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Home , Mindoro (province), Palawan, Romblon, Sibuyan Island, Tablas Island

Philippine Journal of Science 138 (2): 191-204, December 2009 ISSN 0031 - 7683

Georesistivity Signature of Crystalline Rocks in the Island Group,

Leo T. Armada 1,*, Carla B. Dimalanta 1, 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, City, Philippines 1101 2Department of Science and Technology, Bicutan, Taguig City, Philippines 1631

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.

Key Words: Crystalline rocks, georesistivity, Philippines, regolith, tectonics

INTRODUCTION report the results of such investigations in the Philippines. Some works which used the electrical resistivity method In the Romblon Island Group, attempts to provide potable include the resistivity survey done in , Philippines water sources to the rural communities had been carried to constrain the thickness of sand and gravel deposits out under various programs. Some projects involved the (Abarquez 1969). In Paoay, Norte, the thickness drilling of wells to address the scarcity of water in rural and configuration of the sand dune aquifers were communities. Unfortunately, the water wells were poorly delineated through several electrical sounding points located and scientific investigations were not done to (Stirling Edwards and Gonzales 1984). determine the proper sites for the wells. As a result, water extracted from the dug wells were of poor quality (e.g., Based on the groundwater availability map of the some showed fecal contamination and some wells went Mines and Geosciences Bureau (1997), rocks found dry during the summer) (Asian Development Bank 1999). in different parts of the Philippines are characterized in terms of their suitability as aquifers and their Although the resistivity method has been around for potential for storing groundwater. Due to the nature several decades and has been widely used in the search of the underlying rocks, only a small area in Tablas for groundwater, very few papers have been published to Island is underlain by fairly productive aquifers (e.g., *Corresponding author: [email protected] 191 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines sedimentary units) with the rest of the areas in the this block collided with the (Fig. three islands (Tablas, Romblon, and Sibuyan) being 1). The interaction between these two blocks is responsible composed of rocks with limited potential or without for several episodes of collision, subduction, and accretion any known significant groundwater. In 2004, the Local in central Philippines. These processes produced the belt Water Utilities Administration (LWUA) conducted of metamorphic rocks and ophiolitic units in georesistivity surveys in coastal areas underlain by Island, Romblon Island Group, and northwest which sedimentary and alluvial aquifers in Romblon Island to delimits the extent of the arc – continent collision zone identify additional water sources. The sounding data (Ramos et al. 2005; Yumul et al. 2003; 2005; 2008). revealed the presence of possible water-bearing layers which should be confirmed by subsequent drilling The Romblon Island Group in west central Philippines (LWUA 2004). consists of features that attest to the Early Miocene collision between the Micro-continental Block Crystalline rocks (particularly igneous and metamorphic and the arc-related Philippine Mobile Belt. The three big rocks) are generally poor groundwater aquifers due to islands that make up the Romblon Island Group - Tablas, their lack of primary porosity and permeability. This Romblon, Sibuyan - consist mainly of crystalline rocks makes groundwater exploration difficult in hard rock (i.e., ophiolitic units and metamorphic, volcanic and terrains, although groundwater accumulations may intrusive rocks). The sedimentary sequences are found occur in crystalline rocks having limited secondary mostly in (Fig. 2). The distribution of these porosity acquired due to faulting, jointing and lithologic units offers some constraints on the possible weathering (e.g., Owen et al. 2005; Dutta et al. 2006; occurrence of water-bearing units in the area. Yadav and Singh 2008). These localized concentrations of groundwater within a crystalline formation become The search for groundwater in the Romblon Island Group unconventional targets for water resource prospecting. is made difficult by the fact that the islands are dominantly Extraction of groundwater from the weathered and made up of crystalline bedrock (hard-rocks). Units of fractured portions of the crystalline bedrock is reported the Sibuyan Ophiolite Complex are exposed in Tablas by Taylor and Howard (2000). These types of aquifer and Sibuyan Islands. From bottom to top, the sequence usually occur in the permeable zone overlying the is made of harzburgites and dunites, layered pyroxenites, unweathered crystalline bedrock (Taylor and Howard layered and isotropic gabbros, diabase dike swarms, and 1999). This potential groundwater resource, if tapped, basaltic to andesitic pillow lavas and flow deposits (Fig. will provide an important fresh water resource for areas 3a-3b). Units of the ophiolite are seen as tectonic slices underlain by hard rocks (e.g. Adepelumi et al. 2006; bound by thrust faults which generally trend NE and dip Owen et al. 2007; Surrette et al. 2007). NW (Fig. 2 ). This paper discusses the results of the evaluation Aside from the ophiolitic units, there are other volcanic of hard-rocks within a collision zone as potential and intrusive units that are exposed in Tablas and aquifers using electrical resistivity method. There Sibuyan Islands (Fig. 3c-3d). Andesite outcrops which has been relatively little work done on ophiolitic and are fractured and weathered mark the eastern coastline of metamorphic rock aquifers in the Philippines. This Tablas Island. Diorite intrusions are also found in several is one of the few georesistivity investigations carried localities in Tablas and Sibuyan Islands (Fig. 2). The out in collision zones and over hard-rock targets outcrops are highly fractured and weathered especially for groundwater sources in the region. The results in the exposure found in northern Tablas. The fractures obtained from this work suggest that the success rate of generally trend NE, NW, and E-W with SE, NE, and N groundwater exploration in geologically complex areas dips, respectively. Different varieties of metamorphic such as collision zones and in crystalline, hard-rock rocks were mapped in all three islands. These consist of areas may be improved by conducting georesistivity mica, quartz-mica, quartzo-feldspathic, chlorite, and talc- surveys prior to drilling. chlorite schists, and limited exposures of phyllite (Fig. 3e-3f). Marble is found only in Romblon Island (Fig. 2). The metamorphic rocks are complexly folded in some Geologic Setting of the Romblon Island Group places but the general foliation trends are NW (dipping The Philippine Island arc system is an amalgamation of 10-80°NE) and NE (dipping 20-70°NW). blocks of continent- and arc-derivation. The continent- derived block, the Palawan Micro-continental block, is a Clastic sedimentary rocks are generally characterized as fragment of mainland Asia. This piece broke off during the good aquifers, hence, most groundwater exploration work opening of the South Sea (Taylor and Hayes 1980; targets these rock types. In the Romblon Island Group, a Hsu et al. 2004). As it was being translated southward, significant portion of Tablas Island, especially the western

