CHARACTERISTICS OF KARST CAVES INFERRED FROM MICROTREMOR STUDIES: A CASE STUDY FROM CERME CAVE, YOGYAKARTA, INDONESIA ZNAČILNOSTI KRAŠKIH JAM, ZBRANE NA PODLAGI RAZISKAV MIKROTREMORJEV: ŠTUDIJA PRIMERA JAME CERME, YOGYAKARTA, INDONEZIJA Gatot YULIYANTO1* & Muhammad Irham NURWIDYANTO2 Abstract UDC 551.442:550.34(594) Gatot Yuliyanto & Muhammad Irham Nurwidyanto: Charac- teristics of karst caves inferred from microtremor studies: A case study from Cerme Cave, Yogyakarta, Indonesia A microtremor survey based on ground surface data acqui- sition was used to identify and characterize the karst area of Cerme Cave, Yogyakarta, Indonesia, from the entrance to the exit of the cave. The entrance and exit of the cave are used as tie-in points because the characteristics of the two locations can be directly observed. Parameters used in this study include ground vibration amplification, shear wave velocity, and Pois- son’s ratio. The presence of cavities can be characterized by a relatively strong contrast between these physical parameters and their surroundings. The exit of the cave, which can be considered as a sinkhole, has a dominant frequency of 3.2 to 4.6 Hz, which is relatively higher than that of the surrounding area. At the entrance of Cerme Cave, which has a large cavity, a small ground vibration amplification was detected, less than 0.1. The entrance and exit of the cave also exhibit a low shear wave propagation velocity of less than 350 m/s. The presence of a subsurface fluvial channel in Cerme Cave can be character- ized by a high Poisson’s ratio of 0.4–0.5, a gain value of less than 0.1, and a shear wave velocity of less than 350 m/s. Keywords: microtremor, karst, Cerme Cave, amplification, Poisson’s ratio. Izvleček UDK 551.442:550.34(594) Gatot Yuliyanto & Muhammad Irham Nurwidyanto: Značil- nosti kraških jam, zbrane na podlagi raziskav mikrotremor- jev: študija primera jame Cerme, Yogyakarta, Indonezija Raziskava mikrotremorjev, ki temelji na pridobivanju podatkov o površini tal, je podlaga za opredelitev in opis kraškega območja jame Cerme v Yogyakarti v Indoneziji, in sicer od vhoda v jamo do izhoda iz nje. Vhod v jamo in izhod iz nje sta uporabljena kot povezni točki, saj je mogoče značilnosti obeh lokacij opazo- vati neposredno. Parametri, uporabljeni v tej študiji, vključujejo okrepitev vibracij tal, hitrost strižnega valovanja in Poissonovo razmerje. Prisotnost votlin je mogoče opredeliti z razmeroma močnim kontrastom med temi fizikalnimi parametri in njihovo okolico. Izhod iz jame, ki se lahko obravnava kot vrtača, ima prevladujočo frekvenco od 3,2 do 4,6 Hz, ki je razmeroma višja od frekvence okolice. Na vhodu v jamo Cerme, kjer je velika votlina, je bilo zaznano majhna okrepitev vibracij tal, tj. manjša od 0,1. Na vhodu v jamo in izhodu iz nje je tudi majhna hitrost širjenja strižnega valovanja, tj. manjša od 350 m/s. Prisotnost podzemnega rečnega kanala v jami Cerme je mogoče opre- deliti z visokim Poissonovim razmerjem 0,4 do 0,5, vrednostjo okrepitve manj kot 0,1 in hitrostjo strižnega valovanja manj kot 350 m/s. Ključne besede: mikrotremor, kras, jama Cerme, okrepitev, Poissonovo razmerje. ACTA CARSOLOGICA 52/1, 109-119, POSTOJNA 2023 1 Department of Physics, Faculty of Science & Mathematics, Diponegoro University, Semarang, Central Java, Indonesia, e-mail: gatoty@fisika.fsm.undip.ac.id 2 Department of Physics, Faculty of Science & Mathematics, Diponegoro University, Semarang, Central Java, Indonesia, e-mail: irhammn@lecturer.undip.ac.id * Corresponding author Prejeto/Received: 24. 1. 2022 DOI: https://doi.org/10.3986/ac.v52i1.10987 1. INTRODUCTION In general, exploring the presence of subsurface cavities in karst areas is difficult and risky. For geophysical meth- ods, this remains one of the greatest challenges to date, as tools with high power and resolution must be used to image models with good resolution of hollow subsur- face layers with irregular cavity geometry. In many cases, the subsurface cavities of karst areas contain water and/ or air, so the subsurface cavities do not produce easily detectable geophysical anomalies despite the significant difference in the physical properties of the rock relative to the surrounding rock. Currently, several geophysical methods are used to determine the location