![Geologic map of research area [17].](https://www.researchgate.net/profile/Ulva-Irfan/publication/347060289/figure/fig1/AS:973928314974211@1609214013963/Geologic-map-of-research-area-17_Q320.jpg)
Content uploaded by Ulva Ria Irfan
Author content
All content in this area was uploaded by Ulva Ria Irfan on Dec 29, 2020
Content may be subject to copyright.
IOP Conference Series: Earth and Environmental Science
PAPER • OPEN ACCESS
Geology and 2D modelling of magnetic data to evaluate surface and
subsurface setting in Bongongoayu geothermal area, Gorontalo
To cite this article: I N Manyoe et al 2020 IOP Conf. Ser.: Earth Environ. Sci. 589 012002
View the article online for updates and enhancements.
This content was downloaded from IP address 180.249.26.126 on 25/11/2020 at 06:20
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
1
Geology and 2D modelling of magnetic data to evaluate surface
and subsurface setting in Bongongoayu geothermal area,
Gorontalo
I N Manyoe1, D A Suriamihardja2 U R Irfan3 S S Eraku4 S S S Napu1 and D D
Tolodo1
1Geological Engineering, Universitas Negeri Gorontalo, B.J. Habibie Street, Bone
Bolango, Gorontalo, 96119 Indonesia
2Geophysics, Universitas Hasanuddin, Perintis Kemerdekaan Street, Makassar, South
Sulawesi, 90245 Indonesia
3Geological Engineering, Universitas Hasanuddin, Malino Street, Gowa, South
Sulawesi, 92119 Indonesia
4Geography Education, Universitas Negeri Gorontalo, B.J. Habibie Street, Bone
Bolango, Gorontalo, 96119 Indonesia
Corresponding Author: intan.manyoe@ung.ac.id
Abstract. Bongongoayu is one of the regions in Indonesia that has geothermal potential.
Bongongoayu requires surface and subsurface data to support the preliminary data. This research
aims to determine surface and subsurface data conducted by geology and magnetic method. The
surface data, including geomorphology, lithology, hydrology and manifestation. The subsurface
data have taken by the magnetic method. The result showed that the geothermal manifestation
of Bongongoayu is a hot pool. The surface temperature is 43 to 59 ºC. The geomorphology units
is composed of volcanic hills unit and lake plains unit. The lithology of the research area is
composed of granite and alluvial deposits. Based on petrographic analysis, the level of alteration
in granite rocks is 65% and is classified as moderate alteration. The recharge area is in the north
and southwest of the research area. Discharge area is in the central area. Based on 2D magnetic
modeling, there are two subsurface layers. The first layer is alluvial and the second layer is
granite. Rocks that are under the alluvial layer and have been altered are interpreted as a cap
rock. 2D magnetic models show normal faults in the research area as a controlling factor for
geothermal fluid.
1. Introduction
Indonesia affected by three major plates, i.e. SE-Asian Plate, the Indo-Australian Plate, and the Pacific
Plate [1,2]. Sulawesi is formed in the convergence zone of the Eurasian, Pacific, Australian plate
complexes [3] and affected by Philippines plate [4]. Northern part of the North Arm Sulawesi is the
Sulawesi Sea Subduction. Sulawesi Sea Subduction was thought to have been active since the Early
Tertiary and produced Tertiary volcanic arcs that stretch from around Toli-Toli to Manado [5].
The reconstruction in Sulawesi Sea shows that the volcanic arc system surrounding the Sulawesi Sea
has been established since the Late Miocene [6]. Volcanism on North Arm Sulawesi is related to
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
2
Sulawesi Sea Subduction [3]. Geothermal associated with an area of active volcanic [7]. Besides being
associated with active volcanoes, geothermal is also associated with faults [8,9] and intrusions [10].
The volcano-tectonic conditions of Indonesia cause Indonesia, especially the North Arm of Sulawesi,
to have big geothermal potential. This big geothermal potential is being developed by the government
to support national energy. Based on a recent survey from the Geological Agency, the Ministry of
Energy and Mineral Resources [10], there are 331 potential areas that have been identified consisting of
11,073 MW of resources and 17,506 MW of reserves spread across 30 provinces.