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Figure 1. The Romblon Island Group is situated within an arc-continent collision zone. Area encircled in red is the RIG = Romblon Island Group. Yellow shaded region = Palawan Microcontinental Block. Gray shaded region = Philippine Mobile Belt.

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Figure 2. The Romblon Island Group is a crystalline, hard rock area. Exposures of sedimentary rocks are observed only in Tablas Island. Sites occupied during the georesistivity surveys are shown in gray squares. Inset (above) shows the Tablas, Romblon, and Sibuyan Islands (black shaded areas) which are situated within the western part of Central Philippines.

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Figure 3. Photos of the igneous (a-d), metamorphic (e-f) and sedimentary units (g-h) exposed in Tablas, Romblon and Sibuyan Islands.

195 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines side, is underlain by the Early-Middle Miocene, Late In Tablas Island, the survey sites are approximately five Miocene-Early Pliocene, and Late Pliocene to Pleistocene kilometers away from the main population center. These clastic and carbonate sequences of the Binoog, Anahao, areas lie within two adjacent drainage basins which occupy and Peliw Formations, respectively (Fig. 2). These are areas measuring 81 km2 (Anahao) and ~41 km2 (Poctoy) composed of massive to bedded limestone and bedded (Fig. 2). The survey site in Poctoy, situated between conglomerates, mudstones, sandstones, and siltstones latitude 12°24’20”-25’12”N, and longitude 121°59’34”- (Fig. 3g-3h). 122°00’30”E), is located close to the coast. Groundwater from the Anahao drainage basin drains into the Anahao Faults related to the complex tectonic evolution of the River which empties northwestward into the Bay area are mostly observed in the crystalline basement (Fig. 4a). The survey site in this basin (12°22’05”-30”N units. The most significant geologic structures in Tablas and 121°57’40”-58’30”E) is located in the lower reaches and Sibuyan Islands are the thrust faults that bound the of the Anahao River. Both survey sites are underlain by slices of ophiolitic rocks. In Romblon Island, there are sedimentary rocks of the Anahao Formation, which consists numerous faults and fractures which crosscut the island. of interbeds of conglomerates and calcareous to tuffaceous NW- and SW-verging thrust faults, NE-striking normal sandstones and mudstones (Fig. 3). faults, and a generally N-S oriented left-lateral strike-slip fault system were recognized during the mapping (Fig. Metamorphic rocks, specifically marble and schists, 2). The fractured nature of the crystalline rocks attests to underlie the survey areas in Romblon Island (Fig. 2). For the geologic processes that characterize collision zones. The structures that bound and cut these crystalline rocks help pinpoint areas that can be investigated for groundwater sources. Ground fissures such as faults and fractures act as pathways for groundwater. These become important groundwater conduits and reservoirs in areas where the bedrock has low porosity and poor permeability (e.g., Rao et al. 2000; Sharma and Baranwal 2005; Chandra et al. 2006; Surrette et al. 2007). Tensional faults are better targets for groundwater search compared with other fault types. The increased fault/fracture density especially at the intersection of fault systems may improve the ability of rocks to conduct and store large volumes of groundwater. Fractures that penetrate the subsurface deeper also contribute in providing sustainable groundwater sources by producing thicker weathered zones (e.g., MacDonald and Davies 2000; Srinivasa Gow 2004; Owen et al. 2005; Dutta et al. 2006). These structures are good candidates which can be evaluated for groundwater potential in a hard-rock environment.

