Satellite Sensing of Geothermal Activity Literature Review

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Some of these factors enhance the ability to detect geothermal activity in an area, while others impede it. The presence of these factors must be taken into consideration as they relate to the specific area being surveyed. A complete study of these factors and their potential affects on the ability to use remote sensing techniques will have to be explored in order to understand how they affect the use of satellite for remote sensing in Malawi.

The use of thermal infrared (TIR) data from ASTER spaceborne instrumentation was used successfully to detect surface temperature anomalies in the Coso geothermal field in eastern California (Eneva, Coolbaugh, & Bjornstad et al., 2007, pp. 335). One of the key difficulties that was found in the use of TIR data from ASTER is that thermal inertia from different types of vegetative matter can make true geothermal anomalies difficult to identify using spaceborne data alone (Dudley-Murphy & Nash, 2003, p. 645). A study of the thermal inertia of the plant material in Malawi would have to be undertaken so that its affects could be subtracted out of the data in order to detect geothermal anomalies in the area to be surveyed.

Items Affecting Signal Strength and Clarity

As with any spectroscopy technique, a number of elements can affect the ability to detect the desired target. In the laboratory, one has better control over these elements. For instance, one can inject a known gas into the tube, control temperature and take steps to eliminate known contaminants. However, this is more difficult in the natural setting. Spectroscopy techniques from space work in a similar fashion to those in the laboratory except for one important element. Analyses conducted from space do not afford the ability to control the environment in which the analysis is performed. There are a number of factors that can affect the signal and the accuracy of the analysis when it is performed from space. The following will examine these factors and their affect on the ability to perform an accurate assessment.

. Vegetation appears to present one of the key difficulties in anomaly mapping for geothermal exploration. Geobotanical anomalies may accompany mineral deposits and hydrothermal convection systems (Nash & Hernandez, 2001, pp. 1). Therefore, vegetation may not represent unwanted interference all of the time, but may be the clue needed to confirm the anomaly. The affects of vegetation in Malawi will have to be studied to determine the importance of potential interference to signal strength, impedance and phase shift. Colinearity of certain mineral mixes may also have an impact on the spectra. In the case of calcium carbonate, this anomaly caused by colinearity of the complex mixture indicates hydrothermal convection (Nash & Johnson, 2002, pp. 8). In a study conducted in Turkey, mineral identification was impeded by areas of cloudiness as well as dense forests (Dogan, 2008, pp. 224). Geothermal activity may also be responsible for flow patterns in glaciers and ice sheets in Iceland (Bourgeois, Dauteuil, & Vliet-Lanoe. 1999. pp. 74).

Other factors were found to affect the ability to detect geothermal temperature fluctuation anomalies, including elevation and other topographical features, making it difficult to distinguish surface fluctuations from subsurface anomalies (Eneva & Coolbaugh, 2009, pp. 467-470). A comparison of daytime and nighttime images reveals that these comparisons can be an important tool in distinguishing true subsurface geothermal anomalies from surface and atmospheric interference. However, much more work is needed in this area to understand the application of these paired data sets (Eneva, Coolbaugh, & Bjornstad et al., 2007, pp. 1-7).

The pixel in the hyperspectral data set represents a combination of the end-member materials, with those of areal abundance representing the pixel color. However, each pixels distinct and must be analyzed further to determine of indicator minerals for geothermal hotspots are present (Kratt, Calvin & Coolbaugh, 2005, pp. 271-276). This will be an important area for further development of spectral techniques, as many minerals have similar absorption features, as was the case with chalcedony and opal in a recent analysis (Kratt, Calvin, & Coolbaugh, 2004, pp. 1304). Current efforts in research are focusing on the ability to unmix spectra and cancel the affects of atmospheric interference through the use of algorithms (Pal & Nash, 2003, pp. 669-672).