of subsurface voids, including ground penetrating radar (GPR) (Azimah & Widodo, 2017), resistivity method (Vargemezis et al., 2007; Ham- dan et al., 2012), gravity (Braitenberg et al., 2016), mag- netometry (Gibson & George, 2004), electromagnetic (Huang et al., 2020), seismic (Polymenakos, 2017), and Self Potentials (Vichabian & Morgan, 2002). However, due to geological factors, the size, shape, and orientation of subsurface voids, these methods have limitations in terms of penetration depth and resolution accuracy. In addition, gravity methods and seismic methods require a long data acquisition time and are quite expensive. In many cases, a single geophysical method for identifying subsurface cavities in karst areas cannot produce a con- clusive and reliable result. No single geophysical meth- od is capable of identifying subsurface voids with good results; it is common to use more than one geophysical technique to improve the reliability of the resulting in- terpretation. For example, combined resistivity meth- ods with gravity (Hartvich & Valenta, 2011; Putiska et al., 2014), resistivity and LiDAR (Kasprzak et al., 2015; Stafford et al., 2107), resistivity and seismic refraction (Valois et al., 2010), resistivity and magnetism (Balkaya et al., 2012), electromagnetic and GPR (Pueyo et al., 2017), combined methods of Surface Nuclear Magnetic Resonance (SNMR), seismic refraction and seismic re- flection, Transient Electromagnetic (TEM), Multichan- nel Analysis of Surface Waves (MASW), microgravity and magnetic surveys and their combinations (Ezersky et al., 2017), and an integrated survey using Ground Pen- etrating Radar (GPR), Electrical Resistivity Tomography (ERT), and Very Low-Frequency Electromagnetic (VLF- EM) methods (Hussain et al., 2020). In the last decade, passive seismic methods that use ambient noise or microtremor methods have rap- idly evolved. A microtremor is a low-amplitude vibra- tion of about 0.1-1 microns with an amplitude velocity of 0.0001-0.01 cm/s near the ground caused by various natural factors such as wind, ocean waves, vehicle noise, and others (Mirzaoglu & Dykmen, 2003). The concept of the microtremor was introduced by Omori in 1908 and later further developed by Kanai and Tanaka in 1961. In 1970, Nogoshi and Igarashi introduced a technique using the horizontal to vertical spectral ratio of microtremor (HVSR), which was later used by Nakamura (1989) to estimate resonance frequency and local geologic ampli- fication factors. The horizontal-to-vertical spectral ratio (HVSR) is defined as follows: (1) SUD is the Fourier amplitude spectrum of the vertical component of the microtremor; SNS and SEW are two or- thogonal horizontal components [1]. The HVSR micro- tremor method is also known as the single-station micro- tremor method. Data acquisition using this method can be performed quickly, easily, and relatively inexpensively (Sasongko et al., 2000). In its development, this method has been widely used in geotechnical and environmental fields. Some of them are correlation of microtremor mea- surement results with local geological conditions (Fab- rizio et al., 2008), soil and building vulnerability studies, soil shear stresses and sliding surfaces of landslide move- ments (Yuliyanto et al., 2016, 2017, 2018; Sasongko et al., 2020), subsidence studies (Widada et al., 2019), buried objects studies such as archaeological objects (Yuliyanto, 2020; Ubaidillah et al., 2020). On the other hand, the use of microtremor methods for subsurface cavity detection is still relatively small, some of which are studies of caves in coastal areas using microtremor arrays (Nagao et al., 2017), physical modelling and field experiments for sub- surface cavity detection (Kolesnikov & Fedin, 2017), and combined methods employing resistivity with micro- tremors for small caves (Nehme et al., 2013). To determine how the microtremor method re- sponds to the presence of voids below the ground surface with a vertical structure model, several preliminary stud- ies were conducted, including by Justin et al. (2022), who used formerly dug wells and bunker seismometers locat- ed on the Diponegoro University campus and success- fully obtained void spectrum responses with reasonably good sensitivity. In addition, the response