Determining water well sites based on electrical structure in Taobei District of Baicheng

SHEN Xintu, LIU Lijia and LI Peng

College of Geo-Exploration Science and Technology, Jilin University, Changchun 130026, China

Abstract: Drilling a well randomly would lead to high uncertainties and could not meet the demand of water supply from the increasing population. It is in dire need to improve success rate of well drilling. The high-density resistivity method offers us a good choice. In this study, high-density resistivity method is used for groundwater survey in five villages in the Taobei District of Baicheng. The data obtained by high-density resistivity method is inverted through Res2dinv software. It is found that the electrical structure is characterized by a horizontal layer distribution, and the resistivity shows a "high-medium low-low" feature from shallow to deep. Moreover, obvious electrical gradient zones are identified in the strata below each section, which are interpreted as tectonic weak zones, i.e., the faults. The low-resistance anomaly areas are inferred to be favorable aquifers. The results show that high-density resistivity exploration is an efficient and practical method for determining water well sites in rural areas and can provide a guide for finding water resources in the area.

Keywords: high-density resistivity method; electrical structure; water well; electrical gradient zone

0 Introduction

High-density resistivity method was one of the important near surface geophysical methods. In addition to the characteristics of high density of measuring points, large amount of information and accurate measurement, it would not cause damage to the mea-sured object, and the exploration results were relatively intuitive. The method has been widely used in various fields such as urban engineering, geological engineering, hydraulic engineering and archeological engineering. In China, there were many successful cases of using the method in hydrogeological survey and groundwater resource exploration. The application of high-density resistivity method to groundwater resources exploration in different areas had good results(Tianetal., 2008; Chenetal., 2017; Caoetal., 2018; Zhengetal., 2019). The exploration results show that exists in space and state, which provides a reliable basis for engineering safety implementation and hazard prevention(Chenetal., 2015; Zhangetal., 2017; Yangetal., 2018; Jiaetal., 2018). Therefore, in order to meet the water supply of the villagers in Taobei District of Baicheng, the high-density resistivity method is used to detect the underground water in the area.

Baicheng is in the southwest of Songnen Plain and belongs to the western plain of Jilin, i.e., longitude of 121°38′--124°22′E, latitude of 44°13′--46°18′N. It is located on the Taoer River alluvial fan on the Songnen Plain, at the junction of Inner Mongolia Autonomous Region, Heilongjiang Province and Jilin Province. The exploration area belongs to Quaternary (Wangetal., 1985). In recently years, water-well drilling randomly had high uncertainties, which could not meet the demand of water supply from the increa-sing population in the studied area. It is urgent to find a scientific and effective method to improve the well formation rate.

Compared with the conventional resistivity method, the high-density resistivity method included the advantages of both the electrical sounding method and the electrical profiling method. And it can provide conti-nuous change of strata resistivity in profile, with well data stability and good anti-interference performance (Xuetal., 2011). Moreover，Quaternary was composed of loose sediments, which was suitable for electrode arrangement. At the same time, the resistivity difference between aquifer and surrounding rock was obvious, which was suitable for high-density resistivity exploration. Therefore, the high-density resistivity method is used for the field exploration in this research. The locations of the aquifers and the water intake channels are determined by data processing and interpretation, which provide basis for well drilling.

1 High-density resistivity method

High-density resistivity method came from the idea of array electrical method detection in the late 1970s. The electrical sounding system designed by British scholar Dr. Johansson can be regarded as the prototype of high-density resistivity method (Dong & Wang, 2003).

The method was still based on the difference in conductivity between rock and soil. Its basic principle was the same as the resistivity method. The method established an underground electric field through a grounding electrode. Electrical measuring instruments observed the change of underground electric field in the presence of different conductive geological bodies, thus inferred the distribution or the occurrence of the underground geological bodies (Leietal., 2006). Under different conditions, the resistivity of the same geotechnical material had a larger range of variation than other physical quantities. For the geotechnical material being detected, the resistivity characteristics detected by electrical methods had a stronger identification than the physical properties obtained by other geophysical measurements (Xuetal., 2011).

