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GEOMATICS AND ENVIRONMENTAL ENGINEERING
IDENTIFYING GEOLOGICAL FAULT STRUCTURES
USING GGMPLUS SATELLITE DATA AND DERIVATIVE
METHODS TO CHARACTERIZE MOUNT ENDUT
GEOTHERMAL SYSTEMS
VIA 3D-INVERSION GRAVITY MODELING
Abstract:
The geological map shows that the Mount Endut area possesses a
geothermal system, which is suggested by the presence of
geothermal surface manifestations: the Cika- wah and
Handeleum hot springs. The existence of a subsurface geological
fault struc- ture along the manifestations creates good
permeability for the geothermal reser- voir. The purpose of this
study was to utilize Global Gravity Model plus (GGMplus)
gravity satellite data to prove the existence of a geological fault
structure around the manifestation area with the first horizontal
derivative (FHD) and second vertical derivative (SVD) methods;
then, we developed a conceptual model of the geothermal system
from the 3D-inversion gravity method. Results show a cap
suspected of being clay, with a density of 2.52–2.58 g/cm
3
at
depth of 0 – 1250 m. The reservoir layer was suspected to be lava
rock with a density of 2.60–2.66 g/cm
3 at a depth of 1500 – 3000
m; also, the heat source layer was suspected to be an igneous
intrusion with a density of 2.70–2.72 g/cm
3
at depth of 1750–
3000 m.
of subsurface conditions [12, 13].
This research conducted a fault analysis that controlled the
manifestation of Cikawah and Handeleum and carried out 3D modeling using
the gravity method on satellite data that originated from GGMplus.
2. Materials and Method
The gravity method is widely used to identify the presence of
geologi- cal faults [20] as well as the rocks that make up a geothermal
system. However, identifying the presence of fault structures requires
supplementary techniques to offer a clearer representation, so further
methods are needed in order to provide a clear picture; namely, by using
the FHD and SVD methods to further refine fault- boundary detection.
This research was conducted to prove the existence of a fault that controlled
the
manifestation of Cikawah and Handeleum and carried out 3D-inversion
modeling
Identifying Geological Fault Structures Using GGMplus Satellite Data... 3
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using the gravity method on data that originated from GGMplus. Several
scientific and engineering applications require high-resolution and largely
complete gravi- ty knowledge; this is now available through GGMplus
gravity data [7]. A research flowchart is shown in Figure 1.
Fig. 1. Research flowchart
2.1. Geological Sefling
The morphology of the Mount Endut is classified into four units:
complex cone (35%), volcanic cone (25%), weakly undulating hill (32%),
Identifying Geological Fault Structures Using GGMplus Satellite Data... 5
5
is as follows: the Baduy sediment member unit (Tmd), the Bojongmanik
sediment member unit (Tmb), Andesitic intrusion (Ta), Pre-Endut
volcanic rock (Tlpe), Mt. Kendeng lava breccia (Tbr), Mt. Pilangranal lava
(Tlr), Diorite (Td), Granodio- rite (Tgr), Mt. Pilar breccia lava (Qbp), Mt.
Pilar lava (Qlp), Endut lava 1 (Qle1), En- dut Pyroclastic Flow (Qae), Endut
lava 2 (Qle2), Endut lava breccia (Qbe), Endut Lava 3 (Qle3), and Alluvium
(Qal). The geological structure of the Mount Endut area is manifested by the
presence of hill alignments (lineaments), volcanic cones, topo- graphic
alignments, triangular facets, fault scarps, joints, rock offsets, and fault mir-
rors (slickensides) as well as the emergence of geothermal manifestations
and al- tered rocks (Fig. 2). In several areas, Mount Endut has limestone
facies that generally deposit in marine environments. Based on the
geological map of the Leuwidamar Sheet, the limestone facies in the study
area are part of the Bojongmanik and Badui Formations and are in the Bogor
Physiographic Zone [22]. The geothermal manifes- tations of Mount Endut
are in the forms of hot springs that appear at several man- ifestation
locations in the Cikawah and Handeuleum areas (located at the western foot
of Mount Endut) [23].
