Skip to Main Content
Skip Nav Destination

During 1973 and 1974, precision temperatures (±0.02°C absolute) were measured in four drill holes, ranging in depth from 42 to 122 m, in Clear Lake, Lake County, California. The departure of the measured temperatures from their predrilling values was found to vary considerably among the holes. It is a function of the length of time after drilling that the buoy system supporting the plastic casing survived the storms on the lake, as well as the duration of the drilling disturbance. With one exception, CL-73-7, most of the holes were lost within a month after drilling.

Thermal-conductivity measurements on the sediment cores from the holes were measured using the needle-probe technique. These measurements indicate that the sapropelic muds that underlie most of the main body of the lake and the peat of the southeast, or Highlands Arm, of the lake have thermal conductivities only slightly greater than that of water. Conductivities of the coarse-grained sediments associated with the deltaic deposits of Kelsey Creek are two to three times that of water and are not grouped as tightly as the fine-grained muds and peats.

Heat flows calculated from the above measurements are 1.5 to 1.6 heat-flow units (HFU) in the main basin of Clear Lake and about 2.4 HFU in the Highlands Arm of the lake. These values are considerably lower than expected, based on heat-flow measurements in The Geysers 25 km south of Clear Lake. A correction for an average sedimentation rate in the lake of 0.68 mm/yr would raise the observed heat flows about 13 percent. Although the exact thickness of sediment is unknown, a correction for the refraction effect caused by the thermal conductivity contrast between the low-conductivity lake sediments and the higher conductivity surrounding rock would tend to increase the heat flow at depth in the main basin only slightly for any reasonable thickness of sediment. In the Highlands Arm of the lake, the geometry is different, and the correction could raise the observed heat flux at depth as much as three times that observed, again for reasonable sediment thicknesses. However interesting these numbers are, it should be cautioned that sediment composition and thickness below about 200 m in the lake are unknown, and large differences in conductivity could drastically change these corrections. The possibility of such high heat flows, though, is encouraging enough that future measurements at greater depths should be contemplated.

Aside from the refraction effects there are other possible causes of the low heat flows: (1) Clear Lake is bounded by faults and at least one fault is inferred to pass beneath the main body of the lake—downward cold water movement along such faults could absorb heat and decrease the heat flux; and (2) water may be moving down through the sediments, although generally the permeability of lake sediments is rather low. Depending on the velocity of the movement, a substantial reduction in heat flow could result. The downward movement of water over such a large area (114 km2) could be a source of recharge for The Geysers-Clear Lake geothermal system.

You do not currently have access to this chapter.

Figures & Tables




Citing Books via

Related Book Content
Close Modal

or Create an Account

Close Modal
Close Modal