A and B are current electrodes; M and N are receiving electrodes; n is isolation coefficient; a is electrode spacing (n=1).Fig.1 Working diagram of high-density resistivity method

2 Resistivity characteristics and main aquifers of the studied area

Resistivity characteristics were affected by many factors, such as porosity, water content, mineral composition, saturation, and dielectric particle structure. High-density resistivity exploration was based on the theory that media with different lithology have different electrical properties. The research area mainly consisted of sedimentary rocks, including shale, sandstone and glutenite. The apparent resistivity of gravel layer was 100--120 Ω·m, while the apparent resistivity of medium-coarse grained sand layer was 80--100 Ω·m, and the apparent resistivity of medium-fine grained sand layer was 60--90 Ω·m. The apparent resistivity of sub-sand soil was 40--60 Ω·m. The apparent resistivity of sub-clay was 30--50 Ω·m. The apparent resistivity of clay was 20--40 Ω·m. Statistical analysis showed that the apparent resistivity of clay near the groundwater level was 8--40 Ω·m, and was 27--65 Ω·m for sand and gravel below the groundwater level (Jiaetal., 2017; Suetal., 2009).

The main aquifers in the area were as follows. The porous phreatic aquifer consisted of alluvial gravel and sand gravel in Holocene. The phreatic aquifer consisted of sand and loess-like clayed sand in Gu-xiangtun Formation of Upper Pleistocene. The multi-aquifer leakage system consisted alluvial sand, sand gravel, clayed sand and sand clay in Lower and Middle Pleistocene. The porous confined aquifer was composed of sand and sand gravel in Baitushan Formation of Lower Pleistocene. The porous and fissured aquifer was composed of sandstone and sandy conglomerate in Taikang Formation of Neogene (Wangetal., 1985).

3 Water well site determination

3.1 Introduction of fieldwork

The purpose of this geophysical exploration is to locate favorable aquifer and water intake channel for drilling wells in Fajia--, Bajiazi--, Xinxing--, Yonghong villages and Machang (Fig.2a).

This field work uses Duk-2 high-density resistivity measurement system produced by Chongqing Geological Instrument Factory. Its voltage measurement accuracy is 0.01 mV(±1%)and current accuracy is 0.01 mA(±1%). The electrode spacing is set to 10 m. The minimum isolation coefficient is set to 1 and the maximum isolation coefficient is set to 20. The total number of electrodes is set to 120. The electrode arrangement follows Wenner array (AM=MN=NB=na). The supply voltage is set to 75--200 V.

(a) Traffic location map of the studied area; (b) survey line Machang; (c) survey line Bajiazi Village; (d) survey line Xinxing Village; (e) survey line Fajia Village; (f) survey line Yonghong Village.Fig.2 Traffic location map of the studied area (a) and layout of survey lines(b-f)

The layout of survey lines is very important in the onsite engineering exploration, which is closely rela-ted to the exploration effect. The exploration principles of high-density resistivity method are: ① The survey line should be laid as perpendicular to the trend of the anomalous body as possible; ② the length of the survey line should be appropriately selected to be longer than the length of the exploration target area to obtain as much background value as possible; ③ the distance between the measuring points should meet the requirement of having at least 1 or 2 observation data points on the anomalous body; ④ the depth of detection should be greater than the thickness of Quaternary soil layer and the depth of the water table in the area.

Based on the above basic principles, the positions of the five survey lines are determined in advance in the exploration area (Fig.2b-f). The survey lines of Machang (CC’) and Yonghong Village (EE’) are laid from south to north. The survey lines of Fajia-(DD’), Bajiazi-(AA’) and Xinxing (BB’) villages are laid from west to east. Each line is 1 050 m in length, 5 250 m in total.

