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Philippine Journal of Science138 (2): 191-204, December 2009ISSN 0031 - 7683

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

*Corresponding author: [email protected]

Georesistivity Signature of Crystalline Rocksin the Romblon Island Group, Philippines

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 SciencesUniversity of the Philippines, Diliman, Quezon 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.

INTRODUCTIONIn the Romblon Island Group, attempts to provide potable water sources to the rural communities had been carried out under various programs. Some projects involved the drilling of wells to address the scarcity of water in rural communities. Unfortunately, the water wells were poorly located and scientific investigations were not done to determine the proper sites for the wells. As a result, water extracted from the dug wells were of poor quality (e.g., some showed fecal contamination and some wells went dry during the summer) (Asian Development Bank 1999).

Although the resistivity method has been around for several decades and has been widely used in the search for groundwater, very few papers have been published to

report the results of such investigations in the Philippines. Some works which used the electrical resistivity method include the resistivity survey done in Rizal, Philippines to constrain the thickness of sand and gravel deposits (Abarquez 1969). In Paoay, Ilocos Norte, the thickness and configuration of the sand dune aquifers were delineated through several electrical sounding points (Stirling Edwards and Gonzales 1984).

Based on the groundwater availability map of the Mines and Geosciences Bureau (1997), rocks found in different parts of the Philippines are characterized in terms of their suitability as aquifers and their potential for storing groundwater. Due to the nature of the underlying rocks, only a small area in Tablas Island is underlain by fairly productive aquifers (e.g.,

Armada et al.: Georesistivity of Crytalline Rocks in the Romblon Island Group, Philippines

Philippine Journal of ScienceVol. 138 No. 2, December 2009

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sedimentary units) with the rest of the areas in the three islands (Tablas, Romblon, and Sibuyan) being composed of rocks with limited potential or without any known significant groundwater. In 2004, the Local Water Utilities Administration (LWUA) conducted georesistivity surveys in coastal areas underlain by sedimentary and alluvial aquifers in Romblon Island to identify additional water sources. The sounding data revealed the presence of possible water-bearing layers which should be confirmed by subsequent drilling (LWUA 2004).

Crystalline rocks (particularly igneous and metamorphic rocks) are generally poor groundwater aquifers due to their lack of primary porosity and permeability. This makes groundwater exploration difficult in hard rock terrains, although groundwater accumulations may occur in crystalline rocks having limited secondary porosity acquired due to faulting, jointing and weathering (e.g., Owen et al. 2005; Dutta et al. 2006; Yadav and Singh 2008). These localized concentrations of groundwater within a crystalline formation become unconventional targets for water resource prospecting. Extraction of groundwater from the weathered and fractured portions of the crystalline bedrock is reported by Taylor and Howard (2000). These types of aquifer usually occur in the permeable zone overlying the unweathered crystalline bedrock (Taylor and Howard 1999). This potential groundwater resource, if tapped, will provide an important fresh water resource for areas underlain by hard rocks (e.g. Adepelumi et al. 2006; Owen et al. 2007; Surrette et al. 2007).

This paper discusses the results of the evaluation of hard-rocks within a collision zone as potential aquifers using electrical resistivity method. There has been relatively little work done on ophiolitic and metamorphic rock aquifers in the Philippines. This is one of the few georesistivity investigations carried out in collision zones and over hard-rock targets for groundwater sources in the region. The results obtained from this work suggest that the success rate of groundwater exploration in geologically complex areas such as collision zones and in crystalline, hard-rock areas may be improved by conducting georesistivity surveys prior to drilling.

Geologic Setting of the Romblon Island GroupThe Philippine Island arc system is an amalgamation of blocks of continent- and arc-derivation. The continent-derived block, the Palawan Micro-continental block, is a fragment of mainland Asia. This piece broke off during the opening of the South China Sea (Taylor and Hayes 1980; Hsu et al. 2004). As it was being translated southward,

this block collided with the Philippine Mobile Belt (Fig. 1). The interaction between these two blocks is responsible for several episodes of collision, subduction, and accretion in central Philippines. These processes produced the belt of metamorphic rocks and ophiolitic units in Mindoro Island, Romblon Island Group, and northwest Panay which delimits the extent of the arc – continent collision zone (Ramos et al. 2005; Yumul et al. 2003; 2005; 2008).

