pg(s) 255-261 Because
of heterogeneity and anisotropy, it is very difficult to
optimize groundwater exploration drilling locations in karst
areas using only hydrogeological information. However, the
integrated application of the audio frequency telluric method
and electrical resistivity tomography has proved to be
efficient for groundwater exploration in karst areas with or
without thin cover. In the case studies presented here, audio
frequency telluric profiling is used to roughly determine the
location and strike of a karsted or fractured zone where the
cover thickness is less than 30 m, then an electrical
resistivity profile perpendicular to the strike of the zone is
designed to reconstruct the resistivity structure with a
Schlumberger array. By combining the geophysical results with
available hydrogeological data, an optimal drilling site can be
established. This integrated geophysical approach for karst
water exploration has been used in several projects and the
results show that the method is reasonable and useful.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 INTEGRATED GEOPHYSICAL METHODS FOR GROUNDWATER EXPLORATION IN A KARST AREA WITH OR WITHOUT Fuping Gan, Yixiang Chen, Wei Zhao, Yuling Chen, Wei Liu Institute of Karst Geology, CAGS, Guilin, Guangxi 541004, China Karst Dynamics Laboratory, MLR&GZAR, Guilin, Guangxi 541004, China the audio frequency telluric method (AFTM) has been successfully employed to detect water in karst settings of water in combination with the induced polarization (IP) method (Li, 2009). The typical advantages of this method this is countered by disadvantages, including a shallow investigation depth and a lack of anomaly variations with depth. Electrical resistivity tomography (ERT) to delineate aquifers (Kumar, 2012), to search for karst (Metwaly, 2012; Vlahovi 2011) using characteristics of the resistivity variation with depth. Compared with using a single geophysical method alone, the integrated approach usually provides more reliable information, and as a result has been widely applied for groundwater investigations. Alexopoulos (2011) employed both the very low frequency (VLF) electromagnetic method and ERT to map water pathways. Vargemezis (2011) even together to optimize locations for the construction of hydro wells. Two examples presented here to trace karst water show that the AFTM and the ERT methods have the advantages Geophysical methods Audio frequency telluric method (AFTM) This method, which takes advantage of natural telluric current variations with frequencies induced in the earth by phenomena such as solar emission and thunderstorms, detects conductive difference distributions underground and interprets them in terms of geology or hydrogeology. The AFTM is a preferred method for rapid ground reconnaissance and shallow exploration where Abstract Because of heterogeneity and anisotropy, it is very locations in karst areas using only hydrogeological audio frequency telluric method and electrical resistivity exploration in karst areas with or without thin cover. In the case studies presented here, audio frequency location and strike of a karsted or fractured zone where is designed to reconstruct the resistivity structure with a Schlumberger array. By combining the geophysical results with available hydrogeological data, an optimal drilling site can be established. This integrated geophysical approach for karst water exploration has the method is reasonable and useful. Introduction Since 2009, extreme climate events, persistent drought and low rainfall, has made drinking water scarce for human consumption and agricultural purposes in some parts of China, especially in karst regions. At the beginning of 2012, a groundwater exploration team was drought disaster relief. The team traveled to Tai'an city, Shandong Province, in the east of China (Figure 1) to search for promising exploratory sites for karst water using hydrogeological investigations and geophysical optimally positioned and drilled. Subsequently, 21 were tested by pumping, and produced an abundance of water. environments, suitable geophysical methods needed to be 255
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE Apparent resistivity data were collected and stored automatically using 60 electrodes with a 10 m spacing. because of its suitability for the study of horizontal and vertical structures. The results are produced using Res2dinv software, which calculates resistivity and depth values for geological investigation depth mainly depends on the total length of the spread. Case study 1 Geological setting Shangdong Province, China. The widely distributed overburden in the region, with a thickness of about 10 m, is conducted in situ to determine potential gradient measurements along a line, which is usually set up perpendicular to the strike of geological structures. The equipment used to measure the potential gradients was stations were usually 10 m apart. Because telluric currents change with time, the potential gradient between within an hour). In the end, the potential gradient was plotted against the midpoint of the potential electrodes. This method is often used to map shallow subsurface karst features showing relatively low potential values if Electric Resistivity Tomography (ERT) (manufactured by Chongqing Benteng Digital Control 256 Figure 1. Location and geological map of the project area in Taian City, Shandong Province, China 1. Granite, 2-4. Limestone (1, 2, and 3), 5. Dolomite and Limestone (O1) 6. Sandstone and breccia (E), 7. Quaternary, 8. Taian City, 9. Wen River, 10. Fault, 11. Guanlu, 12. Momoshan.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 formed locally, are targeted as karst water reservoirs. A hydrogeological map shows their distribution in Figure 2. Geophysical methods 2 horizontal trend of zones of lower potential and then the ERT method was used to vertically probe lower resistivity variations with depth. The potential curve of the AFTM for line 5 in Figure shows somewhere underground. In the end, the regions of lowest potential from all two main branches of the low potential anomalies in wide in the south. These lower potential anomaly zones, because of their wider distribution, are inferred to be consists of Quaternary deposits of sand and gravel. The bedded limestone or argillaceous limestone embedded Cambrian), as shown in Figure 2. eastward about 600 m to the north, coincides with the south to north following two discharge mechanisms. The scattering discharge as a result of the shallow incision of the valley, the low hydraulic gradient of groundwater, and the wide areal distribution of the groundwater. The second involves abundant volumes of groundwater The condition of the landforms favors groundwater one disadvantage is that contaminated water may intrude into the system from the Wen River as a result of the extraction of karst groundwater. Regional tectonic structures trend from northwest to 257 Figure 2. Hydrogeological map of survey site and geophysical profiles (1. Survey site, 2. Surface water direction, 3. Drill hole, 4. Fault, 5. Quaternary, 6-10. Cambrian Limestone, 11. Recommended borehole, 12. Fracture zone, 13. Profile direction, 14. Station/profile No., 15. Dip direction and dip).
