KPJ 10.
KAPPA JOURNAL
Physics & Physics Education https://e-journal.
id/index.
php/kpj/index Identification Of Sediment Thickness Using The Geomagnet Method in Beleka and Dasan Tapen Villages.
Gerung District.
West Lombok Regency Izza Aulia Putri1*.
Suhayat Minardi2.
Syamsuddin3* Physics department.
Faculty Mathematics and Natural Science University of Mataram.
Indonesia Received: 30 July 2025 Revised: 06 March 2026 Accepted: 08 March 2026 Corresponding Author:
Izza.
Aulia Putri syamsuddin@unram.
A 2026 Kappa Journal is licensed under a Creative Commons AttributionNonCommercial-ShareAlike 4.
International License DOI:
https://doi.
org/10.
29408/kpj.
Abstract: Gerung is one of the sub-districts in West Lombok Regency.
Based on data from BNPB the Gerung sub-district area is an area that is vulnerable to the natural phenomenon of earthquakes, because the soil in this area has a softer texture so that when vibrations occur, the vibrations will be stronger.
Geologically, this alluvium formation is very vulnerable to the natural phenomenon of earthquakes because it is related to the thickness of the sediment.
This research aims to determine the thickness of sedimentary rocks in the Gerung District.
One of the Geophysical Methods used in this research is the Geomagnetic Method.
Based on the 2-Dimensional modeling of the AA' - FF' Incision, it consists of five rock layers, namely the first layer includes Silt Rock, the second layer includes Sand Rock, the third layer includes Clay, the fourth layer includes Breccia Rock and the fifth layer includes Lava Rock.
The composition of the five rock layers, layers including sedimentary rock is found in the first layer to the fourth layer.
The results of the research for sediment thickness from the six incisions ranged from 405 m Ae 485 m.
Based on the results of this research, it can be concluded that Beleka Village and Dasan Tapen Village it is composed of five rock layers with a dominant sediment thickness of 485 m, namely at the AA' incision in the western part of Beleka Village.
Keywords: Earthquake.
Sediment Thickness.
Geomagnetic Method.
Anomaly Value.
Lithological Structure and Susceptibility Introduction Lombok Island is one of the areas prone to natural phenomena of earthquakes.
Lombok Island is flanked by the subduction of the Australian Plate in the south and the Australian Flores fault in the north.
The Flores fault extends from the west-east north of Bali Island to the north of the island in Nusa Tenggara.
As a result of the pressure exerted by the subduction of the Australian Plate south of Lombok Island, the longestablished Flores fault reactivated, which is a sudden re-movement that resulted in devastating shocks on Lombok Island in August 2018.
Administratively.
Lombok Island consists of five regencies or cities, namely.
East Lombok.
Central Lombok.
North Lombok.
West Lombok, and Mataram City.
Based on the geological map (Mangga, 1.
, the region in West
Lombok and Mataram City consists of the Aluvium
Formation This research was conducted in one of the sub-districts in West Lombok Regency, namely Gerung
Sub-district with the more dominant alluvium
Geologically, this alluvium formation is very vulnerable to the natural phenomenon of earthquakes because it is related to sediments which consist of two
___________
How to Cite:
Putri.
Minardi.
, & Syamsuddin.
Identification Of Sediment Thickness Using The Geomagnet Method in Beleka and Dasan Tapen Villages.
Gerung District.
West Lombok Regency.
Kappa Journal, 10.
, 14-18.
https://doi.
org/10.
29408/kpj.
Kappa Journal parameters, namely the type of sediment and the thickness of the sediment.
Therefore, to determine the level of earthquake vulnerability in an area, it is necessary to conduct research related to sediment Sediment thickness is one of the factors that cause local site effects during earthquakes.
Sediment thickness is a part that needs to be considered in determining the location of development because it will affect the durability and safety of the building later.
Sediment thickness is important to study because it is related to earthquake shaking.
If the sediment layer is thick, there will be the repetition of waves, so later the waves will be reflected repeatedly from top to bottom so that in that area the earthquake will be felt longer or called multireflection or repeated reflection.
Based on this description, a study can be conducted that aims to determine the thickness of sediments in the Gerung District area using the geomagnetic method.
Therefore, the title proposed in this research is Identification of Sediment Thickness Using Geomagnetic Method in Beleka Village and Dasan Tapen Village.
Gerung District.
West Lombok Regency.
The purpose of this study is to determine the constituent rocks and the thickness of the sedimentary layer in Gerung District.