Survey Sites and Georesistivity Data Acquisition In order to evaluate the effects of these structures on the groundwater potential of the area, vertical electrical soundings (VES) were carried out in selected areas within the Romblon Island Group. Six survey areas were selected in the islands of Tablas, Romblon, and Sibuyan based on the following criteria: type of rock present in the subsurface, proximity to geologic structures (i.e., faults and fractures), location within a drainage basin, and proximity to population centers. These sites include Poctoy and Anahao in Tablas Island, Bagacay and Sawang Figure 4a. Georesistivity data from a survey site in Tablas Island (black square in inset map). The low resistivity values are in Romblon Island, and Magdiwang and San Fernando in interpreted to correspond to the underlying fine-grained (Fig. 2). sedimentary rocks The zero elevation corresponds to the mean sea level.

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Static water the Bagacay survey site (12°34’33”-17”N, 122°15’55”- 0 mbgs 16’13” E), the groundwater basin from which the town level at 7.6m Soil of Romblon gets most of its potable water requirements covers an area of 3.5 km2. Schists underlie the survey 20 mbgs site in Bagacay. A NE-trending strike-slip fault bisects the survey site (Fig. 5a). South of Bagacay is the Sawang area which lies within a groundwater basin which occupies an 2 40 mbgs Clays area of ~2.5 km . The survey site (123°32’55”-33’10”N, 122°14’40”-16’20”E) is flanking the dried channel of the Sawang River. The surface is composed of loose gravels 60 mbgs and sands. Bordering the survey area are outcrops of fractured schists. 80 mbgs Two large drainage basins in the northern and southern parts of Sibuyan Island were chosen as sites of the georesistivity surveys (Fig. 2). The drainage basin LWUA well no. ROM-ODI-1085 which is the source of groundwater for the Magdiwang population center occupies an area of 80 km2. In the south, Figure 4b. A well log from a drilling conducted by the LWUA in the the Cantingas River forms part of the drainage basin which study area provides constraints on the type of lithologies at depth as well as the static water depth. provides water to the San Fernando community. The drainage basin has an area of 67 km2. Quaternary alluvium and units of the Sibuyan Ophiolite Complex underlie the survey site in Magdiwang (12°28’30” - 12°19’30”N, 122°30’36” - 122°31’53”E), northern Sibuyan. The

Figure 5a. 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.

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Figure 5b. Two section logs generated from the field observations in the area show the weathering profiles near the fault (A) and away from the fault (B). A thicker regolith was formed over the intensely fractured schists. most prominent structural feature in the area is the north- The Direct Current (DC) resistivity method, a non- trending normal fault that bisects the area. Volcanic rocks invasive, non-destructive technique has been widely underlie the area west of the normal fault and peridotites used for a variety of groundwater investigations such of the ophiolite complex consist the east area (Figure 6). as identification of aquifer, determination of the depth In a hard-rock area similar to Sibuyan, recharge is very of the water-bearing strata, geometry of the aquifer and important in order to obtain a continuous water supply delineation of fresh/salt water interface, among others (Sharma and Baranwal 2005). The survey site is located (e.g., McNeill 1990; Sultan et al. 2008; Zouhri et al. 2008). within a large drainage basin system. Meteoric waters The close association among electrical resistivity, rock from the highlands infiltrate into the subsurface, thus, type, and water content makes DC resistivity method the forming the recharge of the groundwater system in the most suited technique in groundwater exploration (e.g., area. The survey area in San Fernando in the southern Subba Rao 2003; Sharma and Baranwal 2005). part of Sibuyan Island (12°18’30” - 12°19’30”N and 122°34’00” - 122°35’22”E) is underlain by Quaternary Forty-two (42) vertical electrical sounding (VES) stations alluvium, units of the Sibuyan Ophiolite Complex, and were occupied during the georesistivity survey which the Romblon metamorphic rocks. It is also situated near was done during the dry season in the month of April the intersection between a NW-trending normal fault and 2006. A Schlumberger array was used with a maximum a nearly north-south curvilinear thrust fault (Fig. 2). spread of 300 meters. This allowed the delineation of vertical variations in electrical resistivities for depths up