Areas identified as having geothermal potential will be explored through scientific survey methods.
The scientific survey method can be in the form of geological and geochemical studies. In addition to
more detailed geological and geochemical studies, various geophysical techniques can be used,
including geomagnetic methods [11].
Geological surveys are preliminary surveys carried out in geothermal exploration by analyzing rocks,
determining morphological characteristics, hydrological conditions, and geothermal systems.
Geophysical survey in the form of magnetic, electrical and other subsurface surveys.
The geomagnetic method utilizes the contrast magnetization of subsurface rocks [12]. In geothermal
areas, geothermal fluids can cause massive changes in the chemical and physical properties of
subsurface rocks. Another change is the magnetic properties of rocks will be down or lost due to heat
generated [13].
Geological surveys have been conducted in several geothermal areas in Gorontalo, one of which is
in the Lombongo geothermal area [14]. Surveys that have been conducted in Bongongoayu are electrical
[15] and magnetic [16] surveys, except that they have not provided surface geology and subsurface
magnetic models. Based on the explanation above, it is very interesting to study the Bongongoayu
geothermal area. This study aims to determine the characteristics of surface and subsurface in the
Bongongoayu geothermal area based on geological and magnetic data.
2. Data and Method
The study area is located at coordinates 0.6230 – 0.7890 N and 122.6940 – 122.4510 E, administratively
included in the Bongongoayu area (also known as Diloniyohu area), Boliyohuto District, Gorontalo
Regency, Indonesia (Figure 1). The research area is located in a depressed area that extends in the central
part of Gorontalo.
Figure 1. Geologic map of research area [17].
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
3
The instruments used in the study are GPS receivers, Proton Precession Magnetometer GSM 19-T
v7.0, Nikon ci-POL Microscope, infrared thermometer, geological hammer and geological compass.
The GPS receiver is to determine the location of base station, mobile stations and observation stations.
One set of Proton Precession Magnetometer GSM 19-T v7.0 is to measure and determine the distribution
of magnetic anomalies. Nikon ci-POL Microscope is to determine mineral composition of rocks.
Infrared thermometer to measure the temperature of geothermal manifestation. Geological hammer is to
take rock samples and a geological compass is to determine direction.
2.1 Data collection and acquisition
Based on the regional geological map, rocks formation in the research area are composed of Bone
Diorite (Tmb), Boliyohuto Diorite (Tmbo), Dolokapa Formation (Tmd), Pinogu Volcanics (Tqpv), and
the Lake Deposit (Qpl). The hot pool is located in the Lake Deposit (Figure 1). The geological data
collection is carried out to determine rock units in the research area. Rock samples were taken in the
research area with the aim to determine the composition of rock minerals. the geological data collection
was also carried out to determine the geomorphological and hydrological data of the research area.
Determination of geomorphological units by conducting field surveys of landforms.
The acquisition data of magnetic is carried out to obtain primary data. A set of Proton Precession
Magnetometer GSM 19-T v7.0 is used for measurements the magnetic observation data of base station
and mobile stations. Acquisition data of magnetic is carried out with a closed loop system (A, B, A), i.e.
measurements always start and end at the same point.
2.2 Processing, analysis and interpretation data
Thin section of rock samples was analyzed in a laboratory using a Nikon Ci-POL Microscope. Analysis
petrography using IUGS (1979) classification. Geomorphology data analysis using Van Zuidam
classification (1985). Hydrological analysis of the research area was conducted to determine the
recharge area and discharge area. This analysis, using field data, topographic data, and lithology.
Magnetic data processing begins by correcting the measurement data in base station and mobile
stations with diurnal variation and International Geomagnetic Reference Field (IGRF) correction.
Magnetic modeling of subsurface data is carried out after data correction. Modeling of subsurface
magnetic data is a continuation of magnetic studies that were not carried out in previous studies [16].
