No single factor is more important to the successful operation of a ground-coupled heat pump system than the efficiency of the heat transfer between the ground loop and the surrounding soil.
Soil thermal properties vary by a factor of ten and the thermal performance of rocks also varies widely. These variations indicate the importance of an accurate estimate before any geothermal loop design can be finalized.
Your contractor might consult a helpful source like the Soil Conservation Service Soil Survey. This is available from local SCS offices located in approximately 3,000 counties across the United States. It gives a detailed layer-by-layer description of the soil down to a five- or six-foot depth, along with any rock content, density, soil types and available water capacity.