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Figure 6. Georesistivity data from the survey site in Magdiwang, Sibuyan Island (red square in inset). Areas near the normal fault display low resistivities. to 60 meters. The apparent resistivity values at depth Inverse models were generated from the apparent were measured using a GEOTRADE GTR-3 Averaging resistivity data using the WinSev 6.1 interpretation Resistivity Meter. The orientation of the spread, wherever software developed by GeoSoft. The calculation of the possible, was parallel to the strike of known geologic and theoretical curve uses the method described by Koefoed structural features in the study areas. This is done to reduce (1979) and Das and Verma (1980). In this method, a the effects of lateral subsurface heterogeneity imparted by preliminary model is initially entered into the software. these structures (Lenkey et al. 2005). Using the least-squares method, the preliminary model

199 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines is automatically adjusted to fit the field data. Based The high resistivity layer is mantled by a less resistive on available geologic information such as underlying layer having resistivity values that range from 50 to lithologies, the thickness of layers based on available 250 Ω-m; this zone is believed to be the water-bearing well data from the Local Water Utilities Administration weathered zone or regolith. A thin, relatively more (LWUA), and depths to the water-table from the National resistive layer (350 to 400 Ω-m) corresponding to the dry Water Resources Board (NWRB) database, the model was regolith, in turn, overlies it. A very resistive layer (1500 further constrained using these set of a priori information. Ω-m) encountered at an elevation of 70 to 80 masl is These geologic constraints are considered during the inferred to be a marble lens overlying the water-saturated modeling process in order to come up with the appropriate regolith of the schist. From the interpreted section, it can model. The processed vertical electrical soundings be seen that the water-saturated regolith is located between acquired at various points in the survey areas were then elevations of 40 to 60 masl. Manifestations of this perched combined to come up with geoelectrical sections of the aquifer are the springs observed in the vicinity of the subsurface. survey area at elevations of about 40 masl. A southwest-northeast georesistvity section in Bagacay, Vertical Electrical Sounding Data Romblon is shown in Fig. 5. In this section, a relatively In Anahao, Tablas Island, a two-layer subsurface is high resistivity layer is identified at a depth of 60 meters. observed from the soundings conducted (Fig. 4a). A more This layer with a resistivity value of ~500 Ω-m is inferred resistive layer overlies a less resistive layer. The more to be the less weathered metamorphic basement. A thick resistive layer has resistivity values ranging from 4 to 6 regolith overlies the basement and is characterized by Ω-m. The bottom layer is typified by resistivity values a wide range of resistivity values (3 to 150 Ω-m). This less than 3 Ω-m. The two layers delineated in the area regolith has an average thickness of 35 meters. This is may correspond to the fine-grained strata of the Anahao believed to be a function of the proximity of the sounding Formation, with the upper layer representing aerated beds point to the NE-trending strike-slip fault. In the case of and the less resistive layer the water-saturated layers. This sounding points near the fault, the interpreted regolith interpretation is constrained by a well log from a drilling is characterized by low values of 3 and 70 Ω-m. On the conducted by the LWUA in the area. Inspection of the other hand, regolith which formed far from this structure lithologic log of well number ROM-ODI-1085 reveals is characterized by high resistivity values of 150 Ω-m and two layers in the subsurface. A thick clay layer is overlain 120 Ω-m. An unsaturated regolith layer is interpreted by ~10 meters of soil. Further, the observed groundwater from the overlying layer with resistivity values ranging depth of 7.6 meters in the drilled well agrees with the from 350 to 360 Ω-m. interpreted depth to the water saturated layer (Fig. 4b). A normal fault is identified within the Magdiwang survey The interpretation of the subsurface rock layers from the area in northern Sibuyan (Fig. 6). As a consequence of georesistvity curves were constrained by a section log the fault, the nearby rocks are fractured. These fractures generated from the field survey of the area. In the Bagacay enhanced the porosity of the rocks and also intensified the area, an uphill geologic traverse along a N-S section near action of chemical weathering in this portion of the study the strike-slip fault reveals the weathering profile of the area. These characteristics of the underlying materials metamorphic rocks (Fig. 5b). Unweathered metamorphic are evident in the low resistivity values observed in the rocks were observed at elevations of 20 meters above soundings. West of the fault, relatively more resistive sea level (masl) up to ~45 masl. Sporadic layers and layers were identified with resistivity values of 200 Ω-m lenses of marble are intercalated with extensive outcrops and 59 Ω-m (Fig. 6). Located east of the fault is another of the chlorite schists. Above these areas, weathered resistive layer observed having a resistivity value of 89 chlorite schists were observed uphill. These weathered Ω-m. Overlying these layers are the less resistive layers metamorphic rocks grade into red soil at elevations greater with thickness varying from 30 meters on the west to 60 than 70 masl. An S-N traverse of an area farther east meters beneath the midsection of the survey site. Their of the fault is characterized by relatively non-fractured resistivities range from 17 to 47 Ω-m. schists. It is noticed from the section log of this traverse that a thinner regolith formed over this crystalline rock The interpretation of the lithologies from the resistivities (Fig. 5b). In the N-S section of the Bagacay survey area generated from the inversion of the apparent resistivity in Romblon Island, the bottom layer is characterized by data are based on the geology of the area, available well very high resistivity values of 1,000 to 2,000 Ω-m (Fig. data and on resistivity values of corresponding rocks 5b). This bottom layer is interpreted to correspond to the from literature. unweathered metamorphic basement rocks in the area.

