Satellite spectroscopy for identifying geothermal targets in Malawi

Satellite Sensing of Geothermal Activity

Using Satellite Images to Identify Anomalous Geothermal Hotspots in Malawi

Finding more efficient and cost effective means to find electrical grade geothermal systems is important for the development of more environmentally friendly generation of electrical power. This is important in Malawi due to its current inability to meet generation demand. The use of satellite systems to detect surface temperature anomalies indicative of areas that deserve further exploration has proven to be an effective means of exploration in the western United States and in other parts of the world. This research will explore the use of satellite spectrometry in the assessment of potential geothermal systems and how this relates to further exploration of geothermal systems in Malawi.

The key question in geothermal exploration is where to drill. Finding efficient means to identify target areas is a key to the success of a geothermal generation project. Satellite data is relatively inexpensive when compared to older exploration methods. Older methods are hit and miss. Satellite data can be used to quickly identify areas that deserve closer exploration through ground verification methods (Jones, 2009, pp. 5). Once a potential site has been located, an exploratory well must be drilled and a flow test performed to assess the potential and quality of the site (Maroake, 2003, pp. 15). Satellite imagery allows for survey of initially larger areas then ground screening methods. At the present time, satellite spectroscopy is used as a screening method to identify targets of interest.

Currently, in the U.S., California is the leader in geothermal power production, but Nevada has tons of hotspots and is expected to rely more on geothermal energy in the past (Tavares, 2009). Geothermal energy is an excellent option for remote areas of the world, such as Eritrea on the Red Sea (Abraha, 2005). This is combined with satellite analysis of ground deformation that is associated with geothermal activity in the area (Fialko & Simons, 2000, pp. 14). These two methods confirmed each other in COSO, California. InSAR observations of ground deformation in the area coincided with satellite thermal imaging.

Geothermal Energy Potential in Malawi

Malawi is located in the southernmost sector of the East African Rift. This area is characterized by rifting, volcanism, crustal thinning and areas of enhanced geothermal gradients. Few volcanoes are still active in the area. However, there are a number of geothermal hotsprings present in the area, making it an area of interest for the generation of geothermal power (Allen, Cawkwell, & Dade et al., 2009, pp. 1-5). A number of surface geothermal spots are located in Malawi, and it is hoped that many more will be found that are not currently apparent on the surface.

Currently, Malawi relies on hydroelectricity for much of its generation. However, environmental degradation has led to a decrease in electrical generation that can result in a decrease of as much as 75%, These decreases have had a negative impact on the industrial sector of the country and the country must now consider searching for other forms of power generation. Their geological location makes geothermal energy a viable alternative to their current crisis (Dulanya, 2006, p. 1). However, methods will have to be developed that will allow for more efficient exploration of their geothermal resources.

All geothermal systems are the result of penetration of meteoric fluids into the crust, causing the subsequent heating and upflow of those liquids (Kratt, Coolbaugh, & Calvin, n.d. pp. 1). A number of hotsprings exist in both the northern and southern portions of Malawi. Many of them contain sulphur compounds. Some are contaminated with human and animal usage. These hot springs range from luke warm to steaming (Dulanya, 2006, p. 2-5). They range from a neutral to basic pH with various potentials for conductivity (Dulanya, 2006, p. 2-5). These hotsprings warrant further analysis for potential usage as power generation sites.

This research will explore the potential for using remote sensing via satellite spectral analysis as a means to locate new locations and to explore existing ones. At present, satellite spectroscopy is being used to locate geothermal hotspots in several areas of the world. However, as a review of current literature on the topic will reveal, analytical methods are not yet precise enough to warrant usage of satellite imagery as a stand alone analytical method for the detection of geothermal hotspots. The literature indicates that several factors affect the ability to remotely sense potential geothermal hotspots. These range from vegetation in the area to atmospheric conditions. This research will result in more effective search techniques using remote satellite sensing to detect potential geothermal sites that are generation quality with as little reliance on ground techniques as possible. It will help to advance knowledge of geothermal hotspots to make it a more reliable detection method for geothermal hotspots in Malawi.

Geomorphology and the Detection of Geothermal Systems.

