Georesistivity Signature of Crystalline Rocks in the Romblon Island Group, Philippines
Leo T. Armada1,*, Carla B. Dimalanta1, Graciano P. Yumul, Jr.1,2, and Rodolfo A. Tamayo, Jr.1
1Tectonics and Geodynamics Group, National Institute of Geological Sciences
University of the Philippines, Diliman, Quezon City, Philippines 1101
2Department of Science and Technology, Bicutan, Taguig City, Philippines 1631
*This email address is being protected from spambots. You need JavaScript enabled to view it.
Georesistivity data from the survey site in Bagacay, Romblon Island (Black Square in inset map). Unweathered crystalline rocks in section B-B’ are characterized by high resistivities (1000 to 2000 Ω-m). The fractured and weathered portions near the fault shown in section C-C’ exhibit lower resistivities.
ABSTRACT
Georesistivity surveys were conducted in the tectonically complex Romblon Island Group, Philippines to assess the groundwater potential of the crystalline rocks found in the area. Vertical electrical sounding (VES) using Schlumberger array with a maximum spread (AB/2) of 300 meters was used during the survey; this array provided vertical images of depth up to 60 meters. The VES results show significantly lower resistivity values for the regolith (~10 to 250 ohm-meters) compared with the resistivity values of the parent units (i.e., ultramafic rocks: ~ 800 ohm-meters and metamorphic rocks: 1000 to 2000 ohm-meters). These resistivity values are attributed to the elevated groundwater content of the regolith compared with the unweathered parent rocks. Furthermore, thick regoliths were formed in areas adjacent to pre-existing faults and fracture zones in the area. The flow of groundwater through the fissures in the crystalline rocks possibly contributes to enhancing deeper levels of weathering to produce the low-resistivity regoliths observed. Also, the regoliths, with an average thickness of 35m, serve as zones of enhanced groundwater potential in the Romblon Island Group because of their relative thick overburden and low resistivity......
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