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DISCUSSIONS portion of the volcanic rocks is inferred at a depth of approximately thirty meters below ground surface and The geologically-complex Romblon Island Group, which is characterized by a resistivity of 200 Ω-m (Table 1). is located within a collision zone, is chiefly characterized This resistivity is less than the >400 Ω-m reported for by crystalline rocks. This impacts the availability of a similar terrain in Zimbabwe (e.g., Owen et al. 2005). freshwater sources particularly in the islands of Romblon This may imply that the volcanic rock at depth is slightly and Sibuyan which are underlain by metamorphic rocks weathered and has elevated fluid concentration than and ophiolitic rocks, respectively. However, the occurrence usual. Furthermore, the area is transected by a normal of collision-related faults and fracture zones within these fault that enhances the porosity of the underlying rocks. rocks provided secondary porosity and permeability that The ultramafic rock in the study area is characterized by improves the groundwater potential in the area. slightly higher resistivities compared with the volcanic The sedimentary rocks located mainly in the Tablas rocks. The ultramafic rocks with resistivities of ~800 to Island are characterized by very low resistivity values as 900 Ω-m were encountered at about 40 to 50 meters below typical of clastic rocks, compared with crystalline rocks ground surface. An increase in the regolith thickness is predominating the islands of Romblon and Sibuyan. This also noted in areas approaching the known location of the distinct difference in resistivity is attributed to elevated fault in the survey area. water content of sedimentary rocks which are natural good In Romblon Island, where suitable aquifers (e.g., groundwater reservoirs because of their high porosity and Quaternary alluvium and sedimentary rocks) are not permeability. On the other hand, crystalline rocks are present, the search for water-bearing layers is focused on characterized by high resistivities, several magnitudes the metamorphic units that exhibit secondary porosity. greater than that of the sedimentary rocks. Localized The presence of geologic structures and the consequent occurrence of groundwater in hard-rock terrains is easily weathering of the metamorphic units through the identifiable owing to the lower resistivities of these percolation of meteoric waters into fractures, as shown by water-rich zones compared with typical high resistivities this study, served to enhance the water-bearing capability of crystalline rocks. Clay deposits in the area are typified of these units. Targeting the regolith as the potential water- by resistivities less than 4 Ω-m. These resistivities are bearing layer has yielded good results (e.g., MacDonald less than the values of 14 to 37 Ω-m for clays reported and Davies 2000; Louis et al. 2002). This has been proven elsewhere (e.g., Israil et al. 2006) (Table 1). This can be by studies that propose greater groundwater flow in explained by the presence of water in the pores of the fissures in the upper layer (regolith) than in the fractured clay deposits and possibly a high concentration of ions basement rock itself (e.g., Dewandel et al. 2005). in these pore waters. Due to recently increasing demand for water, more The metamorphic rocks, specifically the chlorite-schists groundwater exploration activities are being directed in Romblon Island are characterized by resistivity values at hard rock areas (e.g.. MacDonald and Davies 2000; of 1000 to 2000 Ω-m (Table 1). These resistivity values Louis et al. 2002; Owen et al. 2005). The results of the fall within the range obtained by Connell et al. (2000) for georesistivity surveys in selected sites in the Romblon chlorite-schist (360-6600 Ω-m). Overlying the crystalline Island Group show the viability of finding groundwater basement is the regolith with resistivities ranging from 3 aquifers in crystalline rock areas. In the RIG, the possibility to 300 Ω-m. These values are within the range reported of finding groundwater sources in the predominantly for weathered and fractured rocks by other workers (e.g., crystalline rocks that comprise the area is improved due to Mondal et al. 2008) but greater than that reported by the area’s location within a collision zone. Being situated Owen et al. (2005) for regolith of schists (20–100 Ω-m). within a tectonically active, geologically complex area This may be explained by the difference in degree of has led to the formation of faults and fractures within the weathering and also the amount of water within these crystalline rocks (e.g., Dewandel et al. 2005). In areas layers. The weathering action of meteoric waters, which where aquifers with primary porosity are non-existent, percolate through fractures in the crystalline rock, will groundwater exploration normally turn to fracture zones have an effect on its electrical properties (e.g., Taylor and and faults in crystalline rocks. The presence of these faults Howard 1998; 1999). This is evident in the lateral decrease and fractures provide secondary porosity which greatly of resistivities in sections proximal to mapped geologic improve the accumulation of groundwater in these rocks. structures in the area. Recently, however, more importance is given to the role Resistivity signatures of the volcanic rocks and ultramafic of fractures and faults in creating extensive weathered rocks of the ophiolite in Sibuyan Island are interpreted from zones (regolith). Fractures which reach greater depths in data obtained in Magdiwang. A relatively unweathered the bedrock help produce thick, permeable regoliths (e.g.,