characteristics of sinkholes in karst areas were investigated using the mi- crotremor method of Delfira et al. (2022), and the pres- ence of sinkholes can be well imaged. Hana et al. (2021) used the presence of penstock pipelines as a continuous cavity model for the response of subsurface cavities with lateral structures and also obtained good response sensi- tivity with the microtremor method. GATOT YULIYANTO & MUHAMMAD IRHAM NURWIDYANTO ACTA CARSOLOGICA 52/1 – 2023110 CHARACTERISTICS OF KARST CAVES INFERRED FROM MICROTREMOR STUDIES: A CASE STUDY FROM CERME CAVE, YOGYAKARTA, INDONESIA The parameters obtained from the HVSR micro- tremor data are amplification and the dominant fre- quency. The dominant frequency is closely related to the thickness of the sediment. The higher the frequency, the shallower the depth of the bedrock, and vice versa. The amplification value is related to the contrast ratio be- tween the density of the surface layer and the underlying layer (Nakamura, 2000), and the peak of the curve repre- sents the vertical lithological boundary. From the inver- sion of microtremor data, one of the elastic parameters of rocks can be extracted, namely Poisson’s ratio (σ), which is one of the elastic moduli of rocks or subsurface layers. Poisson’s ratio is a combination of Vp and Vs and can be described as follows (Lay & Wallace, 1995): (2) In general, the elastic parameters of rocks depend large- ly on their lithological properties, which include water saturation. In fluid media with zero stiffness, no shear wave can propagate, so the Poisson’s ratio in the fluid is 0.5 (Schon, 2011). At constant pressure, Poisson’s ratio increases with increasing water saturation and poros- ity. Some studies on using microtremors to identify the presence of subsurface layers with high water content such as aquifers include Yulianto et al. (2021), Yuli- yanto et al. (2021), Gulo et al. (2022), and Nurwidyanto (2022). Based on the above description, the parameters of dominant frequency, amplification, shear wave ve- locity, and Poisson’s ratio obtained by the microtremor method can be used to determine the physical charac- teristics of caves or cavities located beneath the surface of the soil layer. 2. STUDY AREA Cerme Cave is located about 20 km south of Yogyakarta (Figure 1). The entrance of the cave is located in Srunggo, Selopamioro Village, Imogiri District, Bantul Regency, and the exit is located in Ploso area, Giritirto Village, Panggang District, Gunungkidul Regency. Topographi- cally, Cerme Cave is a horizontal cave with a length of about 1.0 km and features the beauty of stalactites, stalag- mites, underground rivers and waterfalls. The height of the cave ceiling varies from 1 to 20 meters. Cerme Cave is the main cave, which houses several small caves. The interior of the cave is completely dark and the floor is flooded with groundwater that regularly rises and falls during the rainy season and recedes during the dry sea- son. Figure 1: Location of Cerme Cave, marked with a red circle, located about 20 km south of Yogyakarta. ACTA CARSOLOGICA 52/1 – 2023 111 The Cerme Cave is part of the karst area of the Sewu Mountains. The rocks in this karst area consist of coral- line limestones clustered in the Wonosari Formation and clastic limestones with marl fillings belonging to the Oyo Formation. Both limestones are estimated to be about 5.3 million years old (middle Miocene–Pliocene). Physically, the reef limestone has a rough surface, while the clastic limestone, which is generally hollow, has a dense, imper- meable limestone layer. An example of part of the cham- bers in Cerme Cave is shown in Figure 2. The literature search conducted revealed that there is no research that examines Gua Cerme based on field geophysical studies. The existing studies are more related to areas in Gunung Kidul or Gunung Sewu where the Cerme Cave is located. One of them is Haryono and Hari (2004), who conducted a landscape differentiation study based on surface observations in Gunung Kidul karst. To date, it has not been possible to obtain a base map or floor plan of the Cerme Cave based on previ- ous studies. There are no speleological reference maps or ground plans of Cerme Cave that can be used in this study. There is only a ground plan of Cerme Cave