Application of ground-penetrating radar for shallow subsurface investigation at the coastal area of lhok village, lhoong, aceh besar regency, aceh

. This research aims to identify subsurface layers in the coastal area of Lhok Village, Lhoong, Aceh Besar Regency, Aceh. Ground Penetrating Radar (GPR) was the primary method to extract information concerning concealed geological structures beneath the Earth's surface, including in coastal areas. GPR is a non-destructive geophysical technique that utilizes electromagnetic waves to explore and map subsurface layers. However, data collection involved field surveys utilizing GPR equipment directed at the ground's surface. During the survey, radar data was processed and analysed to pinpoint various subsurface level. The obtained information contributes to a deeper understanding of the region's geological characteristics and subsurface layers. Additionally, it aids in assessing geological risks, facilitating construction planning, and conserving natural resources in the area. Here, the outcomes of this study may also serve as a foundation for further research in geology and geophysics within the Aceh Coastal region.


Introduction
The coastal area represents a dynamic and complex environment with interrelated geology.This transitional zone between the sea and the land continuously adapts to the movements of seawater, encompassing currents, waves, and tides [1].These marine processes carry and deposit sediments comprising fragments of rocks, minerals, and organic matter from diverse sources [2].Various factors, including carriers such as water, wind, and human activities, influence transport processes for sediments [3].
Here, the sediment deposition process in coastal areas involves continuous erosion, transport, deposition, resuspension and redeposition [4].Physical, chemical, and biological factors influence these processes, shaping the characteristics of sediment layers in the region.The accumulation of sediments is an ongoing phenomenon, offering valuable insights into the coastal area's geological history and environmental dynamics [5].
The Ground Penetrating Radar (GPR) is an effective method for investigating Shallow Subsurface and sediment layers.This method applied electromagnetic waves ranging from 1 to 1000 MHz for mapping the subsurface structures [6].GPR offers high-resolution capabilities for distinguishing dielectric properties of materials and shallow subsurface geological formations [7].However, it comes with a limited penetration depth; as the frequency increases, the wavelength decreases, resulting in higher resolution.A few studies used GPR to investigate the subsurface structure, such as [1][2][3] The research used the GPR method to analyze the sediment layers and investigate the shallow subsurface in the coastal area of Lhok Village, Lhoong, Aceh Besar Regency.Furthermore, this area predominantly comprises sand, gravel, mud, and other materials from the Lamno Limestone formation [11] (Fig 1b).Besides, this area has been hit by the 2004 tsunami.The tsunami has left sediments in the area they passed, eroded and covered by coastal sediments due to tides and climate change.
The investigation in this region is important for comprehending sediment layers and subsurface structures.The analysis of GPR data is needed to yield information regarding sediment thickness, identification of sediment layer boundaries, and vertical sediment structure modelling.This information will aid in understanding sediment deposition processes and patterns in the coastal areas of Lhok Village, Lhoong, Aceh Besar Regency.

Methods
The acquisition data was collected in the coastal areas of Lhok Village, Lhoong, Aceh Besar Regency, Aceh.A GPR instrument with Series IDS Opera Duo dual-frequency antenna (250 and 700 MHz) [12] was used to perform the survey.Furthermore, the radar antenna is shielded, enabling simultaneous penetration at shallow and deep depths [13].Further theoretical basis of the GPR method can be found in [7], [14].The transmitter transmits radar waves into the target medium.If there are variations in the electrical properties different from the surrounding medium, the waves are reflected to the surface and received by the receiver [13], [15].