Previous studies have only mapped total magnetic field anomalies without subsurface modeling.
Interpretation of magnetic data uses rock susceptibility values as provided by Telford (1990).
3. Result and Discussion
3.1. Geomorphology
Geomorphological studies in a geothermal field are intended to systematically organize landforms.
Based on Van Zuidam classification [18], the Bongongoayu geothermal area is separated into two
geomorphological units. Two geomorphological units in the Bongongoayu geothermal area are volcanic
hills (Figure 2) and lake plain (Figure 3).
Volcanic hills units occupy the northern and southwestern part of the research area. The elevation of
the volcanic hills unit is 28 to 687.5 meters above sea level. The hills which are located in the north are
part of the north mountain which consists of Mount Tamboo, Mount Dolokapa and Mount Boliyohuto.
The hills in the southwest are part of the Loba Mountains. This geomorphological unit consists of
granite, diorite and soil. On the lower slopes, local people use it as an agricultural area.
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
4
Figure 2. Volcanic hills unit.
Figure 3. Lake plain unit.
The geomorphological unit of the lake plain is composed of alluvial. This unit occupies the central
to southern part of the research area. The elevation of the lake plain unit is about 25 meters above sea
level. Lake plain is part of the depression that stretches from east to west Gorontalo. The unit of landform
of the lake deposit is located in the western part of the depression. Land use in this unit is for settlement,
agriculture/plantation and fisheries.
3.2. Lithology
Bongongoayu geothermal manifestations are hot pool, found in alluvial units. Alluvial units are located
in the southern part of the research area. Alluvial is concentrated around geothermal manifestations
extending to the south, east and west of geothermal manifestations. The color of alluvial deposits in the
research area is blackish brown. The northern part and southwestern part of the research area is
composed of granite units. Granite units are composed of granite and diorite.
Granite is located in the northern part and southwestern part of the research area (Figure 4). Rock
texture is holocrystalline and composed of orthoclase, quartz and plagioclase. Secondary minerals found
in granite rocks are quartz, clay minerals and opaque minerals. The mineral form is anhedral to
subhedral.
Figure 4. Granite (photo direction N 70⁰ E).
Figure 5. Carbonate vein in diorite (photo
direction N 40 ⁰E).
Diorite is located in the northern part of the research area on volcanic hills units. Rock texture is
holocrystalline and composed of orthoclase and quartz. Secondary minerals found in diorite are clay
minerals and opaque minerals. The form of minerals is anhedral and subhedral. Vein Carbonate is found
in diorite (Figure 5).
Microscopic analysis of rock samples in the study area shows that rock samples are alteration
igneous rocks. Rock structure type is massive. Rocks in the study area are granite based on petrographic
analysis of rock samples in the research area.
The mineral composition of rocks in the research area is shown in Table 1. Microscopic analysis of
rock samples in the study area showed that the secondary mineral composition was at least 45% and the
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
5
highest was 65%. Primary mineral forming rocks in the study area are orthoclase, quartz, and
plagioclase. Secondary minerals found in rocks are quartz, clay minerals and opaque minerals.
Table 1. The mineral composition of the rocks in the Bongongoayu geothermal area.
Station
Rocks
Mineral Composition (%)
Primary Mineral
Secondary Mineral
Ort
Qz
Pl
Qz
Cl
Opq
SR.a
Granite
20
10
10
35
15
10
SR.b
Granite
20
10
15
25
15
15
SR.c
Granite
10
20
25
30
10
5
SR.d
Granite
20
15
10
35
15
5
SR.e
Granite
10
25
-
45
15
5
SR.f
Granite
20
10
10
35
15
10
Note: Ort = Orthoclase; Qz = Quartz; Pl = Plagioclase; Cl = Clay Mineral; Opq = Opaque Mineral.
The composition of primary minerals at SR.a is 40%, while the secondary mineral composition is
60%. Primary mineral composition at SR.b is 45%, while secondary mineral composition is 55%. SR.c
has 55% of primary minerals while secondary minerals are 45%. SR.d primary mineral composition is
45%, while secondary minerals 55%. SR.e primary mineral composition is 35%, while secondary
minerals 65%. SR.f shows the composition of primary minerals is 40%, while secondary minerals is
60%.