without scale, which is attached to the information board in front of the cave (Figure 3). There is no information about the method used to create the plan. 3. MICROTREMOR SURVEY Data acquisition with the microtremor method was per- formed at 46 stations (Figure 4) using 4 portable seis- mometer units with VHL PS-2B 2 Hz triaxial geophones and 3 Graphtec GL 240 data loggers and 1 Graphtec GL 840 data logger. The duration of microtremor vibration recording at each station was 20 minutes with a sampling rate of 50 ms. In the subsequent data processing, the vibration data at each station, which is still in the time domain, is converted to a frequency domain. Then the horizontal components are compared with the vertical GATOT YULIYANTO & MUHAMMAD IRHAM NURWIDYANTO Figure 2: Part of the chambers inside the Cerme Cave (http://asc. or.id/asc-jogja/cave-tourism-2-cerme). Figure 3: A ground plan of Cerme Cave with no scale attached to the information board in front of the cave. Based on this ground plan, number 1 is the location of entrance of the cave and number 20 is the exit of the cave. Numbers 7, 14, and 19 are small caves inside Cerme Cave. ACTA CARSOLOGICA 52/1 – 2023112 component and the HVSR curve, i.e., the amplification curve to frequency, is obtained using Geopsypack-2.5.0 software. To obtain the velocity of longitudinal waves (Vp) and the velocities of shear waves (Vs) propagating in the subsurface layer, and the density of the propaga- tion medium, the data were inverted using Vs Dinver Ver 0.5.4 software. The settings in this inversion software in- clude the following: the amount of soil layer, the thickness or depth of the soil layer, the longitudinal wave velocity (Vp), the shear wave velocity (Vs), and the Poisson’s ratio (σ). The result of the inversion at each measurement sta- tion is the value of the compressional wave velocity and the shear wave velocity for a given soil thickness. The cave's entrance and cave's exit serve as reference stations, since both are features that can be observed di- rectly from the ground level. The response spectra and inverse data from both gates are used as forward models to bind responses and compare them with other stations. For this reason, cross-sectional profiles of longitudinal wave velocity (Vp), shear wave velocity (Vs), and Pois- son’s ratio (σ) passing through both gates are created. 4. RESULTS AND DISCUSSION To determine the characteristics of the HVSR curves at the cave's exit and above the cave's entrance, measured near the ground level, the HVSR curves at both cave's gates are compared with several other stations (Figure 5). Stations with measurement trajectories crossing the cave's exit are coded with the alphabetical prefix A, while stations crossing the cave entrance are coded H. Sta- tions A5 and A6 are measurement stations located near the cave's exit, while station H3 is located directly above the cave's entrance. It can be seen that the HVSR curves at the cave's exit and entrance have amplification values below 0.1, which is much lower than the ground vibra- tion amplification of the HVSR curves at other stations. This low amplification appears throughout the frequency range of the HVSR curve. To determine the response of the microtremor spec- trum to lateral depth from Figure 5, the response spec- trum, shear wave velocity, and Poisson’s ratio profiles CHARACTERISTICS OF KARST CAVES INFERRED FROM MICROTREMOR STUDIES: A CASE STUDY FROM CERME CAVE, YOGYAKARTA, INDONESIA Figure 4: (a) Microtremor measurement stations (red circle) and measurement trajectory (blue line), (b) situation around the cave's exit area, (c) location of the cave’s exit, (d) Cerme Cave's entrance situation. ACTA CARSOLOGICA 52/1 – 2023 113 are arranged based on the station sequence, as shown in Figure 6. Both cave entrances appear to have a pattern of spectral response values less than 1 Hz. Near the cave exit, there is considerable anomaly amplification at fre- quencies above 5 Hz. According to local residents, water collects under the rock layer east of the cave exit from various directions, becoming a single source of water that then flows to the cave entrance at a lower elevation. The presence of a vibration amplification value of less than 1 may indicate damping of the energy by dif- fusive