Delineation of Near Surface Water Flow Path of Wahawa Geothermal Field by Using 2D Inversion of Resistivity Data

The Wahawa geothermal field which is located in the Eastern province of Sri Lanka has an average temperature of 60˚C in its surface manifestations. Since the temperature is considerably high, it is important to explore the feasibility of direct utilization of the energy of this geothermal field. In the present study, electrical resistivity measurements were employed in a 20 km 2 region in order to delineate the Wahawa geothermal system and to understand the near-surface fracture pattern. Electrical resistivity mapping of the region has been carried out using Schlumberger array measurements with nominal current array spacing (half spacing) of 150 m and it was observed that there was a path of low (<30 W) apparent resistivity. These results revealed that the hotsprings resting on a hard rock terrain are presumably composed of metamorphic rocks, suggesting lateral movement of hotwater towards the hotsprings instead of a deep-influx. The area of surface manifestations is not suitable for utilization application due to clustering of the main feeding path. The major hotwater feeding path which is extending in the west direction can be recommended as a possible drilling target for direct utilization applications.


Introduction
Heat from the deep earth is exposed to the surface in many ways; the conductive How to cite this paper: Samaranayake, S. A., De Silva, N., Dahanayake, U., Wijewardane, H. O., & Subasinghe, N. D. (2022). Delineation of Near Surface Water Flow Path of Wahawa Geothermal Field by Using 2D Inversion of Resistivity Data. Journal of Geoscience and Environment Protection, 10, 327-339.
https://doi.org/10. 4236/gep.2022.108020 heat flow through the crust, volcanic eruptions, and as heated water. Hotsprings are the features resulting when ground water is heated by geothermal forces and brought to the surface (Hochstein, 1988). The occurrences of hotsprings are not rare in many geological settings in the world, since the Earth is tectonically active with plate margins and active faults. Water in the subsurface is heated basically by two methods. One method is that the water percolating deep into the earth may be heated up under normal geothermal conditions and pumped up to the surface, retaining the heat of the water. The other method is that the near-surface magmatic body or a volcano may act as a source to heat up water at shallow depths giving rise to hotsprings. Also, a slight change of water temperature may occur due to biogenic processes, with no correlation to the heat at depths (Bjorn, 2016). According to the setting of the heat source, there are two types of heat flow patterns that can be identified in a geothermal field (Figure 1). They are the heat source immediately beneath the surface manifestation and vertical heat flow path to the surface (Cox et al., 2015;Barnes & Rose, 1998) and the heat flow in the direction of long-running angle fractures (Kresic, 2010;Kumara & Dharmagunawardhane, 2014).
Detailed information about water flow pattern of the hotspring is crucial for harnessing heat for direct utilization. In particular, this is especially important for demarcating near surface drilling targets. Therefore, this study was focused on understanding the near surface water flow path of the Wahawa geothermal field, which can be identified as one of the suitable geothermal fields for direct utilization applications in Sri Lanka.
The gravity, magnetic and electrical methods play a vital role in geothermal exploration (Shah et al., 2015;Kiyak et al., 2015;Kana et al., 2015). Gravity and magnetic surveys are used to identify the geological structures and related geological features in the field (Blakely, 1996;Hinze et al., 2013). Resistivity techniques Figure 1. Types of hotsprings according to the feeding paths. (a) Heat source just beneath the surface manifestation and vertical heat flow path to the surface (After Cox et al., 2015); (b) heat flow toward the long running angle fractures (After Kresic, 2010). Journal of Geoscience and Environment Protection are used to identify subsurface fracture patterns and reservoir characteristics (Palacky, 1988;Samaranayake et al., 2015). Among the resistivity methods, Direct Current (DC) resistivity method is employed for low depths (Zohdy et al., 1973). Therefore, the majority of geothermal exploration relies on magnetotellurics (Li et al., 2015;Nimalsiri et al., 2015). The broad objective of this study was to delineate the near surface fracture pattern and hence a DC resistivity method was employed (Roy & Apparao, 1971).

Geological Setting
Sri Lanka is an island in the Indian Ocean, near the equator, between 5˚55'N to 9˚55'N latitudes and 79˚42'E to 81˚52'E longitudes. The geographical location of Sri Lanka is comparatively far from known active tectonic plate boundaries. Nearly 90% of Sri Lanka is underlain by late Proterozoic high-grade metamorphic rocks (Kehelpannala, 1997)

Methodology
Electrical resistivity measurements are used in an investigation of a 20 km 2 region in order to delineate the Wahawa geothermal system and to understand the near-surface fracture pattern. Electrical resistivity mapping of the region has been carried out using Schlumberger array measurements with nominal current array (half) spacing of 150 m. Figure    According to Equation (1) and Figure 4, "a" is half of the potential electrode distance [MN/2] and L is half of the current electrode distance [AB/2] (Telford et al., 1990;Reynolds, 2011;Everett, 2013). Advanced Geosciences, Inc. (AGI) mini string 2D resistivity imaging system with 28 electrodes is used for 2D data acquisition.
The hotsprings of the geothermal field are roughly aligned with the NW-SE direction and appear in the shear zone which is driven in the same direction.
Therefore, the EW direction was selected for main profiling and the NS direction was selected for cross profiling. These cross lines were used to control the quality of the profiling. 26 2D profiles were obtained and resistivity profile setup is shown in Figure 5. 1D resistivity profiles were conducted to enhance the data quality by cross checking with 2D profiles.

Results
Twenty six (26)   clearly show the presence of (comparatively) low resistivity zones. Since they occur alternatively, they clearly indicate fracture zones (probably joints) in the rock weathered at shallow depth. They must be tight as they extend to depths and therefore, do not appear as low resistive zones towards depths. This is because high resistivity of country rocks dominates towards depth. The results clearly show that the northern end is acting as a barrier for near-surface fracture propagation.
Zone C: Zone C covers the eastern side of the geothermal field. According to the terrain condition, only 2 profiles were conducted along the direction NS.   Zone E: Extension of western margin of Zone D is named as Zone E. Two (02)2D profiles were conducted accordingly to understand the fracture propagation. The 2D resistivity structures of the Zone E profiles are shown in Figure 10.
Results show that the major fracture continued up to the dolerite dyke and disappears, probably due to deepening of the fracture after the dolerite dyke.

Discussion
The interpretation of the 2D resistivity profiles shows that the Wahawa geothermal springs area has geo-electric layers with low, moderate, and relatively high resistivity zones that could result in from high resistive rocks and fractures.
The resistivity imaging survey has also mapped different weak zones through which the geothermal fluids discharge to the surface. The resistivity data shows the presence of deep extending fracture zone in the hotspring area. The fracture extends to the North West direction starting from the spring field as shown in Figure 11. Then it is further bends to the south direction and passes through the spring field. These fracture zones can act as pathways to thermal water from depths. In addition, the Wahawa geothermal spring is associated with a major fracture which is found to be oriented in the west direction, where both the dolerite dyke and the H/V boundary are located.

Conclusion
In this study, it was intended to understand the near-surface water flow path of the Wahawageothermal springs in Sri Lanka using a resistivity method. The near subsurface of the locality of thermal springs marked by a low resistive zone indicates the channeling of major fracture in the area of surface manifestations.
The NW extension from this low resistive area shows the feeding fracture continuation. The impermeable metamorphic basement located in other cardinal directions with higher resistivity indicates the absence of feeding fracture zones in those directions. The current analysis revealed that the feeding fracture of the Wahawa geothermal field is a western trending dipping fracture and extending up to the dolerite dyke. The results of this study could be used for the direct utilization applications, particularly to identify the drilling targets.