West Lombok Regency using the geomagnet method.
April 2026.
Volume 10 Issue 1, 14-18 Result and Discussion Based on the research conducted, geomagnetic data was obtained in Gerung District.
West Lombok Regency.
Geomagnetic data is processed to obtain measurement results obtained from field magnetic field values and position data (UTM X.
UTM Y).
After initial data processing, the results will obtain the total magnetic anomaly value, reduction to the pole, regional anomaly, and residual anomaly in Oasis Montaj 6.
as follows:
Total Magnetic Anomaly Value This total magnetic field anomaly is obtained from data processing on daily correction and IGRF correction that has been done before.
The corrected data is then displayed in the form of a magnetic anomaly contour The Total Magnetic Field Anomaly Map in Figure 1 is an anomaly resulting from a combination of Residual Anomaly and Regional Anomaly.
Based on Figure 1, there are colors on the contour map which are the values of the magnetic field anomalies.
Method Research conducted to identify the thickness of sediments was conducted in Beleka Village and Dasan Tapen Village.
Gerung District.
West Lombok Regency.
The tools used in this study are a set of PPM (Proton Precession Magnetomete.
units of the GEM - 197 v7.
type used to measure the total magnetic field value of the earth, a GPS (Global Positioning Syste.
to determine the position .
ongitude and latitud.
, elevation and measurement location points, a laptop used to process data, and there are stationery used to record the measurement results.
In this study, measurements were made at 156 points with a space of 100 meters at the research location.
The stages carried out in this study are for the first stage carried out is a literature study, location survey and tool preparation.
Furthermore, for the second stage, data acquisition was carried out at the research location.
This data acquisition was carried out from July 8, 2023 - July 9.
Then the third stage carried out in this study is data processing.
The data processing carried out is daily correction.
IGRF correction, reduction to the pole and upward continuation which is done to separate regional anomalies from residuals.
The next stage is data interpretation where at this stage it is carried out after obtaining the results.
Figure 1.
Map Total Value Magnetic Anomaly Reduction to Poles Reduction to the poles is done to localize areas with maximum or minimum intensity directly above the object causing the anomaly.
Reduction to the Pole is done by changing the parameters of the earth's magnetic field value in the research area to conditions at the pole, namely by transforming the appearance of the dipole into a monopole.
After reduction to the pole, the magnetic anomaly value on the contour becomes larger, for more details can be seen in Figure 2 below.
Kappa Journal Figure 2.
Map of Magnetic Anomalies Reduced to Poles Based on Figure 5.
3, it can be seen that the magnetic anomaly value is larger after the reduction to the poles.
Upward Continuation Upward Continuation is a process that aims to separate magnetic anomalies.
In the upward continuation process, a regional magnetic anomaly will be generated, where the regional anomaly value will be used to obtain the residual anomaly value which is the target of the research survey.
Based on the research location, after the upward continuation is carried out with several heights that have been tried the results of the contour look constant at an altitude of 500m.
The contour map of the upward continuation can be seen in Figure April 2026.
Volume 10 Issue 1, 14-18 Figure 4.
Residual Magnetic Anomaly Map Magnetic Anomaly Modeling After performing several data processing processes, modeling is then carried out to obtain results following the objectives of the study.
Modeling is done by making incisions in several specific lines following the objectives of the study.
The incisions are used to clarify the subsurface lithology.
In this study, there are 6 incisions, namely AA'.
BB'.
CC'.
DD'.
EE' and FF' as Figures Figure 5.
Modeling Incision on Residual Map Figure 3.
Regional Magnetic Anomaly Map After obtaining the regional magnetic anomaly, it is necessary to find the residual anomaly value.
Residual anomalies are the effect or response of shallow magnetic anomalies.
The residual magnetic anomaly is obtained from the subtraction between the total magnetic anomaly and the regional magnetic anomaly.
The residual magnetic anomaly is used as the basis for modeling, for more details the residual magnetic anomaly map can be seen in Figure 4 below.
Figure 6.
Residual Overlay Map with Incision In 2D modeling, it is done by trial and error method, by changing model parameters such as susceptibility value, length, and depth in 2D model.
Here are some 2D models with a depth of 500 m.
Kappa Journal April 2026.
