Karst Detection Research Articles

Application of Electromagnetic Wave Computerized Tomography in Metro Tunnel Detection in Karst Area

When tunnelling in karst areas, a series of geological hazards caused by karst are often encountered, resulting in huge economic losses as well as human casualties. In order to ensure the safety of tunnel construction, risk assessment and early warning of karst-related geological hazards, it is necessary to use geophysical methods to find out the location, size, shape and filling of karst, and to provide necessary reference information for traffic route planning and tunnel construction planning. Based on theoretical model research, the construction plan of electromagnetic wave CT method was designed for data acquisition in an underground tunnel detection project in a city of Jiangsu province based on existing geological data, and the inverse interpretation of the measured data was carried out in combination with borehole core data to study the feasibility, effectiveness and accuracy of the electromagnetic wave CT method for karst detection in urban underground space.

Multi-Mode Imaging of Ambient Background Noise for Karst Detection in the Limestone Area Based on Frequency-Bessel Transform

In response to the challenges of karst geophysical exploration in an environment with strong external interference, this paper proposes a new method, namely the frequency-Bessel transform method, for extracting multi-order dispersion curves of surface waves from background noise to characterize karst. The observation noise data of the Wuhan karst development area are used as an example, where the dolomitic limestone and limestone mixed with dolomite of the Jialing River Formation of the middle lower Triassic are widely developed in the observation area. The frequency-Bessel transform method involves performing a Bessel integral transformation on the cross-correlation coefficient of background noise in the frequency domain. Firstly, by synthesizing theoretical noise data and comparing it with the spatial autocorrelation method—which is currently the main method for extracting the fundamental dispersion curve of surface waves—it is verified that the frequency-Bessel transform method can extract the higher-mode dispersion curve. Then, by taking the actual measured single-point noise data as an example, the effect of applying the frequency-Bessel transform to the actual noise data is tested, and the inversion of the fine structure of the strata by the addition of higher-mode dispersion, the use of the damped least squares inversion method, and the joint inversion of fundamental and higher-mode dispersion curves are analyzed. The higher-mode dispersion curve of Rayleigh surface wave extracted by the frequency-Bessel transform is much clearer, and the 2D shear wave velocity structure profile obtained from inversion explains the karst development area, karst strip area, and thickness of the Quaternary overburden. The inferred results match with the actual borehole data. Multi-mode imaging of background noise based on the frequency-Bessel method can be applied to depict karst in complex backgrounds, and has significant potentiality in the field of ambient seismic noise tomography, providing a new idea and method for karst detection in near-surface engineering.

Application of SPAC method and electromagnetic wave CT in karst detection of Wuhan Metro Line 8

In recent years, karst construction projects in the built-up area of Wuhan (capital of Hubei Province, China) are increasing, and the karst geological disasters have aroused social concerns. The actual engineering projects usually use shallow geophysical exploration methods to explore karst. This paper uses Spatial Auto-Correlation Method (SPAC) and electromagnetic Computerized Tomography (CT) to detect karst in urban built-up areas. Depending on the different physical properties of rock and soil, the SPAC method can better reveal the interface between soil and rock strata and the interface between soil layers. The electromagnetic CT method can identify strata according to the apparent absorption coefficient, which can better reveal the interface between soil and rock, the interface between the more intact and weathered rock. The SPAC method is mainly qualitative to measure the low-speed area, namely, the wrong geological body i.e., karst cave, but also can detect the fracture zone or filling mode of karst cave, and at the same time, cannot use exploration holes or logging observation. The electromagnetic CT method can accurately detect the location and scale of the karst caves and has a higher accuracy detecting karst bands. In addition, exploration holes or well logging observations are also expected to be conducted, and their detection effect is greatly affected by lithology.

