We present a technique for placing a borehole into thermal dis-equilibrium, and thereby interpreting groundwater flow through fractures where it may have been previously undetected. Denoted as Active Line Source (ALS) logging, the method consists of temperature logging while a borehole is heated by the cable and/or during cooling after the heating. With two or more logs collected during either heating or cooling, an estimate of thermal conductivity is obtained. The basic theory, widely used for such things as thermal conductivity probes, is shown to fit the recorded data well. The mechanics of ALS logging are described, and the practical challenges are outlined.

In the absence of groundwater flow in or around the borehole, variations in the thermal conductivity of the rock are largely due to variable water content and the ALS log provides a reasonable surrogate for the neutron log. When groundwater flow dominates the dissipation of thermal energy from the borehole, however, the apparent thermal conductivity is increased. In open boreholes this flow can be both ambient (within the formation itself) and connecting (vertical flow between fractures intersected by the borehole). In cased or lined holes with no connecting flow, ALS logs are particularly useful as detectors of ambient groundwater flow.

Alternative methods for flow detection, such as chemical dilution or flow-meters, require an open borehole and either have poor vertical resolution or require multiple stationary measurements, often with packers to minimize the effects of connecting flow. The ALS technique is a comparatively simple tool, useful in both open and cased or lined boreholes, run continuously down the length of the borehole, with fracture resolution on the order of a few centimeters.

We describe ALS logging of a 75-meter section of a borehole through fractured dolomite which has been lined with a FLUTe sleeve. The ALS results are compared to the geologic units encountered, conventional geophysical logging techniques, time-lapse passive temperature logging, heat pulse flowmeter data and packer testing.

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