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MacDonald and Davies 2000; Louis et al. 2002; Mondal et have low groundwater potential. However, due to brittle al. 2008). The weathered overburden is rendered porous deformation during the emplacement of these crystalline which may enable it to store groundwater, hence, making terrains during the collision event, the rocks became it a possible aquifer (e.g., Kellett and Bauman 2004). fractured. These fractures, associated with the extensive faults cutting through the basement rocks of the Romblon In areas underlain by crystalline rocks such as the Island Group, provided spaces within the crystalline rocks Romblon Island Group, groundwater potential is low. where groundwater accumulated and percolated. These The inherent low porosity and permeability in ophiolite zones of enhanced porosity and permeability are typified rocks and their metamorphic basement contribute to the by localized decrease in resistivities within a resistive difficulty in locating probable aquifers in such areas. area attributed to elevated groundwater concentrations. In this case, the tectonic setting of the area becomes a major consideration in groundwater resource evaluation. Groundwater percolated through the fractures within the The presence of geologic structures (i.e., faults, fractures ophiolitic and metamorphic units thereby inducing the and joints), as a consequence of the tectonic activity in eventual deep weathering of the crystalline basement. the area, provide secondary porosity and permeability Interactions between the percolating waters and the to the crystalline rocks. In areas where these structures rocks along the extensive fracture systems lead to intersect or are concentrated, the secondary porosity and the dissolution and disintegration of the rocks’ labile permeability of the rocks are greatly enhanced. Further minerals. This process resulted in the formation of a more porous and permeable overlying weathered rock, the regolith. This correlation between intense fracturing Table 1. Variations of resistivity with rock type based on the georesistivity and deep weathering is confirmed by the formation of results from the Romblon Island Group. Literature values are thicker regolith over crystalline rocks proximal to a fault shown for comparison (from Telford et al. 1976; Kellett and Bauman 2004; Mondal et al. 2008). or intersection of faults. This is particularly evident in Resistivity values the area of Bagacay in Romblon Island, where thick Literature values ( -m) Rock type ( -m) Ω porous regolith is formed over the intensely fractured Ω Various sources This study schists. These voluminous regolith are characterized Clay <1 – 7 1 – 100 by high concentrations of groundwater. Thus, the main Sand and gravel 55 – 60 10 – 800 groundwater aquifers in crystalline areas like the Romblon Metamorphic rocks 500 – 2000 20 – 100,000 and Sibuyan Islands are made up of the regolith as well as the intensely fractured portions of the crystalline Dry soil 350 – 900 800 – 5000 basement. The results of the georesistivity surveys Volcanic rocks 60 – 200 100 – 1000 obtained from this study confirm the viability of finding Ultramafic rocks 80 – 800 3000 (wet) – 6500 (dry) groundwater aquifers in crystalline rock areas. Weathered and fractured 3 – 300 3 – 400 rocks