Data Acquisition
In this study, data acquisition used the radar reflection profiling system to characterize shallow subsurface layers within the research area.Data was acquired along a 50-meter profile extending from the cave entrance (northwest side) towards the coastline (southeast most) (Fig. 1c).Dual frequencies were used to observe variations in the resulting target distribution.However, based on geomorphological observation, this profile's surface predominantly comprises topsoil, sand, karst and vegetation (Fig 1b & 1c).The GPR measurements produced images of subsurface layers with raw data images.Additional correction data and processing steps are required to eliminate and reduce the influence of noise data or errors that occurred during the measurement process.[11]) (e -f) zoom out area study area

Data Processing
The GPR data was processed in two steps -filtering and visualization.The filtering stage involves separating interference in the measurement data to display the signal from each subsurface layer.In general, this process consists of the following steps in  frequencies within a specific range while diminishing frequencies outside that range [19], [20] d) Background removal: This filter eliminates repetitive noise throughout the entire data profile [21].e) Running average: To smooth the data display and make it more organized [22] f) Fk migration: The filter combines multiple reflected signals to enhance the visibility of the data signal [18 -20] g) Plot scale: Adjusting the colour scale and sharpness of the radar plot makes the radar data easier to interpret The data from the previous step is marked to distinguish individual soil layers during the visualization process.The boundaries separating these layers help clarify the different layers.
The thickness of sediment can be estimated using the layer boundary.

Result and Discussion
The data from acquisition in the field is shown on radargram as a time function (two-way travel time) and distance.The data has flat shapes as the waveform (Fig. 3).On the other hand, if the wave passes through a different material or medium, the shape of the radargram will be associated and change for not flat longer anymore.The interpretation of GPR radargrams showed the distribution of shallow subsurface layers.In Fig 3, the results of the radargram interpreted indicate three subsurface layers.The uppermost layer is a pile of topsoil with a depth of 0-1 m.The second layer is sand with a 1.0 -3.2 m depth along the profile.Moreover, the thickest is fine wet sand that overlaid the rough wet sand below.Finally, the interpretation results indicate that the radargram at the frequency of 250 MHz is better to show the deeper depths but could be clearer at the shallow depth [23].Therefore, this situation is in contrast at a frequency of 700 MHz.In the study of coastal sedimentation, it is recommended to use the frequency of 700 MHz with a good penetration ability to see the sedimentation process.In these antennas, the contact of these deposits shows an increase in the signal amplitude [24].The amplitude analysis of a particular environment and the findings indicated that low amplitude values were correlated with high compaction values.This relationship suggested that high compaction accelerated the decay of signals [25].The results of the identification of radargram show that bedding in coastal areas, e.g., sand material, is on the surface and gravel on the subsurface.Around the coast, we also found a hill or dune (sediment dominated by sand with good sorting) or lagoon (fine deposits and organic material).In addition, sedimentary bedding in this region is also influenced by tidal currents, which can be a small sediment transport medium, such as fine sand and clay.

Conclusions
The interpretation of GPR radargrams has provided valuable insights into the distribution of shallow subsurface layers in the study area.The radargrams revealed the presence of three subsurface layers at different frequencies, with the uppermost layer consisting of topsoil followed by sand and fine wet sand.The result also highlighted the importance of selecting the appropriate frequency for GPR surveys, with the 250 MHz frequency proving effective for deeper depths.In contrast, the 700 MHz frequency offered better clarity for shallow depths.These findings significantly affect coastal sedimentation studies, particularly in identifying sedimentary bedding, such as sand and gravel.The influence of tidal currents on sediment transport, including fine sand and clay, was also observed.Moreover, these results contribute to our understanding of the subsurface characteristics and sediment dynamics in coastal areas, providing valuable information for various geoscience and environmental applications.

Fig. 1 .
Fig. 1.(a) The GPR Unit used (b) Field view of research site with data acquisition process (c) Profile sketch of GPR data acquisition (d) The study area overlaid by geological maps (modified based on [11]) (e -f) zoom out area study area Fig 2:

Fig. 2 .
Fig. 2. Research process steps a) Static correction: Correcting the two-way travel time of the wave to a horizontal reference plane above the ground interface.These corrections are essential for obtaining accurate subsurface images and improving profile interpretation [15 -16] b) Substract-mean (Dewow): A noise removal process that effectively eliminates interfering radar signals from the data [18] c) Bandpass correction: Filtering signals in the frequency domain eliminates unwanted noise, thereby improving the signal-to-noise ratio.Bandpass filters permit the passage of

Fig. 3 .
Fig. 3. Raw data images obtained from GPR data acquisition at frequencies of (b) 250 MHz and (b) 700 MHz, respectively.