Spectral analysis using wavelengths of 0.4 to 2.5 ?m have been used as the means to identify spectral signatures in the laboratory. These same analysis techniques are now being used in spaceborne methods of spectral analysis (Kratt, Calvin & Lutz, n.d. pp.

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1-9). At the present time, these results are being confirmed using handheld GPS and pocket personal computers to provide field verification of results (Coolbaugh, Sladek, & Kratt et al., 2004, pp. 321-324). Spectral analysis of the same set of core samples using both Analytical Spectral Devices (ASD) and Fourier Transform Infrared Spectrometer (FTIR) demonstrated the differences in the two analytical methods (Calvin, Kratt, & Fauldsm 2005, pp. 1-18). Further fieldwork is needed to understand these differences and their affect on data analysis fully.

This section of literature highlights the need for further research in the area of limiting or eliminating disturbances that are due to environmental factors that are beyond the control of the researcher. Currently, researcher is going in the direction of the development of algorithms that correct for many of these factors. However, statistical elimination of these factors has advantages and disadvantages. From one perspective, algorithms can be developed that effectively eliminate the troublesome interference. However, they can also eliminate part of the desirable part of the signal as well. One must do many tests in the area to be assessed in order to confirm the results of statistical elimination of confounding variables in the field. There is still much work to be done in this area regarding remote sensing using satellite imagery. This research hopes to aid research in this direction, with the ultimate goal of making remote sensing by satellite a more reliable means to detect geothermal hotspots in Malawi and in the rest of the world.

The development of remote sensing and correction of interference has become a multi-university effort that is being directed by the Department of Energy in the U.S. It is hoped that by using a multi-university effort, many of the problems currently being experiences, such as vegetation disturbance, atmospheric interference, and signal to noise ratio can be reduced or eliminated to achieve the most useful information from space. The goal is to rely more on spectral analysis and less on ground confirmation and mapping (Pickles, Nash, & Calvin et al., 2003, pp. 673-675). Currently, spectroscopy is being used to identify drill hole samples during drilling operations. It has proven to be a reliable method of assessment in many similar applications (Calvin & Solurn, 2005, pp. 565). This research will contribute to the multi-faceted approach that is being taken to the problem.

Conclusion

Remote sensing via satellite has many advantages and disadvantages over ground mapping methods. The number of things that can affect the accuracy of the readings is a key disadvantage. It represents a cost effective way to explore large areas, prior to the use of field methods. Currently, it is primarily used as a screening method for the selection of smaller areas for further study. It is estimated that many more geothermal resources exist that are yet to be discovered in the U.S. (Coolbaugh & Shevenell, 2003, pp. 13-18). Remote sensing via satellite may provide the solution to this exploration in the U.S. And in other parts of the world, such as Malawi, but only if the sources of interference can be eliminated, or accounted for in the data analysis. Understanding how to use the data is the most important aspect of exploration of geothermal resources via remote satellite sensing.

Remote sensing by satellite is used in two primary fashions. It is used for the mapping of thermal anomalies and for the mapping of indicator deposits of thermal activity. At the present time, satellite mapping is not used as a stand-alone detection method, but is used in conjunction with corroborating evidence from other analytical techniques (Coolbaugh, Raines, & Zehner et al., 2006, pp. 872). For instance, the mapping of rock type, mineral logy, and vegetation stress can be used to provide clues to geothermal activity in an area (Calvin, Coolbaugh, & Vaughan, 2002, pp. 1). At the present time, mineral mapping has proven to be more reliable than vegetative mapping techniques in the identification of potential geothermal targets (Carrnaza & Hale, 2002, pp. 4827). This may be due to the number of environmental factors that can affect vegetation and atmospheric conditions that are not related to geothermal activity.

Despite the research that still needs to be done in the area of satellite mapping of geothermal areas, the use of TIR from the spaceborne ASTER instrument is currently being used in eastern California (Eneva, Coolbaugh, & Combs, pp. 407)......

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