REFERENCES exposure of these fractured crystalline rocks to meteoric ABARQUEZ OE 1969. Resistivity soundings on sand and groundwater percolating through fractures cause and gravel deposits at Malaya, Pililla, Rizal. J Geol dissolution of labile minerals in the rocks, thus resulting Soc Philipp 23, 113-119. in the formation of a more porous and permeable regolith. [ADB] ASIAN DEVELOPMENT BANK. 1999. Project performance audit report on the second island provinces rural water supply sector project in the CONCLUSIONS Philippines. Mandaluyong City, Philippines: Asian Development Bank. 50p. The collision between the Palawan Micro-continental block and the Philippine mobile belt preserved features ADEPELUMI AA, YI M-J, KIM J-H, AKO BD, SON in central Philippines, particularly in the Romblon JS. 2006. Integration of surface geophysical methods Island Group, which include ophiolitic units, intrusive for fracture detection in crystalline bedrocks of and volcanic rocks, metamorphic rocks. Sedimentary southwestern Nigeria. Hydrogeol J 14: 1284-1306. sequences are limited only to Tablas Island. The CHANDRA S, RAO VA, KRISHNAMURTHY NS. 2006. crystalline rocks that dominate the Romblon Island Integrated studies for characterization of lineaments Group, i.e., ophiolitic and metamorphic rocks, originally used to locate groundwater potential zones in a hard

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rock region of Karnataka, India. Hydrogeol J 14: Synthetic simulation approach and case study example. 1042-1051. J Elec Electron Engg Spec Issue, 1-14. CONNELL S, SCROMEDA N, KATSUBE TJ, MACDONALD AM, DAVIES J. 2000. A brief review MWENIFUMBO J. 2000. Electrical resistivity of groundwater for rural water supply in sub-Saharan characteristics of mineralized and unmineralized Africa. Notthingham, United Kingdom: British rocks from Giant and Con mine areas, Yellowknife, Geological Survey. 13p. Northwest Territories. Geological Survey of Canada, [MGB] MINES AND GEOSCIENCES BUREAU. 1997. Current Res (2000-E9) Rpt, 9 p. Water availability map of the Philippines. , DAS UC , VERMA SK. 1980. Digital linear filtering for Philippines: Mines and Geosciences Bureau. computing type curves for the two-electrode system MCNEILL JD. 1990. Use of electromagnetic methods for of resistivity sounding. Geophys Prosp 28: 610-619. groundwater studies. In: Ward SH (ed) Geothechnical DEWANDEL B, LACHASSAGNE P, BOUDIER F, and Environmental Geophysics, Volume 1, Review AL-HATTALI S, LADOUCHE B, PINAULT JL, AL- and Tutorial. Soc Explor Geophys Inv No. 5: 107-112. SULEIMANI Z. 2005. A conceptual hydrogeological MONDAL NC, RAO VA, SINGH VS, SARWADE DV. model of ophiolite hard-rock aquifers in Oman based 2008. Delineation of concealed lineaments using on a multiscale and multidisciplinary approach. electrical resistivity imaging in granitic terrain. Curr Hydrogeol J 13: 708-726. Sci 94: 1023-1030. DUTTA S, KRISHNAMURTHY NS, ARORA T, RAO OWEN RJ, GWAVAVA O, GWAZE P. 2005. Multi- VA, AHMED S, BALTASSAT JM. 2006. Localization electrode resistivity survey for groundwater exploration of water bearing fractured zones in a hard rock area in the Harare greenstone belt, Zimbabwe. Hydrogeol using integrated geophysical techniques in Andhra J 14: 244-252. Pradesh, India. Hydrogeol J 14: 760-766 OWEN R, MAZITI A, DAHLIN T. 2007. The relationship HSU SK, YEH Y-C, DOO W-B, TSAI C-H. 2004. New between regional stress field, fracture orientation and bathymetry and magnetic lineations identifications in depth of weathering and implications for groundwater the northernmost and their tectonic prospecting in crystalline rocks. Hydrogeol J 15: implications. Mar Geophys Res 25: 29-44. 1231-1238. ISRAIL M, AL-HADITHI M, SINGHAL DC, KUMAR RAMOS NT, DIMALANTA CB, BESANA GM, B. 2006. Groundwater-recharge estimation using a TAMAYO RAJ, YUMUL GPJ, MAGLAMBAYAN surface electrical resistivity method in the Himalayan VB. 2005. Seismotectonic reactions to the arc- foothill region, India. Hydrogeol J 14: 44-50. continent convergence in Central Philippines. Res KELLETT R, BAUMAN P. 2004. Mapping groundwater Geol 55: 199-206. in regolith and fractured bedrock using ground RAO YS, REDDY TVK, NAYUDU PT. 2000. geophysics: A case study from Malawi, SE Africa. Can Groundwater targeting in a hard-rock terrain using Soc Explor Geophys Recorder, 24-33. fracture pattern modeling, Niva Pradesh, India. KOEFOED O. 1979. Geosounding Principle, Volume 1. Hydrogeol J 8: 494-502. Amsterdam: 276p. SHARMA SP, BARANWAL VC. 2005. Delineation of LENKEY L, HAMORI Z, MIHALFFY P. 2005. groundwater-bearing fracture zones in a hard rock area Investigating the hydrogeology of a water-supply integrating very low frequency electromagnetic and area using direct-current vertical electrical sounding. resistivity data. J App Geophys 57: 155-166. Geophys 70: 11-19. SRINIVASA GOWD S. 2004. Electrical resistivity [LWUA] LOCAL WATER UTILITIES surveys to delineate groundwater potential aquifers in ADMINISTRATION. 2004. Report on well site Peddavanka watershed, Anantapur District, Andhra evaluation: Georesistivity survey, Romblon Water Pradesh, India. Env Geol 46:118-131. District, Romblon. Technical Report, Quezon City: STIRLING EDWARDS L, GONZALES CP. 1984. Local Water Utilities Administration. 65p. Resistivity survey of the sand-dunes aquifer, Paoay LOUIS IF, LOUIS FI, GRAMBAS A. 2002. Exploring Lake and Currimao, . The Philippine for favorable groundwater conditions in hardrock Geologist 38, 50-69. environments by resistivity imaging methods:

