Geological mapping in western New Hampshire has delineated bodies of orthogneiss lying in structural basins of quartzite and schist. The object of the present investigation was to test the utility of seismic reflection methods for determining the depths of the lower contact of the gneiss at various points and to correlate these data with deductions made from geological observations at the surface. In one of the basins the seismic investigations were confined to one locality. In a second basin four localities were studied.
The surface velocity of the seismic waves in the orthogneiss varied from 11,600 feet per second to 25,250 feet per second, and at one locality it was possible to calculate from the reflected waves an effective average velocity of 17,000 feet per second. Reflections were obtained at all localities, except for one place where the records were illegible. In the basin where the studies were confined to one locality the discrepancy between the geological and seismic data was 20 per cent. In the second basin the average discrepancy was approximately 15 per cent.
The complete mapping of such a basin by seismic means would be difficult because of the nature of the terrain, which impedes the transportation of the equipment. The variation in the velocity of transmission of seismic surface waves introduces a factor of uncertainty. Moreover, the thickness of the glacial drift in many places is a complicating factor.