Karst detection by geophysical methods using microseismic sounding

Research aim is to test a technique for detecting buried karst using microseismic sounding based on spectral amplification of horizontal displacements. The main research tasks include the construction and analysis of geoelectric and microseismic sections. Methodology. The microseismic background was recorded using the OMAR-2s autonomous seismic station (Institute of Geophysics Ural Branch RAS, Ekaterinburg) in the frequency band 1–1000 Hz. Low-frequency horizontal seismic receivers from OYO Geospace Corporation were used as sensors. Vertical electrical sounding (VES) was carried out by a set of ERA-MAX equipment (SPC “ERA”, St. Petersburg) using a symmetrical Schlumberger installation at a frequency of 4.88 Hz, at currents of 5–50 mA. VES processing consisted in quantitative one-dimensional inversion according to the IPI2Win program (Moscow State University, Moscow). Results. Experimental and methodological work on the study of microseisms, accompanied by VES, was carried out on two karst sites of the Sverdlovsk region. Based on the results of processing, geoelectric and microseismic sections were constructed to determine the conditions of occurrence of bedrock and loose sediments covering them, as well as to identify local anomalies associated with karst manifestations. The results of electrical exploration and seismometry are in good agreement with each other and are confirmed by geological data. Conclusions. Vertical electrical sounding makes it possible to distinguish karst by the high contrast of the resistances of the host bedrock (limestone) and terrigenous filler material (clay, loam). Microseismic sounding makes it possible to delineate karst zones by high values of spectral amplification of weakly consolidated karst deposits, relatively low levels of microseisms in bedrock. Thus, on the basis of complex electrical and microseismic soundings, it is possible to confidently identify the areas of development of karst processes.

Prediction and Early Detection of Karsts—An Overview of Methods and Technologies for Safer Drilling in Carbonates

The nature of carbonate deposition as well as diagenetic processes can cause the development of unique geological features such as cavities, vugs and fractures. These are called karsts. Encountering karsts while drilling can lead to serious consequences such as severe mud losses, drops of bottom hole assembly and gas kicks. To improve drilling safety in intervals of karstification, it is important to detect karsts as early as possible, preferably in advance. In this paper, we review methods and technologies that can be used for the prediction and early detection of karsts. In particular, we consider acoustic, resistivity, seismic and drilling-data methods. In addition to the inventions and technologies developed and published over the past 40 years, this paper identifies the advantages, limitations and gaps of these existing technologies and discusses the most promising methods for karst detection and prediction.

Innovative geophysikalische Vorauserkundungstechnologie in verkarstungsfähigen und kristallinen Gesteinen

AbstractIn rock formations prone to karstification there are only limited geological/hydrological principles that allow a safe prognosis of the existence and location of karstic and fault structures. A combination of geophysical borehole radar measurements utilizing both reflection and crosshole probe setups can be used to reliably detect anomalous structures, especially air‐filled karstic cavities and fault zones, when a sensible measuring concept for karstified or crystalline rocks is applied. Besides the determination of the anomalous structure's location it is also possible to distinguish its type of filling. Using examples from the major railway project Albaufstieg between Wendlingen‐Ulm the investigation concepts ahead of the tunnel construction are presented. The borehole radar measurements were successfully utilized during different phases of the tunnel excavation and could be integrated into the construction process without significant delays, thus contributing to a safe construction process and later operation of the tunnels. The reliability and high efficiency of these borehole radar investigation concepts places this technology of karst detection at a significant advantage over the commonly applied seismic methods, especially in karst‐prone and crystalline rocks.

Progressive Fine Integrated Geophysical Method for Karst Detection During Subway Construction

Various geological disasters can easily occur in tunnels that pass through a limestone stratum. The fine detection of the karst system is significant for safe construction. In this paper, a progressive integrated geophysical method is proposed for fine detection of the karst area of an urban subway. The method combines ground-penetrating radar (GPR), transient electromagnetics (TEM), cross-hole electrical resistivity tomography (ERT), and 3D laser scanning to provide quantitative and integrated detection imaging in a shallow karst system. First, 3D images of reflected waves and the apparent resistivity in the study area are determined using GPR and TEM methods, and the key development of abnormal karst bodies along the mileage direction is delineated. Meanwhile, the TEM slice combination results intuitively show the low-resistance anomaly's distribution and development direction. The main water channel is inferred within the detection range. Second, cross-hole ERT exploration is performed to realize the type of judgment and accurate positioning of karst caves. Finally, 3D laser scanning is conducted to obtain the 3D spatial orientation and size of the large-scale caves identified or those that could have a large influence on metro tunnel construction. Geophysical survey results provide a reference for the design and parameter selection of the cave grouting treatment scheme. This method has been effectively applied in a shield section of the R1 line of the Jinan Metro.