203 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines

SUBBA RAO N. 2003. Groundwater prospecting and TELFORD WM, GELDART LP, SHERIFF RE. 1976. management in an agro-based rural environment of Applied Geophysics. Cambridge, United Kingdom: crystalline terrain of India. Env Geol 43: 419-431. Cambridge University Press. 860p. SULTAN SA, MANSOUR SA, SANTOS FAM. 2008. A YADAV GS, SINGH. 2008. Integrated surveys for hydrogeophysical investigation of the Ain Mousa area, delineation of fractures for groundwater exploration near Cairo, Egypt. Bull Eng Geol Environ 67: 111-117. in hard rock areas. J Applied Geophys 62: 301-312. SURRETTE M, ALLEN DM, JOURNEAY M. 2007. YUMUL GPJ, DIMALANTA CB, TAMAYO RAJ, Regional evaluation of hydraulic properties in MAURY RC. 2003. Collision, subduction and variably fractured rock using a hydrostructural domain accretion events in the Philippines: a synthesis. Isl approach. Hydrogeol J 16: 11-30. Arc 12: 77-91. TAYLOR B, HAYES DE. 1980. The tectonic evolution of YUMUL GPJ, DIMALANTA CB, MARQUEZ EJ, the South China Basin. In: Hayes DE (ed) The Tectonic MAGLAMBAYAN VB. 2008. Tectonic setting of a and Geologic Evolution of Southeast Asian Seas and composite terrane: A review of the Philippine island Islands. Am Geophys Monogr Series 23: 89-104. arc system. Geosci J 12: 7-17. TAYLOR RC, HOWARD KWF. 1998. Post-Paleozoic YUMUL GPJ, DIMALANTA CB, TAMAYO RA JR. evolution of weathered land surfaces in Uganda by 2005. Indenter-tectonics in the Philippines: Example tectonically controlled deep weathering and stripping. from the Palawan microcontinental block – Philippine Geomorph 25: 173-192. mobile belt collision. Res Geol 55: 189-198. TAYLOR RC, HOWARD KWF. 1999. Lithological ZOUHRI L, CARLIER E, BEN KABBOUR B, TOTO evidence for the evolution of weathered mantles in EA, GORINI C, LOUCHE B. 2008. Groundwater Uganda by tectonically controlled cycles of deep interaction in the coastal environment: hydrochemical, weathering and stripping. Catena 35 (1): 65-94. electrical and seismic approaches. Bull Eng Geol and Env 67: 123-128. TAYLOR RC, HOWARD KWF. 2000. A tectono- geomorphic model of the hydrogeology of deeply weathered crystalline rock: Evidence from Uganda. Hydrogeol J 8: 279-294.

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