Empirical Analysis of Borehole Temperatures Located

throughout California specifically relating to heat flow.

Stephen Gronow, University of Akron – Ohio

1. ABSTRACT

The National Geothermal Data System, specifically California borehole temperatures, was used to first calculate specific heat and then analyzed to find patterns between heat flow and geologic location. Heat flow data were grouped in three categories: Low (Q<=0.02 W/m2), Medium (.1 W/m2 >Q<.02 W/m2), and high (Q=>.1 W/m2). Boreholes were then plotted using longitude and latitude on a map of California displaying one of the three heat flow categories. Data was inspected through visualization. Regions closer to coast of California had a high value for heat flow while values located centrally had low heat flow values. The difference in the heat flow between the geologic positions was hypothesized to be the result of distance from the subduction of the Farallon Plate. More specifically, the high heat flow region lies on an outer arc while the low heat flow values lie on a fore arc basin.

2. INTRODUCTION

The boreholes in California vary by many parameters, including but not limited to: depth drilled, geographic location, and borehole temperature. There are about 12,000 borehole locations in California. Depending on heat flow, some borehole locations present optimum sites for geothermal energy. With the growing demand for clean energy sources, it important to assess if there are any geological processes that can be correlated to higher valued heat flow locations. Understanding the relationship between heat flow and geologic location is essential for determining cost efficient sites for geothermal energy plants.

3. METHODS

Parameters from the National Geothermal Data System were merged and imported to a spreadsheet for analysis. Data were manipulated in order to solve for heat flow using Fourier equation (Q = -K (ΔT/ΔD). Note that an average of 2.5 W/mK was used for the thermal conductivity. Heat Flow data were organized into three groups: Low (Q<=0.02 W/m2), Medium (.1 W/m2 >Q<.02 W/m2), and high (Q=>.1 W/m2). Heat flow data points were mapped onto California via Longitudes and Latitudes. Data was analyzed by visual inspection.

4. DATA DESCRIPTION

When heat flow was mapped via longitude and latitude on California (Figure 1), patterns emerge. There are prominent regions with high spatial densities of low and high heat flow values that are located spatially close (observe red circle Figure 1). Also shown are each heat flow group graphed individually (Figure 2, Figure 3, Figure 4).

5. INTERPRETATION

The region with high localized heat flow values (Figure 1) is interpreted to have bed rock made up of volcanic rock due to the Franciscan subduction process. (Bartow and Nilsen 1990) The region with low localized heat flow values (Figure 1) is interpreted to have layers of marine sediments due to the Great Valley sequence. (Bartow and Nilsen 1990)

6. DISCUSSION

The region with high heat flow values is geologically located on an outer arc. Specifically, the region was made by the Franciscan subduction process. Due to its location near a subduction complex, the heat flow temperatures are significantly higher here. (Bartow and Nilsen 1990) On the other hand, the region located geologically within the fore arc basin is known as the Great Valley. Due to the basin being further away from the subduction zone the heat flow temps are cooler.(Bartow and Nilsen 1990) Consequently, it would be more efficient to put a geothermal energy plant on an outer arc (See Figure 5 for general picture of subduction zone)

7. CONCLUSION

The purpose of this lab was to better understand the geothermal energy of different geologic locations within California based on heat flow. It was found that the region that had high heat flow values were geologically located on an outer arc. Adversely, the region with low heat values was geologically located on a fore arc basin. Based on the findings, a possible location to build a geothermal energy plant in California would be on the outer arc, specifically near Santa Rosa.

8. CITATIONS

Bartow, J. and T. Nilsen. 1990. Review of the Great Valley sequence, eastern Diablo Range and northern San Joaquin Valley, central California. Department of the Interior U.S. Geological Survey: 1-25.

William P. Elder, Geology of the Golden Gate Headlands, National Park Service, Golden Gate National Recreation Area, from, Stoffer, P.W., and Gordon, L.C., eds., 2001, Geology and Natural History of the San Francisco Bay Area: A Field-Trip Guidebook, U. S. Geological Survey Bulletin 2188, p. 61-86.

APPENDIX A

Figure 1. Borehole locations in California classified by heat flow, notice region encircled by red boundary.

Figure 2. Borehole locations in California regarding high heat flow, notice location of high values are along the west coast.

Figure 3. Borehole locations in California regarding high medium flow, notice medium values are located near the west coast.

Figure 4. Borehole locations in California regarding low heat flow, notice low values are located in the Great Valley.

Figure 5. Great Valley Sequence presented by Geocaching.com taken from William Elder.