3.2 Basis of water-well site determination

Fault is a passage for fluid migration such as groundwater, which can also be used as a structure with water storage under certain geological background. When there is a water-bearing stratum, the water can be separated out, transported and stored in the fault. So the kind of faults can be used as water intake channel. In this paper, the well site is determined based on the location of such fault. In other words, the site of water well drilling on the ground generally corresponds to the location of the underground fault. The depth of well drilling depends on the depth of aquifer, that is, the water-well depth should be over the upper boundary of the aquifer. Generally, fault is characterized by an electrical gradient zone or an electrical boundary zone in geoelectric cross section. The high-density resistivity method has a good effect in fault identification, which can provide an important basis for determining well site.

3.3 Data processing method

After the high-density resistivity measurement was completed, the obtained data was the apparent resistivity measured by each electrode at different positions. The data need to be calculated by inversion to obtain the resistivity of the underground geological body in the form of a graph. The instruments for high-density resistivity survey were generally equipped with built-in inversion processing software. Currently the most widely used two-dimensional resistivity inversion software was the Res2dinv developed by Dr. M. H. Loke of Sweden. This program was based on the least-square method of smoothness constraint to optimize the model. It automatically determined a two-dimensional resistivity model based on the apparent resistivity data of the survey line profile, which was effective (Yanetal., 2012).

In this survey, Res2dinv software is used to process the raw field data of apparent resistivity at different depth points on the ground. In data processing, it is necessary to eliminate the distorted data points caused by poor coupling between the electrodes and the ground. The number of iterations is set to 3 and 5 to obtain the resistivity profile of the inverse model. Finally, the interpretation of the underground electrical structure combines with the main aquifers and resistivity characteristics of the studied area.

3.4 Geophysical interpretation and result analysis

Geophysical data and geological and geomorphological observation data were acquired in the field survey. After quality analysis, data processing and resistivity inversion, the section of inverse model resistivity can be obtained. It is the basis for identifying underground geological bodies. Any influence of topography and geological conditions must be taken into account in the geophysical interpretation. In the process of data interpretation, the geological and geomorphic information of the studied area should be made full used of for comprehensive interpretation and tried to exclude the influence of natural factors.

It is shown in the geoelectric cross sections that the stratum is nearly horizontally layered distribution(Fig.3). The sedimentary thickness of the stratum is slightly thicker in the west in Bajiazi- and Xinxing villages, which is slightly thicker in the north in Machang and uniform in Fajia- and Yonghong villages. The low-resistance abnormal areas surrounded by the white thin dotted lines are judged to be the favorable aquifers. According to the sequence of Bajiazi-, Xinxing villages, Machang, Fajia- and Yonghong villages, it is can be seen from the sections that the aquifers buried depth are 57 m, 70 m, 80 m, 70 m and 50 m below the survey lines, respectively(Fig.3).

The electrical gradient zones marked by the white thick dotted lines are referred as the faults in the stratum. The recommended water-well sites in each survey line are located at where the electrical gradient zones with obvious electrical change and shallow bu-ried aquifers. And they are arranged at 310 m, 530 m, 250 m, 280 m and 580 m along the survey lines, respectively. The depth of the proposed well site should exceed the buried depth of the aquifer at its location, which is 57 m, 80 m, 80 m, 80 m and 50 m below the survey line, respectively.

(a) Bajiazi Village; (b)Xinixng Village; (c) Machang; (d) Fajia Village;(e) Yonghong Village.Fig.3 Inverse model resistivity sections

4 Conclusions

In the water-well drilling research in five villages of the Taobei District of Baicheng, it is confirmed that the high-density resistivity exploration can effectively identify the faults and the aquifers within 100 m underground. Therefore, the method provides a good choice for determining the water well sites.

(1) Data collected by the high-density resistivity survey lines are processed using Res2dinv. The geo-electric cross sections are obtained.

(2) Examining the geoelectric cross sections, it can be concluded that ① the electrical structures of the strata generally show a horizontally layered distribution; ② the shallow part shows high resistance and the middle part shows medium and low resistance; ③the low resistance abnormalities in the deep part are the electrical reflection of the aquifers, which distri-bute in the depth range of 50--80 m; ④ there are electrical gradient zones in each section, which are inferred to be the reflection of the faults.

(3) Finally, considering the basis of water-well site determination and result analysis, the proposed well site and depth of the five villages are confirmed.