Methodological Considerations in Cover-Collapse Sinkhole 1 Analyses : A Case Study of Southeastern China ’ s Guangzhou City 2

Abstract. Cover-collapse sinkholes can present significant hazards to human habitation and communal facilities in soil-covered karst regions. Therefore, for human security and land-use planning in sinkhole-prone areas, appropriate approaches are required prior to construction in order to understand the cover-collapse sinkhole genesis and its likely evolution. The study seeks to contribute to performing an integrated analysis of karst hazards in mantle karst regions where karst evidence can be masked, with the ultimate goal of developing a methodological framework utilizing different techniques and approaches. A small area located in Guangzhou City of southeastern China’s Guangdong Province was analyzed. The detailed typology, morphometry, and chronology inventory of 49 cover-collapse sinkholes in the study area were analyzed using various surface investigation methods, such as field surveys, aerial photography, and photogrammetry. The Quaternary deposits and indicators of the active underground karst features in the aforementioned mantle karst region were geotechnically characterized using drilling and geophysical techniques. These techniques included ground penetrating radar (GPR), electrical resistivity imaging (ERI), natural source audio frequency magnetotellurics (NSAMT), and micro-tremors. During this study’s investigations, three karst fissure zones covered by Quaternary soil were observed using multiple techniques. In addition, it was found that the groundwater dynamic monitoring data confirmed that the sinkholes in the study area were closely related to changes in groundwater levels. Therefore, the efforts which have been made to investigate and monitor the sinkhole development will be required to continue into the immediate future.



27
As near-surface indicators of active karst features in soil-covered karst regions, cover-collapse 28 the 318 sites. The sites were spaced 5 m apart, and the single point collection time was 20 minutes. 141

Electrical resistivity imaging (ERI) 142
Electrical resistivity imaging (ERI) is a technique in which many individual resistivity measurements 143 are combined to produce a resistivity cross-section of the subsurface. Electrical parameters, such as 144 resistivity or conductivity, are very sensitive to formation properties. Therefore, ERI methods have been 145 effectively used for differentiation processes related to rock layers. Electrical resistivity tomography 146 profiling (surface electrode arrays) is also commonly used for sinkhole investigations as a means of 147 identifying shallow limestone deposits, large dissolution feature zones, and underlying cavities (Fabregat 148 et al., 2017). In the present study, two ERI (electrical resistivity image) profiles, with a total length of 500 149 m and spacing of 30 m, were conducted in the study area. The resistivity lines of this pattern were acquired 150 utilizing a WDJD-3 Supersting multi-channel and multi-electrode resistivity system designed in China, 151 equipped with 60 electrodes spaced at 5 m intervals along each line. The data were inverted using 152 RES2DINV software. 153

Natural source audio frequency magnetotellurics (NSAMT) 154
Audio frequency magnetotelluric (AMT) methods involve surface-based electromagnetic sounding 155 techniques which use fixed grounded dipoles as signal sources (CSAMT), or alternatively, the 156 naturally-occurring fields of the Earth's atmospheric system (NSAMT). The higher frequency 157 audio-magnetotelluric (AMT) methods are able to detect the ranges of karst fissure zones based on the 158 different electrical conductivity of the underground rock strata. Once water flows into caved and fractured 159 zones, the resistivity of those areas will rapidly decrease. These are referred to as low-resistivity anomaly 160 zones. In this study, the naturally-occurring electromagnetic fields were used as the signal sources. Then, 161 NSAMT (Natural Source Audio Frequency Magnetotelluric) profiles with total lengths of 500 m were 162 conducted in the areas coinciding with the ERI profiles. The NSAMT data were collected using a 163 https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License. Geometrics StrataGem EH4 system in the study area. Then, an EH-4 conductivity imaging system 164 manufactured by EMI and Geometrics (US), was adopted in this study as the electromagnetic geophysical 165 detection system for the auto data acquisition and processing procedures. reflection method is similar to that of surface-based GPR, with the exception that reflectors may occur on 178 all sides of the borehole recording line. Planar features, such as fault surfaces, which may be intersected by 179 a borehole, will appear as V-shaped reflections in a single-hole radar section. The images of the point 180 reflectors (for example, karst caves) are hyperbola. In the current study, a MALA system equipped with 181 100 MHz borehole antennae was used to acquire all of the radar data. The single-hole full-waveform data 182 were recorded in all six holes utilizing transmitter and receiver antennae separated by 2.75 m. 183