Volume 10 Issue 1, 14-18
CCAo
0,001
0,032
0,045
0,055
0,081
0,001
0,032
0,047
0,073
0,084
Silt
Sandstone Clay
Breksi
Lava
Silt
Sandstone Clay
Breksi
Lava
0 Ae 151
7 Ae 224
164 Ae 366
283 Ae 464
338 Ae 500
0 Ae 131
93 Ae 209
191 Ae 370
302 Ae 444
372 Ae 500
EEAo
0,020
0,035
0,045
0,055
0,074
Silt
Sandstone Clay
Breksi
Lava
0 Ae 151
42 Ae 284
182 Ae 340
282 Ae 417
364 Ae 500
FFAo
0,022
0,034
0,057
0,076
0,096
Silt
Sandstone Clay
Breksi
Lava
0 - 128
34 - 242
118 - 362
252 - 465
336 - 500
DDAo Figure 7.
Pemodelan 2D pada Sayatan ao Based on the modeling in Figure 6, it can be seen that the modeling consists of five layers.
The first layer has a susceptibility value of 0.
021 (SI), the layer is estimated to include siltstone at a depth of 0 m - 179 m.
The second layer has a susceptibility value of 0.
031 (SI),
estimated to include Sandstone at a depth of 11m 234m.
Then the third layer has a susceptibility value of 042 (SI), estimated to be included in Clay Rocks at a depth of 157 m - 380 m.
The fourth layer in the modeling has a susceptibility value of 0.
052 (SI) which is estimated to include Breccia Rock with a depth of 290 m - 485 m and the last in the fifth layer has a susceptibility value of 0.
077 (SI) which is estimated to include igneous rocks, namely Lava rock at a depth of 424 m - 500 m.
The modeling in Figure 6 is a susceptibility value of 0.
077 (SI) which is estimated to include igneous rocks at a depth of 424 m - 500 m .
The modeling in Figure 6 is modeling for incision AA', then for the susceptibility value and depth for modeling on incision BB' - FF' can be seen in Table 1 below.
Table 1.
Susceptibility Values of Rocks in the Research Area Sayatan ao BBAo Suseptibilitas (SI) Litologi Kedalaman .
0,021 Silt
0 Ae 179
0,031
0,042
0,052
0,077
Sandstone Clay
Breksi
Lava
11 Ae 234
0,003
0,029
0,049
0,060
0,070
Silt Sandstone Clay Breksi Lava 0 Ae 84 20 Ae 225 235 Ae 405 337 Ae 500 Based on Table 1, it can be seen that rocks that include sediments are in the first layer to the fourth layer, namely siltstone, sandstone, clay and breccia rock.
for the fifth layer, it does not include sedimentary rocks because these rocks include igneous rocks.
From Incision AA' to Incision FF'have different depths, so the thickness of each incision has a different thickness.
The thickness of the sediments in the incisions can be seen in Table 2 below Table 2.
Sediment thickness at the incision at the research location Sayatan ao
BBAo
CCAo
DDAo
EEAo
FFAo
Sedimentary Rock
Silt
Sandstone Clay
Breksi
Silt
Sandstone Clay
Breksi
Silt
Sandstone Clay
Breksi
Silt
Sandstone Clay
Breksi
Silt
Sandstone Clay
Breksi
Silt
Thickness Layer
0 Ae 179
11 Ae 234
157 Ae 380
290 - 485
0 Ae 84
20 Ae 225
137 Ae 271
235 - 405
0 Ae 151
7 Ae 224
164 Ae 366
283 - 464
0 Ae 131
93 Ae 209
191 Ae 370
302 - 444
0 Ae 151
42 Ae 283
182 Ae 340
282 - 417
0 Ae 128 Kappa Journal April 2026.
Volume 10 Issue 1, 14-18
Sandstone Clay
Breksi
34 Ae 242
118 Ae 362
252 - 465
Based on table 2, it can be seen that the thickness of sediment in the research location is The smallest sediment thickness is in the area with the BB 'incision of 405 m, then for the medium-value sediment thickness is in the area with the FF 'incision with a sediment thickness of 465 m, and for the largest sediment thickness is in the area with the AA 'incision amounting to 485 m.
Conclusion Based on the in terpretation results obtained, it can be concluded that the results of 2D modeling at the research location there are five types of constituent rock layers, namely, siltstone, sandstone, clay, breccia rock and lava rock.
Then for the thickness of sediments at the research location in the AA 'incision to FF' incision has a sediment thickness that ranges from 405 m - 485 Of the six modeling on each incision, the incision that has a very large thickness is in the western part of Beleka Village with incision AA'.
Acknowledgments The author would like to thank friends and seniors who have helped during data collection and taught ho w to process data so that the author can complete this References