Fig. 2. Geological map of Mount Endut
Source: [21]
modeling using SRTM (Shuttle Radar Topography Mission) global
topography. ERTM has a spatial scale with a spherical harmonic coefficient
of up to 2160 degrees, which is used in making GGMplus gravity maps on
short scales from 10 km to 250 m [25, 26].
In selecting GGMplus for the gravity-data analysis in this study, several
addi- tional factors beyond those that were mentioned before warranted
consideration. First, GGMplus offers a global gravity-field model that is
continuously updated, thus ensuring that the data reflects the most current
understanding of our gravita- tional variations. This is particularly important
in geothermal studies, where sub- tle changes in gravity can indicate the
presence of geothermal reservoirs or fault lines [27, 28]. Furthermore,
GGMplus incorporates a multi-resolution approach, thus allowing for the
integration of both high-resolution local data and broader re- gional data sets
[29]. This capability enables researchers to analyze gravity anoma- lies at
various scales, thus facilitating a more comprehensive understanding of the
geological context.
Identifying Geological Fault Structures Using GGMplus Satellite Data... 9
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Additionally, GGMplus provides access to a wealth of ancillary data,
including topographic and geological information (which can be crucial for
interpreting gravity anomalies of surface features) [6, 30]. The software also
supports advanced filtering techniques that can enhance the clarity of the
gravity data by minimizing noise and artifacts, thus improving the accuracy
of subsequent analyses [31]. Moreover, the user-friendly interface of
GGMplus allows for efficient data manipulation and visu- alization, making
it easier for researchers to communicate their findings and engage
interactively with the data [32]. Last, the strong community support and
extensive documentation that are associated with GGMplus foster a
collaborative environ- ment for researchers, thus enabling them to share
insights and methodologies that can enhance the overall quality of
geophysical research. These factors collectively underscore the rationale for
choosing GGMplus as a pivotal tool in the investigation of the geothermal
systems at Mount Endut.
Traditional gravity methods, which typically entail the collection of
gravity data from ground stations, can be labor-intensive and have limited
spatial coverage. In contrast, GGMplus integrates satellite data, thus
providing a more comprehen- sive view of gravitational anomalies across
vast areas without the need for extensive ground surveys. This capability is
crucial for the preliminary mapping and under- standing of geological
features before conducting more-invasive ground-based in- vestigations [33,
34]. For example, GGMplus data has been successfully utilized in studies
that have identified geothermal systems and fault structures, thus demon-
strating its effectiveness in enhancing traditional methodologies [27, 35].
2.3. Data Corrections
The data that was obtained when downloading from GGMplus was still
in the form of gravity disturbances and not yet in the form of a free-air
Identifying Geological Fault Structures Using GGMplus Satellite Data... 11
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Free-Air-Anomaly Correction
Free-air-anomaly correction has an influence that originates from a
gravity dis- turbance (δg). To get the free-air anomaly at a specific position,
it is necessary to correct any free-air anomalies.
The following is the formula for finding free-air-anomaly values [37]:
FAA 0.3086 h g FAC g (2)
where
: FAA – free-air anomaly [mGal],
FAC – free-air correction [mGal],
δ g – gravity disturbance value from GGMplus data [mGal],
h – altitude or elevation value that is obtained from geoid [m].
Terrain Correction
Terrain correction is a correction that is caused by the differences in
irregular topographical forms at measurement points, such as mountains,
hills, and valleys; this affects the gravity value that is obtained. For this
reason, terrain corrections are carried out in order to obtain a value that is
close to the rock configuration [33]. Terrain corrections can be obtained
using the Hammer chart method or DEM (digital elevation model) maps.
The calculations of terrain corrections in this study used Geosoft software by
entering the data in the form of coordinates ( X , Y ) and DEM.
