POTENTIAL APPLICATIONS OF GROUND-PENETRATING RADAR PROFILES TO SURFICIAL AND BEDROCK GEOLOGY MAPPING IN NEW HAMPSHIRE

New Hampshire has a rich natural and glacial history resulting in complex surficial and bedrock geology. Much of the state has been successfully mapped through the federally funded STATEMAP program. However, due to vegetation cover, challenging terrain, and many geological complexities, some ambiguities, simplifications, and unknowns exist within New Hampshire geology interpretations. Here we explore the potential of ground-penetrating radar (GPR) as a tool for aiding surficial and bedrock geology mapping efforts in New Hampshire and we provide case studies in support of these efforts.

We have collected ~200 km of GPR data with center frequencies between 80-400 MHz within the Upper Connecticut River Valley and regions surrounding the Presidential and Franconia Mountain Ranges of New Hampshire. Profiles were collected on class II-VI roads, hiking trails, and off-trail, during the months of February-September 2012. Lower frequency profiles revealed maximum penetration depths over 30 m with greater penetration over solid bedrock and shallower penetration generally occurring over till. Attenuation rates vary considerably depending on water content, till cover, and bedrock type. However, the wet till sequences are generally the most lossy with meta-volcanic bedrock also exhibiting higher attenuation rates than granites. Relative permittivity contrasts between till, granite, and fractures within granite allow for easy delineation of till over bedrock (aiding estimates of till volume) and constituents within granite fractures (i.e. water or air). In some instances, a contrast in reflection characteristics between bedrock type is evident (e.g. granite versus metamorphic-volcanic bedrock) allowing the potential sub-surface delineation of bedrock type. These results suggest that GPR may be useful for delineating some rock types at depth, till depth and associated till volume, aquifer dimensions, and water volume based on till-bedrock geometry and calculated relative permittivity values.