Cross-hole radar 184
Cross-hole GPR is a trans-illumination survey method in which two antennae are lowered down into 185 adjacent parallel boreholes (Bachrach et al., 2005;Cordua et al., 2009). Then, by transmitting radar signals 186 https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License. from one borehole to another, the electromagnetic EM wave velocity and attenuation between the two 187 boreholes can be estimated. The high-resolution imaging of subsurface electromagnetic EM wave 188 velocities has proven to be effective in detections conducted in the majority of water-filled areas, such as 189 water-filled faults and caves, in which the low-speed zones represent the water filled areas (Tan et al., 190 2012). In this study, three pairs of boreholes were used for cross-hole radar surveys, taking advantage of 191 the adopted MALA system with 100 MHz borehole antennae.

InSAR 202
Interferometric Synthetic Aperture Radar (InSAR) analysis methods can be used to screen large areas 203 for anomalous vertical movements, as well as to guide intensive field investigations and detection 204 processes to areas where significant changes are occurring (Intrieri et al., 2015). In addition, the mapping 205 of ground displacements may assist in the identification of locations prone to future cover-collapse 206 sinkholes. In the study area, InSAR ground deformation data were obtained with a 5 m pixel size and a 207 vertical accuracy higher than 3 m. Then, three RADARSAT-2 Ultra Fine images from November 27 th of 208

Soil layers 237
The thicknesses and structures of the soil layers in the study area were obtained according to the 238 results of the drilling, micro-tremors, and electrical resistivity imaging (ERI). 239

Quaternary soil thicknesses 240
The drilling profiles showed that the thicknesses of the quaternary soil layers in the collapsed 241 https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License. single-hole radar measurements showed that there were linear anomalies located around boreholes ZK1, 265 ZK5 and ZK6, indicating the existence of karst cracks in those areas.

297
In accordance with the information obtained from the local residents and staff, the daily water output 298 of a waterworks located 800 m east of the study area was approximately 1,200 to 6,000 m 3 . The change of 299 water output was related to the water consumption of the residents. the water levels of the local wells had 300 https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License. dropped by about 7 m in early 2015 when a large scale karst collapse had occurred in the study area. Also, 301 on the basis of the hydrodynamic monitoring data, it was confirmed that there was a relationship between 302 groundwater level changes and the aforementioned collapse in the study area, as shown in Fig. 8. In

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In accordance with surveys in China, the analysis processes for cover-collapse sinkhole conditions 320 should involve three main steps, with each step built upon the previous one, as follows: 321

Geomorphic analysis 322
This study illustrated that geomorphic mapping which utilizes historical aerial photographs and 323 unmanned aerial vehicle (UAV) images may be essential for investigations of cover-collapse sinkholes. 324 The UAV images were found to have advantages over the satellite images, due to the fact that they had 325 captured aerial images from certain flying heights with flexible flying missions and time frames. However, 326 the effectiveness of the aforementioned approach may be quite limited in areas where the geomorphic 327 expressions of sinkholes have been obliterated by natural processes or anthropogenic fill. Therefore, on 328 this basis, thorough reconnaissance of the ground would be required to locate sinkholes not identifiable on 329 aerial photographs due to high vegetation cover. It was also determined that information from local 330 residents in the area was conducive to ascertaining the precise spatial distributions of the complex sinkhole 331 clusters, especially concealed sinkholes which may be masked by anthropic landforms. One of the 332 meaningful aspects of this case study was that the geomorphic model produced by combining data from 333 aerial photographs and field surveys could potentially constitute a basis for accurately designing future site 334 investigations and interpreting the results, such as implementing geophysical profiles and borehole data. 335

