Theoretical analysis of sounding anomaly and field application of the natural electric field frequency selection sounding method in groundwater exploration

doi:10.13745/j.esf.sf.2020.6.34

Abstract

Abstract:

The frequency selection method is a further development of the audio frequency telluric electricity field method. In this report, we illustrate the usefulness of the method in shallow groundwater exploration through field application examples. We studied the relationship between potential electrode spacing MN and groundwater depth and carried out preliminary theoretical analysis. Firstly, under the combined action of horizontal alternating electric and magnetic fields, we established a simplified geophysical model of low resistivity conductive sphere in homogeneous half space, and performed forward calculation on the sounding curve. We then recorded water yields of 131 rural drinking wells in Guangxi Province. In addition, we performed detailed tabular statistical analysis of 98 drilling wells, and compared the relationship between potential electrode spacing MN in abnormal sounding curve and actual drilling water depth. Theoretical analysis and field application show that the frequency selection method can be very useful in shallow groundwater exploration for determining shallow groundwater well location. At the same time, the tabular statistics show that there is 1∶1 approximation between the size of potential electrode spacing MN in the anomaly curve and actual drilling water depth, validating the theoretical simulation results. Moreover, we show that the observed value of telluric electric field is related not only to the Earth's resistivity and signal frequency, but also to the potential electrode spacing in a shallow (< 200 m) magnetotelluric exploration.

Key words: frequency selection method for natural electric field (FSMNEF), groundwater, sounding, electromagnetic field, stray current

CLC Number:

YANG Tianchun, CHEN Zhuochao, LIANG Jing, DAI Shixin, ZHOU Lin, YANG Zhui. Theoretical analysis of sounding anomaly and field application of the natural electric field frequency selection sounding method in groundwater exploration[J]. Earth Science Frontiers, 2020, 27(4): 302-310.

Figures/Tables 9

Fig.1 A schematic diagram of a profile detection device (a) and a FSMNEF sounding device (b)

Fig.2 Profile survey (a) and FSMNEF sounding (b) results for Zhoujia village

Fig.3 A sphere in a naturally alternating electromagnetic field in half space

Fig.4 A sphere in an uniformly alternating magnetic field in z direction

Fig.5 The FSMNEF forward calculation curve

Table 1 The change of MN/h0 at the abnormal points of FSMNEF sounding curves with spherical resistivity ρ1

ρ₁ /(Ω·m)	150	100	80	50	40
MN/h₀比值	1.20	1.07	1.00	0.80	0.60

Table 2 The change of MN/h0 at abnormal points of FSMNEF sounding curves with surrounding rock's resistivity ρ2

ρ₂ /(Ω·m)	4 000	1 500	1 000	800	500
MN/h₀比值	1.00	1.00	0.933	0.933	0.80

Table 3 Summary of FSMNEF application in the rural drinking water safety project in Guangxi Province

序号	应用区域	钻孔数量/个	钻孔深度/m	岩性概况	不同出水量的钻井数量/个
序号	应用区域	钻孔数量/个	钻孔深度/m	岩性概况	≥5 m³/h	>1 m³/h~ <5 m³/h	≤1 m³/h
1	武宣县	84	80.0~142.8	灰岩	52	22	10
2	平乐县	17	75.2~135.0	灰岩	10	3	4
3	恭城县	25	80.3~126.6	西岭乡虎尾村GC8-1钻孔处的基岩为灰绿色泥岩、泥质砂岩;西岭乡虎尾村GC8-2钻孔处的基岩为灰岩与泥质粉砂岩互层;其余钻孔处基岩皆为灰岩	20	3	2
4	藤县	2	96.0, 125.9	砾岩		1	1
5	容县	1	102.6	风化砂质泥岩	1
6	覃塘区	1	98.5	灰岩	1
7	港北区	1	115.4	灰岩	1
合计					85	29	17

Table 4 Characteristics of some typical borehole

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