high probability of a soil wedge failure is on the east?facing cliff edge close to the observation post that could damage or destroy the building. The mass movement potential at the Rudder?s command post area is low to moderate. The greatest risk is associated with soil wedge failures at the top of the cliffs. The resistivity geophysical data add great value to the natural geohazard assessment at Pointe du Hoc and constitute an integral component of an interdisciplinary approach to the problem of cultural resource preservation at the site. Geophysics is a non?invasive and relatively inexpensive technology that provides unique constraints which are unobtainable using traditional engineering geology methods for site characterization. However, the technology is difficult to master and the inherent limitations must be carefully understood to ensure a reliable geotechnical interpretation.

Azimuthal resistivity was performed at the King site, near Birsay, Saskatchewan to characterize the orientation and extent of fracturing in a glacial till. The target consisted of shallow (less than 4m deep) fractures in the upper oxidized portion of the Battleford Formation.<p> The fractures were visible in soil cores, but their orientation and extent were not known at the time of investigation. It was hoped that if the azimuthal resistivity method could be successfully applied at this site it could be used as an in situ fracture mapping tool at other sites. <p> Preliminary azimuthal resistivity surveys failed to detect a consistent anisotropic pattern that could be attributed to fracturing. A refined method of azimuthal resistivity was developed that built a 3D resistivity model of the site assuming a heterogeneous, isotropic earth. This model was used to predict and remove the effect of heterogeneity on the azimuthal resitivity observations. The results from the refined method also failed to detect a consistent anisotropic pattern. <p>Any single azimuthal resistivity observation from either the preliminary or refined surveys would have provided data that could have been interpreted as anisotropy due to fractures. It was only by comparing many azimuthal observations across the site that the lack of consistent azimuthal pattern became apparent. <p>It is recomended that an analysis of several observations be made before any interpretation of anisotropy is made for azimuthal resistivity sureveys in general.

Azimuthal resistivity was performed at the King site, near Birsay, Saskatchewan to characterize the orientation and extent of fracturing in a glacial till. The target consisted of shallow (less than 4m deep) fractures in the upper oxidized portion of the Battleford Formation.<p> The fractures were visible in soil cores, but their orientation and extent were not known at the time of investigation. It was hoped that if the azimuthal resistivity method could be successfully applied at this site it could be used as an in situ fracture mapping tool at other sites. <p> Preliminary azimuthal resistivity surveys failed to detect a consistent anisotropic pattern that could be attributed to fracturing. A refined method of azimuthal resistivity was developed that built a 3D resistivity model of the site assuming a heterogeneous, isotropic earth. This model was used to predict and remove the effect of heterogeneity on the azimuthal resitivity observations. The results from the refined method also failed to detect a consistent anisotropic pattern. <p>Any single azimuthal resistivity observation from either the preliminary or refined surveys would have provided data that could have been interpreted as anisotropy due to fractures. It was only by comparing many azimuthal observations across the site that the lack of consistent azimuthal pattern became apparent. <p>It is recomended that an analysis of several observations be made before any interpretation of anisotropy is made for azimuthal resistivity sureveys in general.

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As part of the southern Ontario POLARIS project, this thesis uses magnetotelluric methods to investigate the lithospheric architecture of the Proterozoic Grenville Province and its margin with the Archean Superior Province. The first multi-dimensional crustal and lithospheric resistivity images for this region are presented. The resistivity structure of the Phanerozoic sedimentary rocks in the lower Great Lakes region was determined using 1-D methods. The responses are strongly affected by a 20-23 S conductive layer within the sedimentary rocks, interpreted to be associated with Upper Ordovician shale units. This layer excludes resolution of resistivity structure of underlying crust. The resistivity structure of the Precambrian crust and lithosphere was determined using 2-D methods. Different strike azimuths were determined for the crust, the upper lithospheric mantle and the deeper mantle layer. The crustal resistivity model for a profile from 50oN79oW to 43oN76oW images resistive Laurentian margin rocks dipping southeast to the base of the crust, bounded by the Grenville Front and the Central Metasedimentary Belt Boundary Zone. In a 2-D model of the mantle lithosphere for the same profile, a conductor at 70-150 km depth, located along-strike from the Mesozoic Kirkland Lake and Cobalt kimberlite fields, is interpreted to be due to mantle re-fertilization. Results from multiple MT profiles indicate conductive (<10 Ω.m) lithospheric mantle beneath the Central Metasedimentary Belt and show that the northwestern Grenville Province is characterized by large-scale, resistive lithosphere (>10,000 Ω.m) extending for about 300 km beneath the Grenville Province and 800 km along strike. Lithospheric thickness is interpreted to be 280 km; local decreases in this depth are attributed to refertilization of the lower mantle lithosphere by fluids associated with Cretaceous kimberlite magmatism. Anisotropic 2-D modeling reveals minimal electrical anisotropy (<10%) at mantle depths in contrast to the factor of 15 anisotropy determined in earlier 1-D studies. This result suggests that observed MT response anisotropy is caused by large-scale structures. Strike direction in the upper lithospheric mantle is interpreted to be related to the Archean fabric of the Superior craton and in the deeper, conductive, mantle it is interpreted to have been established in the Cretaceous.

Historically pore pressure evaluation in exploration areas was based on empirical relationships between drilling parameters, wireline logs and the mud weight. Examples include Eaton's Ratio and the Hottman & Johnson Methods, which were based on data from the Gulf of Mexico. These methods are not readily transported to other areas, such as the North Sea Basin, where the sediments are different in character and where burial and temperature histories are distinctly different. Data from several offshore North Sea wells, with high quality wireline and associated data have been analysed to determine the most appropriate method to estimate pore pressure in mudrocks. The data have led to an understanding of the key parameters for successful pore pressure estimation. The most effective method is shown to be the Equivalent Depth Method, but only where disequilibrium compaction is the source of the overpressure in the mudrocks. Core samples from 576 British Geological Survey sites in the offshore area of the British Islands were compared with > 10,000 porosities collected from the deep oceans (DSDP/ODP sites), which show that the porosities in the shallow section in the North Sea are anomalously low. The shallow section of the North Sea includes large volumes of Pleistocene-Recent sediments deposited as glacial and inter-glacial deposits. Frequency analysis (Cyclolog) of the wireline data covering this interval in several North Sea wells revealed a pattern in the relative featureless original data. Comparison with the global signature for oxygen isotopes for the same time period suggests that there have been ten cycles of ice sheet build up (Glacial period) followed by melting (Interglacial period) during the last one million years. Glacial deposits from 10 individual glacial cycles have therefore been identified in several exploration wells in the North Sea. Implications of loading/unloading of ice for the migration and trapping of hydrocarbons in the North Sea Basin are assessed.