The Romblon Island Group in west central Philippines consists of features that attest to the Early Miocene collision between the Palawan Micro-continental Block and the arc-related Philippine Mobile Belt. The three big islands that make up the Romblon Island Group - Tablas, Romblon, Sibuyan - consist mainly of crystalline rocks (i.e., ophiolitic units and metamorphic, volcanic and intrusive rocks). The sedimentary sequences are found mostly in Tablas Island (Fig. 2). The distribution of these lithologic units offers some constraints on the possible occurrence of water-bearing units in the area.

The search for groundwater in the Romblon Island Group is made difficult by the fact that the islands are dominantly made up of crystalline bedrock (hard-rocks). Units of the Sibuyan Ophiolite Complex are exposed in Tablas and Sibuyan Islands. From bottom to top, the sequence is made of harzburgites and dunites, layered pyroxenites, layered and isotropic gabbros, diabase dike swarms, and basaltic to andesitic pillow lavas and flow deposits (Fig. 3a-3b). Units of the ophiolite are seen as tectonic slices bound by thrust faults which generally trend NE and dip NW (Fig. 2 ).

Aside from the ophiolitic units, there are other volcanic and intrusive units that are exposed in Tablas and Sibuyan Islands (Fig. 3c-3d). Andesite outcrops which are fractured and weathered mark the eastern coastline of Tablas Island. Diorite intrusions are also found in several localities in Tablas and Sibuyan Islands (Fig. 2). The outcrops are highly fractured and weathered especially in the exposure found in northern Tablas. The fractures generally trend NE, NW, and E-W with SE, NE, and N dips, respectively. Different varieties of metamorphic rocks were mapped in all three islands. These consist of mica, quartz-mica, quartzo-feldspathic, chlorite, and talc-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 places but the general foliation trends are NW (dipping 10-80°NE) and NE (dipping 20-70°NW).

Clastic sedimentary rocks are generally characterized as good aquifers, hence, most groundwater exploration work targets these rock types. In the Romblon Island Group, a significant portion of Tablas Island, especially the western



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.

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Figure 4a. Georesistivity data from a survey site in Tablas Island (black square in inset map). The low resistivity values are interpreted to correspond to the underlying fine-grained sedimentary rocks The zero elevation corresponds to the mean sea level.

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side, is underlain by the Early-Middle Miocene, Late Miocene-Early Pliocene, and Late Pliocene to Pleistocene clastic and carbonate sequences of the Binoog, Anahao, and Peliw Formations, respectively (Fig. 2). These are composed of massive to bedded limestone and bedded conglomerates, mudstones, sandstones, and siltstones (Fig. 3g-3h).

Faults related to the complex tectonic evolution of the area are mostly observed in the crystalline basement units. The most significant geologic structures in Tablas and Sibuyan Islands are the thrust faults that bound the slices of ophiolitic rocks. In Romblon Island, there are numerous faults and fractures which crosscut the island. NW- and SW-verging thrust faults, NE-striking normal faults, and a generally N-S oriented left-lateral strike-slip fault system were recognized during the mapping (Fig. 2). The fractured nature of the crystalline rocks attests to 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 AcquisitionIn 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 in Romblon Island, and Magdiwang and San Fernando in Sibuyan Island (Fig. 2).

In Tablas Island, the survey sites are approximately five kilometers away from the main population center. These areas lie within two adjacent drainage basins which occupy areas measuring 81 km2 (Anahao) and ~41 km2 (Poctoy) (Fig. 2). The survey site in Poctoy, situated between latitude 12°24’20”-25’12”N, and longitude 121°59’34”-122°00’30”E), is located close to the coast. Groundwater from the Anahao drainage basin drains into the Anahao River which empties northwestward into the Odiongan Bay (Fig. 4a). The survey site in this basin (12°22’05”-30”N and 121°57’40”-58’30”E) is located in the lower reaches of the Anahao River. Both survey sites are underlain by sedimentary rocks of the Anahao Formation, which consists of interbeds of conglomerates and calcareous to tuffaceous sandstones and mudstones (Fig. 3).