Figure 6. Photomicrograph of sample in the study area. Sample SR.a up to SR.f are granite.
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
6
Alteration intensity is defined as the level change based on the percentage of secondary minerals.
Morrison [19] provides alteration intensity based on the percentage of secondary minerals. Not altered,
if no secondary mineral. Weak altered if secondary mineral is 25%. Moderate altered if the secondary
mineral percentage is about 25 to 75% and strong altered if secondary mineral is >75%.
If all primary minerals are altered (except quartz, zircon and apatite) but the primary texture is still
visible, the intensity of the altered is called intense altered intensity. The total altered intensity is all
primary minerals are changed (except quartz, zircon and apatite) and the primary texture is not visible.
Microscopic analysis of samples in the research area showed that the secondary mineral composition
was at least 45% and the highest was 65% (Figure 6). Based on the classification given by Morrison
[19] the intensity of rock altered in the study area is moderate altered.
3.3. Hydrology
Annual rainfall in the research area ranges from 1244 to 2311 mm per year. The highest monthly rainfall
in eight years generally occurs in May and the lowest monthly rainfall generally occurs in August.
Cumulative rainfall a month with rainfall 90 mm occurred in August and September while the
cumulative monthly rainfall >100 mm occurred in October – July [20]. The level of rainfall can affect
the height of the water table [21]. Thus, high rainfall in the research area can affect groundwater
availability.
The northern part of the Bongongoayu geothermal area is a protected forest area, production forest
and Nantu reserved forest [22]. Vegetation in the forest can accelerate the process of rainwater
infiltration. Plant roots forest vegetation opens the way for rainwater to be infiltrated. The infiltration
process is one of the determinants of water availability.
The next determining factor is the shape of the landscape and permeability. According to Noor [23],
water will flow faster on steep slopes than moderate sloping. It means that the rainwater infiltration
process takes place in the northern part of the study area. The nature of rock that can carry water is
another determining factor for infiltration. The fractures in the rocks in the northern part of the
Bongongoayu geothermal area [17] are potential infiltrations. Rainwater infiltrates into the soil through
fractures in rocks.
According to Rezky et al. [24] shallow groundwater conditions can be seen from the presence of
large and watery rivers throughout the year. The rivers surrounding the Bongongoayu geothermal area
are the Bongo River, Diloniyohu River, Buliya River and Paguyaman River. Bongo River, Diloniyohu
River and Buliya River are watery rivers throughout the year and head into the Paguyaman River.
Recharge areas in the Bongongoayu geothermal area are in the northern part of the Bongongoayu
geothermal area which has moderate slopes and rocks that have fractures. Discharge area is located in
the central part, which is located in the lake plain unit. The hydrological conditions in the research area
have caused a large amount of surface and groundwater reserves in the Bongongoayu geothermal area.
According to Marshak [21] on a local scale, a recharge area occurred at the top of a hill and discharge
area in the valley. This means that on a local scale, the recharge area in the research area is on volcanic
hill near from hot pool. Discharge area is in the valley where geothermal manifestations occur.
3.4. Geothermal manifestation
Geothermal manifestation in the Bongongoayu area is hot pool (Figure 7). The hot pool is located at
coordinates 0.67785 N and 122.5911 E. The hot pool is located at an altitude of 24 meters above sea
level.
The research area consists of granite units and alluvial units. Granite and diorite are included in
granite units. Hot springs are located in alluvial units. The northern part of the hot pool is the granite
and diorite volcanic hills. The wide area of a hot pool is + 600 m2.
Measurement of surface temperature in hot pool using an infrared thermometer. The surface
temperature of the hot pool ranges from 43 to 59 ⁰C. Physical conditions of hot pool are tasteless and
odorless. The appearance of the color of hot water is clear with a neutral pH 6.
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
7
Figure 7. Hot pool as a geothermal manifestation in research area.
