CHARACTERIZATION OF SUBSURFACE ARCHITECTURE OF HUMMOCKY TERRAIN USING ELECTRICAL RESISTIVITY IMAGING

Hummocky terrain genesis still is unresolved. Some researchers continue to map hummocky terrain as stagnation features formed near ice-margins, but increasing evidence points towards a subglacial origin. That is to say, hummocks are not always composed of slumped glacial sediments but rather, may be composed of a variety of materials some of which were glaciotectonically modified. One of the barriers to better understanding how these hummocky landforms were formed is the lack of 3D subsurface information. The majority of studies undertaken on hummocky terrain have been based on 2D outcrops. Since geological structures by nature are 3D, a 3D geophysical survey will provide useful information about the subsurface sediment architecture.

Electrical resistivity is a function of porosity, saturation, resistivity of the pore fluids and the solid phase, and the material texture. Resistivity data was collected using the Sting-Swift R1 IP resistivity meter. As the project focused on 3D architecture, a hybrid 3D ERI array with a sparse net of parallel and perpendicular 2D lines was utilized. The net reduces the number of measurements required without seriously degrading the quality of the model. This format allowed a single hummock to be surveyed in only two days. This layout utilizes the ease and speed of 2D surveys while producing 3D images. Elevations for each electrode position were used to correct topographic distortion in the inversion process.

The hybrid 3D ERI array was successful in imaging the subsurface architecture of hummocky terrain. Tills, fluvial and lacustrine sediments, bedrock, and structural features such as faults, exhibit large contrasts in their physical properties. Therefore, electrical resistivity is well suited to resolving architectural features in hummocky landforms. Electrical resistivity imaging when integrated with traditional techniques provides enhanced insight for inferring the processes of sediment emplacement. Preliminary results indicate that the sediments that make up the hummocky landforms were not emplaced by supraglacial meltout as with traditional models, but underwent subglacial emplacement and deformation. This change is significant in that it casts doubt on deglacial reconstructions that rely on traditional models for hummock formation.