Heat flow transducer for use in measuring the geothermal heat flow in the earth. The transducer utilizes copper alloy flexible wire bundles at opposite ends of an elongated body to conduct heat from the strata surrounding a bore hole, into the body of the transducer, and through thermally conductive elements, into contact with a solid state thermopile. The thermopile includes a plurality of semi-conductor elements, electrically in series and thermally in parallel, to produce a usable electrical output at very low heat flows. Multiple modules of semi-conductor elements are employed. The modules are separated by a heat conductive, but electrically insulated layer between modules.

A method for measuring the thermal conductivity of well casing in situ. The instrument consists of a probe several feet in length lowered into a well and decentralized so that it maintains contact with the casing wall. Two temperature sensors situated adjacent opposite ends of the probe are thermally insulated from an electrically heated intervening portion of the probe body. The probe is moved from one depth to another at a constant predetermined velocity, and the sensors continuously measure the temperature of the casing wall before and after passage of the heated probe portion thereover. The effective thermal conductivity of the casing is directly proportional to the flux applied by the heater and inversely proportional to the temperature change as the heater passes a point along the casing.

A method and apparatus for determining vertical heat flux of a geothermal field, and mapping the entire field, is based upon an elongated heat-flux transducer (10) comprised of a length of tubing (12) of relatively low thermal conductivity with a thermopile (20) inside for measuring the thermal gradient between the ends of the transducer after it has been positioned in a borehole for a period sufficient for the tube to reach thermal equilibrium. The transducer is thermally coupled to the surrounding earth by a fluid annulus, preferably water or mud. A second transducer comprised of a length of tubing of relatively high thermal conductivity is used for a second thermal gradient measurement. The ratio of the first measurement to the second is then used to determine the earth's thermal conductivity, k.sub..infin., from a precalculated graph, and using the value of thermal conductivity thus determined, then determining the vertical earth temperature gradient, b, from predetermined steady state heat balance equations which relate the undisturbed vertical earth temperature distributions at some distance from the borehole and earth thermal conductivity to the temperature gradients in the transducers and their thermal conductivity. The product of the earth's thermal conductivity, k.sub..infin., and the earth's undisturbed vertical temperature gradient, b, then determines the earth's vertical heat flux. The process can be repeated many times for boreholes of a geothermal field to map vertical heat flux.

A method for determining the thermal conductivity of the earth in situ is based upon a cylindrical probe (10) having a thermopile (16) for measuring the temperature gradient between sets of thermocouple junctions (18 and 20) of the probe after it has been positioned in a borehole and has reached thermal equilibrium with its surroundings, and having means (14) for heating one set of thermocouple junctions (20) of the probe at a constant rate while the temperature gradient of the probe is recorded as a rise in temperature over several hours (more than about 3 hours). A fluid annulus thermally couples the probe to the surrounding earth. The recorded temperature curves are related to the earth's thermal conductivity, k.sub..infin., and to the thermal capacity per unit volume, (.gamma.c.sub.p).sub..infin., by comparison with calculated curves using estimates of k.sub..infin. and (.gamma.c.sub.p).sub..infin. in an equation which relates these parameters to a rise in the earth's temperature for a known and constant heating rate.

A geothermal heat flux transducer includes a plurality of insulating slats on which constantin wire is wound and each half turn is coated with a silver layer to produce a hot junction and a cold junction per turn of wire. The slat with wire thus coated is assembled with insulating material between adjacent slats such that silver coatings on adjacent slats face each other and also uncoated wire half turns on adjacent slats face each other. This allows a multiplicity of thermal junctions thus formed to be assembled in a small package which may be suspended in earth bore holes in mine shafts or otherwise below or above ground for measurement of heat flow emanating from sources far below the earth's surface. Variable heat flow occasioned by diurnal and/or seasonal variations are accounted for in ascertainment of that steady heat flux flowing from geothermal sources alone. The transducer is of special dimensional proportions with parts specially related both structurally and thermally.