Joint tomography of multi-cross-hole and borehole-to-surface seismic data for karst detection

Two-dimensional (2D) cross-hole seismic tomography is widely used in karst exploration. However, this method only obtains subsurface geological structures from the profiles of two adjacent wells and cannot acquire geological information from the lateral profile direction. The tomographic imaging area and accuracy are also limited owing to sparse ray density in the near-surface area. This approach is therefore not conducive to the spatial identification and evaluation of subsurface karst structures. To address this problem, we conducted multi-hole combined seismic tomography experiments and obtained high-density and high-quality seismic spatial data using an optimized observation system and acquisition parameters. We propose a new method of joint tomography of multi-cross-hole and borehole-to-surface seismic data. We performed 2D seismic tomography inversion by combining cross-hole data with borehole and surface data based on the theory of seismic tomography, and then used the data fusing from multiple wells based on the theory of kriging method. A pseudo-three-dimensional (3D) spatial velocity field of the experimental site is established. The results show that (1) the combined borehole and surface data provide richer ray-tracing information than the cross-hole dataset alone, (2) the pseudo-3D spatial velocity field and spatial distribution of seismic anomalies (karst) can be obtained using the proposed method, and (3) a comparison with actual borehole logging data verifies the effectiveness and accuracy of the proposed method in obtaining subsurface geological structures and detecting the spatial distribution of karst, which has significant practical and application potential.

A borehole multifrequency acoustic wave system for karst detection near piles

Karst cavities at or near pile locations pose a substantial danger to underground construction projects. To locate karst cavities at pile locations, advance drilling boreholes are commonly used, but a cavity can be missed due to its limited size. In this paper, we present a new borehole-based technology employing multifrequency acoustic wave (MFAW) detection of karst cavities near drilling boreholes. First, we developed an MFAW detector and signal processing software. The high- and low-frequency transmitters operate in the frequency range of 0.05 kHz to 10 kHz and the range of 1 kHz to 50 kHz, respectively. Next, we evaluated the performance of the prototype in a controlled borehole model. Lastly, the MFAW detection system was tested at a field site. A nearby borehole core was used to verify the MFAW detection results. The testing of the prototype detection system revealed the following findings. (1) The integrity of the rock surrounding the borehole can be assessed by the energy of the high-frequency zero-offset data. The zero-offset section has a high correlation with the energy of the high-frequency detection signal. The rock is intact when the high-frequency detection signal is strong. The rock is fragmented when the test signal exhibits amplitude variance over a short-range away from the borehole. The test signal becomes weak when the borehole meets a karst cavity filled with sand, clay or water. A color bar showing the signal amplitude can represent the integrity of the borehole wall more accurately than a core sample. (2) A karst cavity near the borehole can be detected by low-frequency waves. The low-frequency acoustic detection wave field includes a reflected guided wave from the interface that the borehole meets, the wave in the borehole, and the effective reflected body wave from the karst cavity. The integrity of the rock around the borehole wall obtained by high-frequency acoustic detection can help identify the reflected guided wave. Median filtering suppresses the strong amplitude borehole wave and helps reveal the weaker reflected waves. The reflected wave from the karst cavity near the borehole can be extracted after filtering. (3) The azimuth of the cavity is identified by the different detection recordings of the azimuthal receivers. Reflected wave amplitudes obtained by the azimuthal receivers facing the karst cavity are greater than those obtained by the receivers facing away from the karst cavity. The azimuthal angle can be obtained by the three-dimensional electric compass in the downhole tool. The MFAW detection system is shown to be an effective tool for investigating the geological environments at pile locations in detailed geo-investigation periods.

Assessment of freely available DSMs for automatic karst feature detection

The purpose of this work is to present the detection and qualification of natural karst depressions in southern Ksiromero, SW Aitoloakarnania Prefecture, Western Greece, by the use of freely available digital surface model (DSM) data in line with the geographical information systems—GIS. The study area is a part of the Ionian geotectonic zone whose geological background consists of the Triassic evaporates and Triassic carbonate breccias. The after-Triassic carbonate series, which consist of the Jurassic limestones, the Cretaceous limestones and the Eocene limestones, are also presented in the study area. Several DSMs such as ASTER GDEM, SRTM DEM, ALOS Global DSM, and a DSM generated from Sentinel-1 data were evaluated so as to be used for the automatic karst detection. Furthermore, a karst detection procedure applied on DSMs was performed with the aid of analogue and digital air photos. Additionally, the aforementioned procedure was applied also on DEM with the aid of digitized contours referring to topographic maps characterized by a scale of 1/50,000. All the detected features have been also validated via in situ observations. Finally, all DSMs presented herein are characterized by similar percentage of success in terms of automatic karst detection, regardless their spatial resolution and height accuracy. Their rate varies from 66 to 73%.