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE karst and valley topography. The maximum elevation difference in the region is about 150 m and the valley positioned with only small changes in elevation, are located across a hill slope showing bare karst rocks. argillaceous limestones or edgewise limestones embedded with thin shale (upper Cambrian). Karst moves from north to south in a direction that is nearly opposite to the dip direction of the beds. A map of the hydrogeology is shown in Figure 5. a depth of about 100 m and the location of the survey to search for groundwater resources. Since 1949, three dry wells have been drilled nearby. From a landform perspective, high in the north and low in the south, it is likely that groundwater in this region moves southward Therefore, appropriate geophysical methods can be chosen to outline these favorable zones. Geophysical methods constrain the position and orientation of groundwater runoff zones, then the ERT method was applied to Figure 5. The zone of lower potential anomaly is (see Figure 6). Because of site limitations, only one ERT array resistivity contours in Figure 7 (upper) readily ) Regions of lower resistivity determined by the integrated favorable well positions. Because of site limitations, the the similar characteristics of the anomaly there. Subsequently, the ERT line was set up along the strike of the lower potential anomaly. In Figure 4 (upper), Schlumberger array apparent resistivity contours, taken from the ERT for line 5, show a clear contact between In particular, the striped low resistivity zone between in Figure 4 (lower) was predicted from the Schlumberger resistivity section in the ERT to consist of a subsoil of alluvial clay, sand and gravel that is about 26 m thick. 26 70 m and 90 110 m, respectively. These predictions were tested by hammer drilling. The resulting well is 150 m deep, with a static water level highly fractured nature of the rock that caused the pipe to stick often. After pumping tests, the pumping capacity was set at 480 m Case study 2 Geological setting at Momoshan village, approximately 20 km west 258 Figure 3. Potential curves of the AFTM for lines 1, 2, 3, 4 and 5.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 259 Figure 4. Resistivity imaging (upper), Geological interpretation for profile 5 (lower) 1. Fine sand with gravel, 2. Gravel with fine sand, 3. Limestone, 4. Karst fissures, 5. Interbedded limestone and shale. Figure 5. Hydrogeological map of the survey site and geophysical profiles 1. Survey site, 2. Surface water direction, 3. Fault, 4. Quaternary, 5-8. Cambrian Limestone, 9. Fissure zone, 10. Survey line direction, 11. Station/ Line No., 12. Recommended borehole, 13. Dip direction and dip.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE Conclusions m) have shown that an integration of AFTM and ERT to optimize data acquisition and provide crosschecks can produce promising results. Fieldwork procedures usually consist of the following steps. 1. collected primarily to roughly establish the that may focus karst development and result in depth estimates are not generally possible with this procedure alone. 2. Second, to constrain the geometry of horizontal and vertical geological structures, ERT data low potential anomalies, thereby enabling the detailed interpretation of karst geological structures by using information about resistivity variations with depth. For a Schlumberger array consisting of 60 electrodes spaced at 10 m the maximum depth of penetration is about 150 m The well depth is 190 m with a static water level at a depth of about 80 m. After pumping tests, the yield was determined to be 280 m drawdown of 40 m. The flow comes mainly from units at depths of about 127 and 161 m. 260 Figure 7. Resistivity imaging (upper), Geological interpretation for line 1 (lower) 1. Clay, 2. Limestone, 3. Karst fissures, 4. Interbedded limestone and shale. Figure 6. Potential curves of the AFTM for lines 2, 3, and 4.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 Metwaly M, Elawadi E, Moustafal SR. 2012. Saudi Arabia. International Journal of Physical Sun JL.2002.Applicaton of natural audio frequency islandsthe island of Olib, Croatia.Environ Monit Assess.184:6211. Available from: karst cave probing. Site Investigation Science and The two methods taken together constrain lower potentials distributed horizontally (along strike) and similar changes in resistivity varying vertically. When combined with hydrogeological information, this procedure provides important and effective geophysical indications of the geological setting, and enables the optimal determination of well positions. References Chen S. 1988. The use of selected frequency of natural exploration in the arid and semiarid areas tomography technique in mapping shallow Zaidi FK, Kassem OMK. 2012. Use of electrical resistivity tomography in delineating zones of groundwater potential in arid regions: a case study from Diriyah region of Saudi Arabia. Arabian focusing approach to ground water detection by means of electrical and EM methods ,the geophysical surveys to assess the structural conditions of a karstic cave of archaeological Sciences 5:17. Alexopoulos JD, Dilalos S, Vassilakis E.2011. Meteora). Advances in the Research of Aquatic Environment Vol. 2. Available from: prospecting in granite region by comprehensive geophysical methods. Site Investigation Science 261
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