Metamorphic rocks, specifically marble and schists, underlie the survey areas in Romblon Island (Fig. 2). For



LWUA well no. ROM-ODI-1085

20 mbgs

0 mbgsStatic waterlevel at 7.6m

40 mbgs

60 mbgs

80 mbgs

Soil

Clays

Figure 4b. A well log from a drilling conducted by the LWUA in the study area provides constraints on the type of lithologies at depth as well as the static water depth.

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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the Bagacay survey site (12°34’33”-17”N, 122°15’55”-16’13” E), the groundwater basin from which the town of Romblon gets most of its potable water requirements covers an area of 3.5 km2. Schists underlie the survey 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 area of ~2.5 km2. 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 and sands. Bordering the survey area are outcrops of fractured schists.

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 which is the source of groundwater for the Magdiwang population center occupies an area of 80 km2. In the south, the Cantingas River forms part of the drainage basin which 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 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.

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most prominent structural feature in the area is the north-trending normal fault that bisects the area. Volcanic rocks underlie the area west of the normal fault and peridotites of the ophiolite complex consist the east area (Figure 6). In a hard-rock area similar to Sibuyan, recharge is very important in order to obtain a continuous water supply (Sharma and Baranwal 2005). The survey site is located within a large drainage basin system. Meteoric waters from the highlands infiltrate into the subsurface, thus, forming the recharge of the groundwater system in the area. The survey area in San Fernando in the southern part of Sibuyan Island (12°18’30” - 12°19’30”N and 122°34’00” - 122°35’22”E) is underlain by Quaternary alluvium, units of the Sibuyan Ophiolite Complex, and the Romblon metamorphic rocks. It is also situated near the intersection between a NW-trending normal fault and a nearly north-south curvilinear thrust fault (Fig. 2).

The Direct Current (DC) resistivity method, a non-invasive, non-destructive technique has been widely used for a variety of groundwater investigations such as identification of aquifer, determination of the depth of the water-bearing strata, geometry of the aquifer and delineation of fresh/salt water interface, among others (e.g., McNeill 1990; Sultan et al. 2008; Zouhri et al. 2008). The close association among electrical resistivity, rock type, and water content makes DC resistivity method the most suited technique in groundwater exploration (e.g., Subba Rao 2003; Sharma and Baranwal 2005).

Forty-two (42) vertical electrical sounding (VES) stations were occupied during the georesistivity survey which was done during the dry season in the month of April 2006. A Schlumberger array was used with a maximum spread of 300 meters. This allowed the delineation of vertical variations in electrical resistivities for depths up



Figure 6. Georesistivity data from the survey site in Magdiwang, Sibuyan Island (red square in inset). Areas near the normal fault display low resistivities.

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to 60 meters. The apparent resistivity values at depth were measured using a GEOTRADE GTR-3 Averaging Resistivity Meter. The orientation of the spread, wherever possible, was parallel to the strike of known geologic and structural features in the study areas. This is done to reduce the effects of lateral subsurface heterogeneity imparted by these structures (Lenkey et al. 2005).

Inverse models were generated from the apparent resistivity data using the WinSev 6.1 interpretation software developed by GeoSoft. The calculation of the theoretical curve uses the method described by Koefoed (1979) and Das and Verma (1980). In this method, a preliminary model is initially entered into the software. Using the least-squares method, the preliminary model



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is automatically adjusted to fit the field data. Based on available geologic information such as underlying lithologies, the thickness of layers based on available well data from the Local Water Utilities Administration (LWUA), and depths to the water-table from the National Water Resources Board (NWRB) database, the model was further constrained using these set of a priori information. These geologic constraints are considered during the modeling process in order to come up with the appropriate model. The processed vertical electrical soundings acquired at various points in the survey areas were then combined to come up with geoelectrical sections of the subsurface.

