A hole is drilled below the depth of the annual in-ground temperature cycle. A small pipe filled with water is place in the hole and the hole is backfilled. After stabilization, a calibrated temperature probe is run inside the pipe and the temperature as a function of depth is measured.
A heat source, either convection along a fault, conduction from a deep-seated source through hard rocks to the surface, or a subsurface steam pocket is to be detected.
Permits to drill, complete and abandon the holes as well as a rig capable of dealing with the highly variable subsurface conditions are required. The annual cycle often penetrates to 30 meters so a popular depth for this type of drilling is 60 meters.
Calibration of the temperature probe, possibly at high temperatures (150-200C), can be a problem. Water inflow, drilling problems, and backfill stabilization can cause mediocre results. The thermal conductivity of the materials penetrated should be monitored as misleading interpretations can result from the assumption of homogeneity. Convection is prevalent in geologic materials.
After drilling, the crew size is one person. The equipment is portable but access is generally not a problem after a sizable rig has been to the site. To verify stabilization the measurements may be made several times: after drilling, 24 hours later, and 72 hours later.
Profiles of temperature vs. depth for mapped boreholes. If thermal conductivity is measured, heat flow may be estimated. If sufficient holes are drilled, the data may be contoured and interpreted in terms of the mapped geologic structure and the location and quality of the heat source.