Georesistivity Signature of Crystalline Rocks in the Romblon Island

Transcription

Philippine Journal of Science
138 (2): 191-204, December 2009
ISSN 0031 - 7683
Georesistivity Signature of Crystalline Rocks
in 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
1
Tectonics and Geodynamics Group, National Institute of Geological Sciences
University of the Philippines, Diliman, Quezon City, Philippines 1101
2
Department 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 preexisting 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
In 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
*Corresponding author: [email protected]
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.,
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Vol. 138 No. 2, December 2009
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 Group
The Philippine Island arc system is an amalgamation of
blocks of continent- and arc-derivation. The continentderived 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,
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Armada et al.: Georesistivity of Crytalline Rocks
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 talcchlorite 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.
195
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.
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
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
in Romblon Island, and Magdiwang and San Fernando in
Sibuyan Island (Fig. 2).
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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.
Static water
level at 7.6m
0 mbgs
Soil
20 mbgs
40 mbgs
Clays
60 mbgs
80 mbgs
LWUA well no. ROM-ODI-1085
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.
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 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 northtrending 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).
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The Direct Current (DC) resistivity method, a noninvasive, 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.
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 Data
In 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.
200
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.
DISCUSSIONS
The 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 waterbearing 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
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 type
Resistivity values
Literature values (Ω-m)
(Ω-m)
Various sources
This study
Clay
Sand and gravel
<1 – 7
1 – 100
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.
CONCLUSIONS
The collision between the Palawan Micro-continental
block and the Philippine mobile belt preserved features
in central Philippines, particularly in the Romblon
Island Group, which include ophiolitic units, intrusive
and volcanic rocks, metamorphic rocks. Sedimentary
sequences are limited only to Tablas Island. The
crystalline rocks that dominate the Romblon Island
Group, i.e., ophiolitic and metamorphic rocks, originally
202
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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204

Georesistivity Signature of Crystalline Rocks in the Romblon Island

Transcription

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