Vertical Electrical Sounding DataIn Anahao, Tablas Island, a two-layer subsurface is observed from the soundings conducted (Fig. 4a). A more resistive layer overlies a less resistive layer. The more resistive layer has resistivity values ranging from 4 to 6 Ω-m. The bottom layer is typified by resistivity values less than 3 Ω-m. The two layers delineated in the area may correspond to the fine-grained strata of the Anahao Formation, with the upper layer representing aerated beds and the less resistive layer the water-saturated layers. This interpretation is constrained by a well log from a drilling conducted by the LWUA in the area. Inspection of the lithologic log of well number ROM-ODI-1085 reveals two layers in the subsurface. A thick clay layer is overlain by ~10 meters of soil. Further, the observed groundwater depth of 7.6 meters in the drilled well agrees with the interpreted depth to the water saturated layer (Fig. 4b).

The interpretation of the subsurface rock layers from the georesistvity curves were constrained by a section log generated from the field survey of the area. In the Bagacay area, an uphill geologic traverse along a N-S section near the strike-slip fault reveals the weathering profile of the metamorphic rocks (Fig. 5b). Unweathered metamorphic rocks were observed at elevations of 20 meters above sea level (masl) up to ~45 masl. Sporadic layers and lenses of marble are intercalated with extensive outcrops of the chlorite schists. Above these areas, weathered chlorite schists were observed uphill. These weathered metamorphic rocks grade into red soil at elevations greater than 70 masl. An S-N traverse of an area farther east of the fault is characterized by relatively non-fractured schists. It is noticed from the section log of this traverse that a thinner regolith formed over this crystalline rock (Fig. 5b). In the N-S section of the Bagacay survey area in Romblon Island, the bottom layer is characterized by very high resistivity values of 1,000 to 2,000 Ω-m (Fig. 5b). This bottom layer is interpreted to correspond to the unweathered metamorphic basement rocks in the area.

The high resistivity layer is mantled by a less resistive layer having resistivity values that range from 50 to 250 Ω-m; this zone is believed to be the water-bearing weathered zone or regolith. A thin, relatively more resistive layer (350 to 400 Ω-m) corresponding to the dry regolith, in turn, overlies it. A very resistive layer (1500 Ω-m) encountered at an elevation of 70 to 80 masl is inferred to be a marble lens overlying the water-saturated regolith of the schist. From the interpreted section, it can be seen that the water-saturated regolith is located between elevations of 40 to 60 masl. Manifestations of this perched aquifer are the springs observed in the vicinity of the survey area at elevations of about 40 masl.

A southwest-northeast georesistvity section in Bagacay, Romblon is shown in Fig. 5. In this section, a relatively high resistivity layer is identified at a depth of 60 meters. This layer with a resistivity value of ~500 Ω-m is inferred to be the less weathered metamorphic basement. A thick regolith overlies the basement and is characterized by a wide range of resistivity values (3 to 150 Ω-m). This regolith has an average thickness of 35 meters. This is believed to be a function of the proximity of the sounding point to the NE-trending strike-slip fault. In the case of sounding points near the fault, the interpreted regolith is characterized by low values of 3 and 70 Ω-m. On the other hand, regolith which formed far from this structure is characterized by high resistivity values of 150 Ω-m and 120 Ω-m. An unsaturated regolith layer is interpreted from the overlying layer with resistivity values ranging from 350 to 360 Ω-m.

A normal fault is identified within the Magdiwang survey area in northern Sibuyan (Fig. 6). As a consequence of the fault, the nearby rocks are fractured. These fractures enhanced the porosity of the rocks and also intensified the action of chemical weathering in this portion of the study area. These characteristics of the underlying materials are evident in the low resistivity values observed in the soundings. West of the fault, relatively more resistive layers were identified with resistivity values of 200 Ω-m and 59 Ω-m (Fig. 6). Located east of the fault is another resistive layer observed having a resistivity value of 89 Ω-m. Overlying these layers are the less resistive layers with thickness varying from 30 meters on the west to 60 meters beneath the midsection of the survey site. Their resistivities range from 17 to 47 Ω-m.