Geological analysis 336
Due to the complex and sometimes chaotic underlying geology observed in mantle karst areas, 337 investigations which combine several methods are generally the only way to achieve satisfactory 338 geological models for such areas. 339 Borehole drilling processes are performed in mantle karst regions in order to geotechnically 340 characterize the stratigraphic information and calibrate and validate the geophysical detection results. 341 However, drilling activities are expensive and time-consuming techniques, and the limited drill footage 342 may potentially have a high degree of uncertainty for the complex underlying geology in karst areas. 343 However, the punctual information derived from limited numbers of boreholes could be extended 344 laterally using borehole geophysical investigations, such as single-hole radar and cross-hole radar. In this 345 way, other karst caves and fissures around the borehole clouds may be discovered using geological 346 borehole radar techniques. 347 In the present study, based on the limited borehole data, micro-tremor explorations were used to 348 estimate the sediment thicknesses, thereby making it possible to reconstruct the bedrock morphology 349 beneath the entire study area. The non-disturbed areas were represented by the general horizontal bedding 350 of the Quaternary deposits. Therefore, any local thinning or thickening of the Quaternary deposits observed 351 using the mirco-tremor Nakamura technique were believed to indicate the presence of serial sediment 352 within active karst areas. In the present study, the aforementioned techniques were examined in order to determine the most 360 advantageous synergistic approach in the study area. It was expected that the limitations observed in each 361 examined method would be balanced out by the advantages observed in the other methods. 362 https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License.

Dynamic monitoring 363
In order to understand the causes of cover-collapse sinkholes, and to assess and predict the kinematics 364 of the subsidence phenomena, it is generally considered that monitoring methods are necessary. Since karst 365 cover-collapse sinkholes are known to be caused by declines in groundwater levels, a sound knowledge of 366 the short-and long-term dynamics of the effected hydrogeological systems are essential for sinkhole 367 hazard assessments. Hydrodynamic monitoring methods focus on the potential relationships between 368 hydrological changes and the development of cover-collapse sinkholes. The interpretations of the 369 groundwater level monitoring data allow the hydrogeological behaviors of the groundwater to be 370 accurately reconstructed. As a result, the kinematics of the subsidence phenomena can be assessed. In 371 addition, the accurate mapping of ground displacements may serve to identify the locations of future 372 cover-collapse sinkholes and guide future intensive field investigations. Therefore, it was found in this 373 study that monitoring of ground anomalous vertical movements by Interferometric Synthetic Aperture 374 Radar (InSAR) analysis could be an effective approach. 375

376
(1) In mantle karst regions, cover-collapse sinkholes are considered to be major geohazards due to the 377 large and increasing impacts of sinkhole damages. In this study, based on an appropriate methodological 378 framework, it was found that sinkhole condition analyses were conducive to human security and land-use 379 planning in sinkhole-prone areas. 380 (2) The multi-disciplinary approach adopted in this study was determined to the most effective 381 method for identifying and understanding cover-collapse sinkhole phenomena in a complex geological 382 frameworks, such as southeastern China's Bumei Village in the presented case study. The present study's 383 goal was to contribute to deepening the understanding the genesis and early-stage evolution of a sinkholes 384 by utilizing geological, geomorphological, and hydrodynamic integrated methodologies. Special focus was 385 paid to the contributions of the various examined methods to overcome the limitations of the other 386

methods. 387
In this case study, a mapping procedure was introduced which combined data from aerial photographs 388 and intensive field investigations. The results clearly indicated the characterization of the cover-collapse 389 sinkholes in the study area. In addition, data interpretations from borehole drilling activities and different 390 geophysical approaches were performed in order to reconstruct the Quaternary deposit features, rock head 391 morphology, and karst features. These examples also indicated why multi-disciplinary and complementary 392 data acquisition approaches were necessary in order to ensure accurrate interpretations in mantled karst 393 settings. For this reason, due to the results obtained in this study, the adopted methodological approach 394 could successfully be extended to other areas characterized by similar geological and hydrogeological 395 characteristics. 396 (3) In the study village area, the integration of borehole, geophysical, and hydrogeological data 397 suggested that aquifer pumping had triggered the loss of hydrostatic support and accelerated the 398 washing-out processes. As a result, cover sagging and suffosion sinkholes had been generated in the 399 mantled karst region. Although the groundwater levels had been restored at the time of this study, the 400 sinkholes had the potential to again impact the local residents. Therefore, efforts to investigate and monitor 401 the sinkhole development processes in the region will be required to continue into the immediate future. https://doi.org/10.5194/nhess-2020-53 Preprint. Discussion started: 28 February 2020 c Author(s) 2020. CC BY 4.0 License.