Full elastic finite-difference modeling and interpretation of karst system in a subsalt carbonate reservoir

We evaluated an advanced forward-modeling-based reservoir characterization technique that uses full elastic finite-difference simulation to investigate the limits of karst identification in stacked seismic data. Identification of karsts is important for field development in carbonated reservoirs because paleokarst features can result in a loss of circulation and/or sometimes lost drill bits. Our primary objective was to verify whether we can detect and interpret the location and size of karsts from seismic data, especially given a complex overburden. We constructed an elastic reservoir model consisting of compressional velocity ([Formula: see text]), shear velocity ([Formula: see text]), and density for the study area using interpreted horizons and well log information. Karsts with varying widths, thicknesses, dip angles, and porosities were inserted to generate multiple versions of the model. We also evaluated the imaging impact of overlying faults and salt on karst detection. Full elastic simulation was performed on the various reservoir models using a realistic acquisition geometry to generate gathers, which were then prestack time migrated to quantify the impact of different karst properties on the seismic images and study the effect of reservoir property changes on the seismic response. Finally, a wave-equation target-oriented analysis was presented to improve the understanding of subsalt amplitude and illumination. From the finite-difference modeling and analysis that we performed, we obtained an uncertainty range on karst property estimation from seismic images and gained insights into future survey design for subsalt interpretation and amplitude analysis. For our specific model, we found the limit of karst identification from seismic data is a 30-m-wide horizontal karst or a 500 m karst dipping at 60°. Also, the karst image width reflected its true width only when the actual karst width was larger than the P-wave wavelength (240 m in this case). With a dipping overburden above the reservoir, apparent positions of karsts were shifted in the updip direction by prestack time migration up to 50 m from their true position. This lateral uncertainty should be kept in mind in well planning to avoid karst features interpreted from a time-migrated seismic section.

The Application of Ground Penetrating Radar in the Karst Detection of Tunnel Bottom

In order to ascertain the karst development status under tunnel arch, take the appropriate engineering measures according to the different karst state and ensure the tunnel structure safety. Based on the difference electric properties of electromagnetic wave in the different media, for karst development features of large dam tunnel area, applying geological radar, the karst development status under the tunnel arch dam bottom was detected. Geophysical interpretation show the karst of the rock under arch dam bottom develops well and exist karst structure. The tunnel is not safe. In construction, engineering measures such as excavation and backfill are taken. The results show that: the geological radar is a good resolution for karst development situation; the engineering measures adopted are effective.

Deep confined karst detection, analysis and paleo-hydrology reconstruction at a basin-wide scale using new geophysical interpretation of borehole logs

summary Deep karst voids can be identified by a new method of geophysical interpretation of commonly used borehole logs in deeply confined carbonate aquifers. We show that deep, buried karst voids can be characterized by combining this geophysical interpretation together with geological and hydrological data, and with known speleological constraints. We demonstrate how this characterization can reveal past hydrological regimes and allow mapping of karst distribution on a basin-wide scale. A combined analysis of geophysical, geological, hydrological, and speleological data in the confined Yarkon–Taninim aquifer, Israel, led us to reconstruct past groundwater levels at different relief and sea levels, with the karst voids as a marker for long-term flow close to the water table. Paleo-canyons along the Mediterranean Sea shoreline strongly affected the region’s paleo-hydrology, by serving as major outlets of the aquifer during most of the Cenozoic. We conclude that intensive karstification was promoted by flow periods of longer duration and/or higher flux and flow velocities close to the aquifer’s past and present outlets. In addition, we suggest that karst voids found under shallow confinement were developed by renewed aggressivity due to hypogene water rising in cross-formational flow becoming mixed with fresh lateral water flow from the east.