The interpretation of the lithologies from the resistivities generated from the inversion of the apparent resistivity data are based on the geology of the area, available well data and on resistivity values of corresponding rocks from literature.



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DISCUSSIONSThe geologically-complex Romblon Island Group, which is located within a collision zone, is chiefly characterized by crystalline rocks. This impacts the availability of freshwater sources particularly in the islands of Romblon and Sibuyan which are underlain by metamorphic rocks and ophiolitic rocks, respectively. However, the occurrence of collision-related faults and fracture zones within these rocks provided secondary porosity and permeability that improves the groundwater potential in the area.

The sedimentary rocks located mainly in the Tablas Island are characterized by very low resistivity values as typical of clastic rocks, compared with crystalline rocks predominating the islands of Romblon and Sibuyan. This distinct difference in resistivity is attributed to elevated water content of sedimentary rocks which are natural good groundwater reservoirs because of their high porosity and permeability. On the other hand, crystalline rocks are characterized by high resistivities, several magnitudes greater than that of the sedimentary rocks. Localized occurrence of groundwater in hard-rock terrains is easily identifiable owing to the lower resistivities of these water-rich zones compared with typical high resistivities of crystalline rocks. Clay deposits in the area are typified by resistivities less than 4 Ω-m. These resistivities are less than the values of 14 to 37 Ω-m for clays reported elsewhere (e.g., Israil et al. 2006) (Table 1). This can be explained by the presence of water in the pores of the clay deposits and possibly a high concentration of ions in these pore waters.

The metamorphic rocks, specifically the chlorite-schists in Romblon Island are characterized by resistivity values of 1000 to 2000 Ω-m (Table 1). These resistivity values fall within the range obtained by Connell et al. (2000) for chlorite-schist (360-6600 Ω-m). Overlying the crystalline basement is the regolith with resistivities ranging from 3 to 300 Ω-m. These values are within the range reported for weathered and fractured rocks by other workers (e.g., Mondal et al. 2008) but greater than that reported by Owen et al. (2005) for regolith of schists (20–100 Ω-m). This may be explained by the difference in degree of weathering and also the amount of water within these layers. The weathering action of meteoric waters, which percolate through fractures in the crystalline rock, will have an effect on its electrical properties (e.g., Taylor and Howard 1998; 1999). This is evident in the lateral decrease of resistivities in sections proximal to mapped geologic structures in the area.

Resistivity signatures of the volcanic rocks and ultramafic rocks of the ophiolite in Sibuyan Island are interpreted from data obtained in Magdiwang. A relatively unweathered

portion of the volcanic rocks is inferred at a depth of approximately thirty meters below ground surface and is characterized by a resistivity of 200 Ω-m (Table 1). This resistivity is less than the >400 Ω-m reported for a similar terrain in Zimbabwe (e.g., Owen et al. 2005). This may imply that the volcanic rock at depth is slightly weathered and has elevated fluid concentration than usual. Furthermore, the area is transected by a normal fault that enhances the porosity of the underlying rocks. The ultramafic rock in the study area is characterized by slightly higher resistivities compared with the volcanic rocks. The ultramafic rocks with resistivities of ~800 to 900 Ω-m were encountered at about 40 to 50 meters below ground surface. An increase in the regolith thickness is also noted in areas approaching the known location of the fault in the survey area.

In Romblon Island, where suitable aquifers (e.g., Quaternary alluvium and sedimentary rocks) are not present, the search for water-bearing layers is focused on the metamorphic units that exhibit secondary porosity. The presence of geologic structures and the consequent weathering of the metamorphic units through the percolation of meteoric waters into fractures, as shown by this study, served to enhance the water-bearing capability of these units. Targeting the regolith as the potential water-bearing layer has yielded good results (e.g., MacDonald and Davies 2000; Louis et al. 2002). This has been proven by studies that propose greater groundwater flow in fissures in the upper layer (regolith) than in the fractured basement rock itself (e.g., Dewandel et al. 2005).