reflection of the vibration energy in the cave cavity due to the morphology, geometry of the cave, and rocks in the cave. The extreme amplification anomaly due to attenuation and absorption of vibrational energy near the bottom of the cave exit is likely due to the water source in the cave, in addition to the morphology, geometry, and rocks that make up the cave. To determine the hardness of the rock, the value of the shear wave velocity (Vs) can be used as a parameter. In this case, SNI 1726-2019 is used as a reference stan- dard. Hard rock sites have Vs above 1500 m/s, medium- hard rock sites have Vs between 750-1500 m/s, hard, firm, low-hard rock sites have Vs 350-750 m/s, medium- hard soil sites have Vs 175-350 m/s, and soft soil sites GATOT YULIYANTO & MUHAMMAD IRHAM NURWIDYANTO Figure 5: The HVSR curve on the measurement path passing through the cave exit, encoded with the alphabetical prefix A (left) and the one passing through the cave entrance is encoded with the alphabetical prefix H (right). The HVSR curves of both cave gates have amplification values below 0.1, which is much lower than amplification at other stations. Figure 6: Spectral response pro- file for station trajectories passing through the cave exit (top) and passing through the cave entrance (bottom). Both caves have an am- plification value of less than 1. ACTA CARSOLOGICA 52/1 – 2023114 CHARACTERISTICS OF KARST CAVES INFERRED FROM MICROTREMOR STUDIES: A CASE STUDY FROM CERME CAVE, YOGYAKARTA, INDONESIA Figure 7: Shear wave velocity profile. The cave entrance is characterized by a low shear wave velocity below 350 m/s (top). The contour above the cave’s entrance shows a layered pocket-like loop pattern with less than 750 m/s (bottom). Figure 8: Poisson’s ratio profile for measurement trajectories passing through the cave's exit (top) and those passing through the cave’s entrance (bottom). Both cave gates have higher Poisson’s ratio values than Poisson’s ratio values at other stations. ACTA CARSOLOGICA 52/1 – 2023 115 have Vs below 350 Vs m/s. A shear wave velocity profile with the same trajectory as in Figure 6 is shown in Fig- ure 7. The shear wave velocity in the rock layer around the cave exit is 750-1800 m/s. Below this rock layer is a soil layer with Vs 128-750 m/s, with the lowest Vs at the spring site below the cave exit. The low Vs is probably due to the fluid filling the rock pores and the spaces be- tween the rock fragments at this location. The study area had a Poisson’s ratio value of 0.18-0.5 (Figure 8). According to Gercek (2007), the value of Poisson’s ratio of limestone ranges from 0.11 to 0.33, indicating that the Poisson's ratio is suitable for field conditions. In the area of the cave exit, there is a high Poisson’s ratio value of 0.4-0.5 at a depth of 20 m below the surface. Based on observations in the field, a water source was found at this depth, on the inside of the cave's GATOT YULIYANTO & MUHAMMAD IRHAM NURWIDYANTO Figure 9: Model of the trajectory of the underground river flow inside Cerme Cave. The poor contour pat- tern located at the top right and bottom right is caused by the ab- sence of data in this section. Figure 10: Model of 3D isosurface based on Figure 9, which shows the existence of a pattern of cavities in Cerme Cave. The isosurface value used in this model is 0.6. ACTA CARSOLOGICA 52/1 – 2023116 CHARACTERISTICS OF KARST CAVES INFERRED FROM MICROTREMOR STUDIES: A CASE STUDY FROM CERME CAVE, YOGYAKARTA, INDONESIA exit. At the measurement site above the cave's entrance, a Poisson’s ratio value of 0.4-0.5 was recorded at a depth of 27 m. This correlates with the response spectrum profiles in Figures 5 and 6, which show a low ground vibration amplification. In addition, a longitudinal sec- tion of the ground vibration amplification model with very low vibration amplification can be interpreted as an underground river flow passage inside Cerme Cave, as shown in Figure 9. To confirm the interpretation of Figure 9, a 3D model of the 0.6 amplification isovalues can be interpreted as the presence of subsurface cavities (Figure 10). 5. CONCLUSIONS Surface surveys can effectively use microtremor methods to characterize and identify subsurface cavities in Cerme Cave. 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