Groundwater investigation in Mankayan mining area, Philippines

Last update time : 09/10/2019

Author : zhang jie

Invited by Philippine DDCP Company, Geomative arranged Geophysical Instruments that is GD-10 High-density electrical equipment operation training. Also by the training, Geomative assistted DDCP to complete the investigation of the water content and surrounding geological stability of the dam foundation in Mankayan mining area. Meanwhile the investigation provided location reference for the later construction of roads near the abutment.

The geographical location and surrounding conditions of the dam are as follows. The dam body has no concrete structure and it is a simple earth-rock dam. Both sides of the east and west are high-lying slopes. 2D ERI The two measuring lines are laid out as follows: ERT1, ERT2.

Electrical resistivity tomography working principle

The resistivity method is a geophysical method based on the difference in dielectric resistivity. The detection principle of the DC resistance method is to establish an artificial electric field by using a direct current to pass a pair of current poles A and B into the ground. By the difference in conductivity between the interlayer media, another potential pair M, N can be used to measure the potential difference between the electric field between M and N, thereby obtaining the apparent resistivity of the formation and estimating the underground formation. Conductivity distribution.

It is often presented as a cross-sectional equivalent image. Physical units often use resistivity (ohm-m) to represent different levels of color with different tones.

ERT1 line (Survey for water content in dam foundation)

Ⅰ. Purpose of investigation
Through the electrical resistivity survey, the water content of the base of the dam can be made out, thus, the stability of dam body can be understood indirectly.

Ⅱ. Investigation plan

Detection was performed using geophysical exploration with Electrical Resistivity tomography. The ERT1 line was laid along the dam top,with a length of 370m and an electrode spacing of 10m. According to the change of the topography of the line, GPS positioning of the corresponding electrode position is required to perform terrain correction. The array types :wen-sch, sch, and dipole-dipole.

ERT Line 1 Investigation result

As shown in the inversion results on the right: the depth of the detection is 70 meters. The results of the three electrode devices indicate that the horizontal layer of the survey area is obvious. The first layer is the surface interference layer with an average thickness of about 3.5 meters.
the dispaly of the resistivity imaging measurement .The second layer is presumed to be the foundation of the dam body, which is compact , so it presents a relatively high-resistance layer with an average thickness of 25 m. The buried depth of 30 m or less is presumed to be a relatively low-resistance layer with a high water content, which may be the natural sedimentary layer of the original valley; It is also possible to enrich the adsorption layer of metal elements in the mining area.
     According to the ERT method and the distribution of the vertical water content, the stability of the dam can be analyzed.

Wenner-Schlumberger
Schlumberger
Dipole-Dipole

ERT2 line (Geological stability detection around the dam)

I. Purpose of investigation
   1. Purpose of investigation
     Through geophysical investigation, the geological stability of the eastern slope of the dam abutment was studied, and the exploration of the depression zone and the fault zone were simulated.

Ⅱ. Investigation plan
With Electrical Resistivity tomography, the ERT2 line with the total length 150m was laid from west to east; the electrode spacing was 5m and the total electrodes number was 31. Along the variation of the topography of the line, GPS positioning of the corresponding electrode position is required to perform terrain correction of the inversion process.

ERT line 2 Investigation result

As shown in the inversion results on the right: the depth of the detection is 15 meters. The results of the three electrode devices indicate that there is a clear karyo-difference boundary in the stratum. The upper apparent resistance is relatively high and the lower layer is relatively low.

          According to the surface topography, it is inferred that the low-resistance stratum may be a metal mine interference zone, a fissure-filled high water content zone, and possibly a hollow water-bearing zone. It is necessary to identify by using drilling.
          According to the ERT method and the existence of the vertical lithology interface, the slope stability can be analyzed initially.

Advantages of geophysical methods

It can carry out two-dimensional geoelectric section measurement, and has the functions of section method and sounding method. It is an effective means for stratigraphic division, detection of hidden fault structures, karst cavities and geological landslides.
It can effectively determine the size, shape, distribution range and projection position of the underground goaf and roadway, and provide suggestions and guidance for the construction of the owner.
Compared with other traditional methods, this method has the advantages of low cost, high efficiency, rich geoelectric information and high detection accuracy, and is worthy of popular use in engineering geological exploration.

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Last update time 09/10/2019

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