For the conventional needle probe, the standard mode (line source in an infinite medium) is achieved simply by inserting the probe into unconsolidated material or into a hole drilled in harder material.A half space mode is achieved by embedding the needle probe in an epoxy material of low thermal conductivity (<0.2 Wm 1 K x ) and grinding the material away until the needle is flush with a flat surface. The sample to be measured is placed on the half space surface, and the conductivity is measured in the usual way. Either configuration is capable of yielding values of thermal conductivity of consolidated rocks comparable in accuracy to values obtained with the steadystate, divided-bar technique most commonly used for these rocks. The linesource techniques are of particular advantage for materials that prove difficult to machine into the cylindrical disk specimens required for the steady-state apparatus. The half space probe is of additional value for rocks that are too friable to machine, but too hard to allow the drilling of the long, small diameter (38 x 1 mm) holes required for the standard application of the needle probe.

Subsurface t h e d wasur-ts are the most effective, least ambiguous tools for identifying and delineating possible geothermal resources. M e a s m m t s of thermal gradient in the upper few tens of meters generally are *sufficient .to outline the major anomalies, but it is always desirable to combine these gradients with reliable estimates. of the-conductivity to provide data on* the'energy flux and to constrain mdels for the heat s m c e s responsible for the observed, near-thermal anomalies. The major problems associated with heat-flaw measurements in the geothermal exploration mode are concerned with the economics of casing and/or grouting holes, the repeated site visits necessary to obtain equilibrium temperature values, the possible legal liability associated with 'the disturbance of underground aquifers, the surface hazards presented by pipes protruding fnrm the ground, and the security problems associated with leaving cased holes open for periods of weeks to months. We have developed a technique which provides reliable llreal-time** , determinations of temperature, thermal conductivity, and hence, of heat unconsolidated sediments. A t h e d conduct-1 say, 50 to 150 meters provide a high-quality heat-flow determination at costs comparable to those associated with a standard cased "gradient hole" to COmpaTable depths. The hole can be backfilled and abandoned upon cessation of drilling, thereby eliminating the need for casing, grouting, or repeated site visits.

Subsurface thermal measurements are the most effective, least ambiguous tools for locating geothermal resources. Measurements of thermal gradient in the upper few tens of meters can delineate the major anomalies, but it is also desirable to combine these gradients with reliable estimates of thermal conductivity, to provide data on the energy flux and to constrain models of the heat sources responsible for the anomalies. Problems associated with such heat flow measurements include the economics of casing or grouting holes, the long waits and repeated visits necessary to obtain equilibrium temperature values, the possible legal liability arising from disturbance of aquifers, the hazards presented by pipes protruding from the ground, and the security problems associated with leaving cased holes open for periods of weeks to months.

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