The following is the equation for field corrections [38]:
TC G ( r
r )
where:
TC – field correction [mGal],
G – Newton’s gravitational constant [Nm
2
/kg
2
] ( G = 6.67430∙ 10
− 11
r
2 z
2
r z
Nm
2 /kg
2 ),
ρ – rock density [kg/m
3 ],
θ – angle formed [°],
r 1
- radius of inner circle [m],
r 2
- radius of outer circle [m],
z – altitude/depth of field [m] (an absolute difference between the
station
elevation and the average elevation ( z = | z stations
z average
This equation estimates the gravitational effect of the terrain in each
sector of the Hammer chart; the total terrain corrections are obtained by
summing up the con- tributions from all of the sectors. The average altitude
within a single compartment is estimated from the contour lines within this
compartment and then subtracted from the station altitude. The difference in
altitude ( z ) is used to calculate the terrain- correction contribution for each
compartment. To obtain the total terrain correction, we summed up all of the
contributions from the innermost sector to the outermost sector, resulting in
the final terrain correction value.
Complete Bouguer Anomaly Correction
Complete Bouguer anomaly (CBA) correction is the sum of the simple
Bouguer anomaly and terrain corrections. CBA describes the density
conditions below the surface of the study area (which later requires
separations between the regional and residual anomalies for further analysis)
[39]. The following is the equation for ob- taining CBA correction:
CBA SBA TC (6)
where
: CBA – complete Bouguer anomaly
[mGal], SBA – simple Bouguer
anomaly [mGal], TC – terrain
correction [mGal].
Identifying Geological Fault Structures Using GGMplus Satellite Data... 15
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z (^) 0
2.4. Energy Spectrum Analysis
Spectrum analysis is performed to separate regional and residual
anomalies so that the estimated depth of an anomaly can be determined.
This analysis is done with the Fourier transform, which is used to convert
time-domain data into a fre- quency domain. Systematically, the spectrum
value results from the gravitational potential derivative in the horizontal
plane. The Fourier transform can be written with the following equation
[39]:
F [ g ] 2 G e
| k |( z 0
z )
with z′ > z (7)
where:
F [ g z
] – Fourier transform of gravitational acceleration derivative
(vertical component) [s
G – Newton’s gravitational constant [Nm
2 /kg
2 ] ( G = 6.67430 ∙ 10
− 11
Nm
2
/kg
2
),
μ – body mass [kg],
z 0
- height of measurement point [m],
z ′ – anomaly depth [m].
The processing of the energy-analysis data uses Oasis Montaj software
to pro- duce a radially averaged power spectrum (RAPS) curve from the
Fourier transform process, which is displayed in the logarithmic value of the
normalized energy spec- trum at each radial frequency value. To carry out a
depth analysis with wave num- ber ( k ) and amplitude ( A ) values, it can be
done to separate the regional and resid- ual anomalies by determining the
cutoff area of the existing spectrum analysis [39].
2.5. Bandpass Filters
A gravity anomaly is the sum of the sources of an anomaly under a
surface [40]; so, it is necessary to separate the regional anomaly, residual
Identifying Geological Fault Structures Using GGMplus Satellite Data... 17
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of the geological structure of the gravity anomaly. The FHD-value equation is
ob- tained from the following equation [43]:
FHD (8)
where ∂ g /∂ x and ∂ g /∂ y are the first derivatives of the gravity anomaly in the
x and y directions, respectively. In cross-sectional modeling, only the x
direction is used; so, this formulation is as follows:
g
FHD
x
(9a)
This equation can be written as follows:
g
g ( i 1)
g i
,
x x
g g
FHD
( i 1) i
x
(9b)
where
: FHD – first horizontal
derivative,
g – anomalous value [mGal],
∆ x – difference in distance along the track [m].
Second Vertical Derivative (SVD)
SVD is used in interpreting structures that are insufficient in the Bouguer
g
2
(^) g
2
(^)
x
y
anom- aly map to separate the shallow and deep structural effects that result
in regional and residual anomalies. A residual anomaly describes a structure
that is close to the surface but has not provided specific results; therefore, a
second vertical descent is needed for a more specific structural effect [44].
This second vertical descent is a high-pass filter that provides a clear
picture of the residual anomalies in shallow structures; therefore, SVD is
used to identify the type of down fault or up fault. In the SVD method, the
existence of a fault boundary is indicated by an SVD value of zero (or close
to zero). The SVD equation is obtained through the horizontal deriv- ative
using the Laplace equation for gravity anomalies; namely, the following
[18]:
g 0 ,
2
g
2
g
2
g
x
2 y
2 z
2