3.5. Magnetic model
The 2D modelling of magnetic data is shown in Figure 8 and Figure 9. The first model is in the Northeast
part of the research area stretching from Northwest-Southeast. The second model is in the Southwestern
part of the research area stretching from Northeast-Southwest.
Figure 8. 2D modelling of first anomaly.
There are two layers in the first model consisting of alluvial and granite. The total layer depth in the
model is 62.5 m. The first layer has 0.0118 susceptibility value. The first layer is at a depth of 0 to 62.5
m. Based on the value of susceptibility, the first layer is interpreted as alluvial. The second layer has
0.5440 susceptibility value. The second layers are at a depth ranging from 62.5 m. Based on the value
of susceptibility, the second layer is interpreted as granite.
In the second model, there are two layers. The first layer has a value of 0.0118 susceptibility value.
The first layer is at a depth from 0 to 43.75 m. Based on the value of susceptibility, the first layer is
interpreted as alluvial. The second layer has a value of 0.5440 susceptibility value. The depth of the
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
8
second layer starts at a depth of 43.75 m. Based on the value of susceptibility, the second layer is
interpreted as granite. Lithology and susceptibility value refer to Telford et al. [25].
Figure 9. 2D modeling of second anomaly.
2D magnetic models show that alluvial deposits are surface layers. The layers beneath the alluvial
are granite. Some minerals in granite rocks have been altered. The altered part of Granite just below the
alluvial layer is interpreted as a cap rock. 2D magnetic models also show the geological structure.
Magnetic modeling show that the structure contained in the study area is a normal fault. Geothermal
fluids from reservoirs exposed to the surface are controlled by this fault.
4. Conclusion
Geomorphology of the research area is volcanic hills and lake plains. The lithology of the study area is
composed of alluvial and granite. The alteration intensity in the research area is included in moderate
rock alteration intensity. Recharge area is located in the northern part, while the discharge area is in the
valley. The valley is located in the central part of the research area, where hot pool appears. The surface
temperature of hot pool ranges from 43 to 59 ⁰C. The physical condition of water in a hot pool is tasteless
and odorless. The color appearance of hot pool is clear with neutral pH 6. Based on the 2D model, there
are two layers, alluvial with susceptibility value 0.0118 and granite with susceptibility value 0.5440. 2D
models show that the normal fault in the research area is a controlling factor for geothermal fluids.
5. References
[1] Katili J A 1980 Geotectonics of Indonesia, a modern view (Jakarta: Directorate General of Mines)
pp 271
[2] Kreemer C, Holt W E, Goes S and Govers R 2000 Active deformation in eastern Indonesia and
the Philippines from GPS and seismicity data J. of Geophys. Res. 105 663-680
[3] Carlile J C, Digdowirogo S and Darius K 1990 Geologic setting, characteristics and regional
exploration for gold in the volcanic arcs of North Sulawesi, Indonesia. In: Hedenquist, J.W.,
White, N.C., Siddeley, G. (Eds.), Epithermal Gold Mineralization of the Circum Pacific:
Geology, Geochemistry, Origin and Exploration I. J. Geochem. Explor 35 105– 140
[4] Verstappen H T 2010 Indonesian landforms and plate tectonics Indonesian Journal on
Geoscience 5 197-207
[5] Simandjuntak T O 1986 Sedimentology and tectonics of the collision complex in the East Arm of
Sulawesi Indonesia (United Kingdom: University of London, Royal Holloway and Bedford
New College)
[6] Lee T Y and Lawver L A 1995 Cenozoic plate reconstruction of Southeast Asia Tectonophysics
251 85-138
[7] Wilson M E and Moss S J 1999 Cenozoic palaeogeographic evolution of Sulawesi and Borneo
Palaeogeography, Palaeoclimatology, Palaeoecology 145 303-37
[8] Sukamto R 1973 Reconnaissance geologic map of Palu Area, Sulawesi — scale 1:250,000
(Bandung: Geol. Res. and Dev. Cent)
TREPSEA 2018
IOP Conf. Series: Earth and Environmental Science 589 (2020) 012002
IOP Publishing
doi:10.1088/1755-1315/589/1/012002
9
[9] Boedihardi M, Mulyono A, Ginting A, Mosby M D and Radja V T 1992 Geology, energy
potential and development of Indonesia’s geothermal prospects. In: Tectonic Framework and
Energy Resources of the Western Margin of the Pacific Basin Bull. Geol. Soc. Malays. Spec.