Due to recently increasing demand for water, more groundwater exploration activities are being directed at hard rock areas (e.g.. MacDonald and Davies 2000; Louis et al. 2002; Owen et al. 2005). The results of the georesistivity surveys in selected sites in the Romblon Island Group show the viability of finding groundwater aquifers in crystalline rock areas. In the RIG, the possibility of finding groundwater sources in the predominantly crystalline rocks that comprise the area is improved due to the area’s location within a collision zone. Being situated within a tectonically active, geologically complex area has led to the formation of faults and fractures within the crystalline rocks (e.g., Dewandel et al. 2005). In areas where aquifers with primary porosity are non-existent, groundwater exploration normally turn to fracture zones and faults in crystalline rocks. The presence of these faults and fractures provide secondary porosity which greatly improve the accumulation of groundwater in these rocks. Recently, however, more importance is given to the role of fractures and faults in creating extensive weathered zones (regolith). Fractures which reach greater depths in the bedrock help produce thick, permeable regoliths (e.g.,



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MacDonald and Davies 2000; Louis et al. 2002; Mondal et al. 2008). The weathered overburden is rendered porous which may enable it to store groundwater, hence, making it a possible aquifer (e.g., Kellett and Bauman 2004).

In areas underlain by crystalline rocks such as the Romblon Island Group, groundwater potential is low. The inherent low porosity and permeability in ophiolite rocks and their metamorphic basement contribute to the difficulty in locating probable aquifers in such areas. In this case, the tectonic setting of the area becomes a major consideration in groundwater resource evaluation. The presence of geologic structures (i.e., faults, fractures and joints), as a consequence of the tectonic activity in the area, provide secondary porosity and permeability to the crystalline rocks. In areas where these structures intersect or are concentrated, the secondary porosity and permeability of the rocks are greatly enhanced. Further

have low groundwater potential. However, due to brittle deformation during the emplacement of these crystalline terrains during the collision event, the rocks became fractured. These fractures, associated with the extensive faults cutting through the basement rocks of the Romblon Island Group, provided spaces within the crystalline rocks where groundwater accumulated and percolated. These zones of enhanced porosity and permeability are typified by localized decrease in resistivities within a resistive area attributed to elevated groundwater concentrations.

Groundwater percolated through the fractures within the ophiolitic and metamorphic units thereby inducing the eventual deep weathering of the crystalline basement. Interactions between the percolating waters and the rocks along the extensive fracture systems lead to the dissolution and disintegration of the rocks’ labile minerals. This process resulted in the formation of a more porous and permeable overlying weathered rock, the regolith. This correlation between intense fracturing and deep weathering is confirmed by the formation of thicker regolith over crystalline rocks proximal to a fault or intersection of faults. This is particularly evident in the area of Bagacay in Romblon Island, where thick porous regolith is formed over the intensely fractured schists. These voluminous regolith are characterized by high concentrations of groundwater. Thus, the main groundwater aquifers in crystalline areas like the Romblon and Sibuyan Islands are made up of the regolith as well as the intensely fractured portions of the crystalline basement. The results of the georesistivity surveys obtained from this study confirm the viability of finding groundwater aquifers in crystalline rock areas.

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Table 1. Variations of resistivity with rock type based on the georesistivity results from the Romblon Island Group. Literature values are shown for comparison (from Telford et al. 1976; Kellett and Bauman 2004; Mondal et al. 2008).

Rock typeResistivity values

(Ω-m)This study

Literature values (Ω-m)Various sources

Clay <1 – 7 1 – 100

Sand and gravel 55 – 60 10 – 800

Metamorphic rocks 500 – 2000 20 – 100,000

Dry soil 350 – 900 800 – 5000

Volcanic rocks 60 – 200 100 – 1000

Ultramafic rocks 80 – 800 3000 (wet) – 6500 (dry)

Weathered and fractured rocks 3 – 300 3 – 400

exposure of these fractured crystalline rocks to meteoric and groundwater percolating through fractures cause dissolution of labile minerals in the rocks, thus resulting in the formation of a more porous and permeable regolith.

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