Geomorphology is the most common means of exploring areas that have the potential for development as sources for geothermal electrical generation. Several algorithms have been developed based on geomorphologic components to help detect areas of interest. However, many variables exist that can limit the efficiency of these methods (Nash & Johnson, 2003, pp. 663-666). Geographic Information Systems (GIS) have traditionally been underused as a means to detect geothermal hotspots. However, improvements in these systems increase their potential to provide an efficient means for exploration (Nash, 1999). GIS systems were found to be superior to CAD generated maps due to their connection to a relational database (Nash & Adams, 2001, pp. 1). The connection with the database allows the researcher to examine the underlying thermal data that is associated with the landform.

Remote satellite sensing was found to be a useful tool in the detection of a geothermal area in Indonesia. However, this method continues to suffer from interference from sun anomalies due to the high sun elevation angle in the tropical regions. It also suffered from the inability to obtain accurate data due to continual clouds due to volcanic feathers in the region (Urai, Muraoka, & Nasution, 2002, pp. 99-108). These same concerns may affect the ability to use this technology in Malawi. Although, Malawi has not currently active volcanoes, it has several hotsprings that produce heavy steam. This may limit the ability of satellite sensory equipment to penetrate the steamy cover. Malawi may also be subject to high sun elevation and the anomalies that are caused by it. This factor will have to be carefully examined in the course of the study.

As geothermal water evaporates, it many leave behind certain mineral deposits, such as borate minerals, whose spectral fingerprint can be used to identify likely sites of geothermal activity (Kratt, Coolbaugh, & Calvin, 2006, pp. 435). The ProSpec TIR VS2 was successfully used to identify a new blind geothermal target in the Columbus Salt March Playa in Nevada. Indicator minerals were the key method used to identify the presence of geothermal activity (Kratt, Coolbaugh, & Peppin et al., 2009, pp. 481). Boron levels were used to detect the presence of thermal springs and wells in the Great Basin region (Coolbaugh, Kratt & Sladek et al., 2006, pp. 395). Mapped imagery taken during the day and night can reveal surface anomalies due to geothermal activity by examining known geothermal resources for the presence of hydrothermal alteration of clays and sulfates (Calvin & Coolbaugh, 2003, p. 1).

Remote mapping of minerals was achieved using the Spatially Enhanced Broadband Array Spectrograph System (SEBASS). Atmospheric conditions had to be subtracted out in order to gain a clear view of the surface radiance using this system (Vaughn, Calvin, & Taranik, 2003, pp. 55). The data was verified using a ground study. Several materials were found to interfere with the accuracy of the readings. These included natural vegetation, soil types, man-made materials, roofs, roads, mine tailings and other materials. However, for the most part, these presented a specific signal and could be differentiated from ground level mineral concentrations.

The presence of surface calcium carbonate deposits is associated with elevated subsurface geothermal reservoir temperatures and the presence of hotsprings on the Pyramid Lake Paiute Reservation. However, those associated with these calcium carbonate deposits are typically not electrical grade (Coolbaugh, Lechler, & Sladek, et al., 2009, pp. 461-463). Remote sensing and hyperspectral imagery with a spectral resolution of 15-20 nm was used to identify areas of interest prior to field reconnaissance work (Coolbaugh, Faulds, & Kratt et al., 2006, pp. 61). However, it may be noted that not all calcium carbonate formations represent active geothermal activity. In the case of TIR data over the Buffalo Valley, these deposits were the result of a dry lake (Littlefield & Calvin, 2009, pp. 498).

The enhancement of oxides and hydroxyls, along with subdued vegetative spectral characteristics was used to successfully analyze hydrothermal activity in Baja California, Sur Mexico (Bancora Prol-Ledesma, 2008, pp. 184). Aside from spectral anomalies, satellite magnetic data was also used to detect heat flux underneath the Antarctic Ice Sheet (Maule, Purucker, & Olsen et al., 2005, pp. 404). These studies suggest that thermal fingerprints of an area can be affected by several factors. 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.

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). It has also been used in East Africa's Rift Valley to identify potential volcanic risks and geothermal resources (GIS and Science, 2010, p. 1). Satellite -- based spectral reflectance has been used to successfully identify geothermal hotspots in remote Russia (Crowe, Jordan, & Baker et al., 2008, p. 565). These successes present a hopeful future in the area of remote sensing using satellite imagery.

The ability to explore below the surface for hidden geothermal resources will be the key to successful geothermal exploration in the future. Improving satellite resolution is a key obstacle that will have to be resolved in order to use satellite imagery to go beyond geothermal resources that can be seen on the surface (Taylor, 2007, pp. 20). This research will help to further this field by exploring the conditions that will affect satellite analysis and the use of it as a tool for geothermal exploration in Malawi.