Publ. 33 369–385
[10] Kementerian Energi dan Sumber Daya Mineral 2017 Potensi panas bumi Indonesia jilid 2
(Jakarta: Kementerian Energi dan Sumber Daya Mineral)
[11] Árnason K and Gíslason G 2009 Geothermal surface exploration Makalah disajikan dalam Short
Course on Surface Exploration for Geothermal Resources (El Salvador: United Nations
University Geothermal Training Programme and LaGeo) p 17-30
[12] Gupta H K and Roy S 2006 Geothermal energy: an alternative resource for the 21st century
(United Kingdoom: Elsevier)
[13] Arsadipura S, Kholid M, Djukardi D 2011 Penyelidikan geofisika terpadu gaya berat, geomagnet
dan geolistrik daerah panas bumi Kepahiang, Kecamatan Kaba Wetan, Kabupaten
Kepahiang, Provinsi Bengkulu Proc. Hasil Kegiatan Pusat Sumber Daya Geologi Tahun
2010
[14] Manyoe I N 2017 Kajian geologi daerah panas bumi Lombongo Kabupaten Bone Bolango
Provinsi Gorontalo Jurnal Geomine 5 10-18
[15] Manyoe I N 2016 Model inversi data geolistrik untuk penentuan lapisan bawah permukaan daerah
panas bumi Bongongoayu, Gorontalo Jurnal Sainstek 8 358-371
[16] Manyoe I.N, Irvan U R and Suriamiharja D A 2015 Distribusi anomali magnetik daerah panas
bumi Bongongoayu, Gorontalo Proc. Seminar Ilmiah Nasional. Universitas Hasanuddin
[17] Bachri S, Sukido and Ratman N 1993 Peta geologi lembar Tilamuta, Sulawesi (Bandung: Pusat
Penelitian dan Pengembangan Geologi)
[18] Zuidam R A Van and Zuidam F I van Cancelado 1985 Aerial photointerpretation in terrain
analysis and geomorphologic mapping (Enschede: ITC)
[19] Morrison K 1997 Hydrothermal minerals and their significance (Auckland: Geothermal and
Mineral Service Division of Kingston Morrison Ltd)
[20] Stasiun Meteorologi Jalaluddin Gorontalo 2014 Data curah hujan daerah Gorontalo tahun 2005
– 2013 (Gorontalo: Badan Meteorologi, Klimatologi dan Geofisika Stasiun Meteorologi
Jalaluddin Gorontalo)
[21] Marshak S 1955 Earth portrait of a planet (New York: W.W Norton and Company)
[22] Direktorat Bina Rencana Pemanfaatan Hutan Produksi 2010 Peta sebaran spasial IUPHHK
HA/HT/HTR (Jakarta: Kementerian Kehutanan)
[23] Noor D 2011 Geologi untuk perencanaan (Yogyakarta: Graha Ilmu)
[24] Rezky A, Hasan A R and Dirasutina S 2005 Penyelidikan geologi daerah panas bumi Suwawa
Kabupaten Bone Bolango – Gorontalo Presented in Kolokium Badan Geologi 2005 Pusat
Sumber Daya Geologi Badan Geologi.
[25] Telford W M, Geldart L P and Sheriff R E 1990 Applied geophysics (Cambridge: Cambridge
University Press)
Acknowledgments
We would like to express our deep gratitude to KEMENRISTEKDIKTI for the research fund. We would
like to extend our thanks to the staff of the Geological Engineering Laboratory and Geography
Laboratory of Universitas Negeri Gorontalo for their help in acquisition data of magnetic.
