The parallel electrical technology was first proposed by Anhui University of Science and Technology [8] . And it was verified in a large number of the models and field tests. This method achieved significant effect [9] [10] . The observation system of the two-gateways parallel 3-D electrical technology depends on the roadways. Usually electrical lines are laid along the air roadway (the roadway of F) and the conveyor roadway (the roadway of J) in the roadway floor. At the same time, the power supply electrode B is placed in the opposite lane corresponding (Figure 1), and plus 1 infinite electrode N. The parallel electrical instrument is used in data collection. It is applied to observe the changes of underground space electric field surrounding each electrode in the line. When one electrode is powered, the remaining electrodes simultaneously measure the potential changes of natural electric field, the first field and the second field. The multiple coverage greatly improves the detecting accuracy. When a station data acquisition is completed, back line is moved to the next station data acquisition while covering a certain area of the former station until the whole detection task is finished.

Depending on the different arrangements of electrodes, the data acquisition of the parallel electrical technology works in two ways: single point power field (AM) and opposite electric field power supply (ABM).

1) Operating mode of AM

In AM acquisition, the opposite electrode B is arranged in the position corresponding in the oppositional roadway. Each electrode in the line combining B pole supplies power once, at the same time the other electrodes in the line are collecting the electric field data. Through the professional software decoding, we can get the data of the double-pole device, the three-pole device and the composite profiling device in the resistivity profile method. The potential field it measures is a point electric field (Figure 2). The method is suitable for long line detection task on account of its single point of power supply, fast data acquisition and greatly-improved working efficiency.

2) Operating mode of ABM

The observation system of ABM is similar to that of AM. The only difference is that ABM works in a way that two electrodes supply power, while the other electrodes collect data synchronically. Via the parallel elec-

trical collection system, a measurement can achieve the effect of all kinds of four-pole device of high density electrical prospecting, which improves the collection efficiency highly, reduces the system error, and acquires plenty of data. Compared with the AM method, ABM takes longer to get the results but more precisely. The ABM method of data collection is a dual specific point source electric field (Figure 3).

The data of the parallel electrical technology is inversion by the professional soft-ware AGI. The data collected by the two-gateways parallel 3-D electrical technology is decoded and extracted to getting different devices’ currents and potential data. Typically, the currents and potential data will be combined and used 3-D resistivity inversion. The general form of 3-D resistivity inversion problem is expressed as:

In the equation: G is a Jacobi matrix; is the difference between the observed data of d and the residual vector forward theoretical value of d₀. For the three-dimensional problem, the model is divided into three dimensional grids, and the parameters of each grid cell are the conductivity values. At the same time the observation data of 3-D inversion is a measurement of the potential difference. Because of the large variation, logarithm is used to calibrate the inversion data and model parameters in order to ensure the stability of inversion. AGI inversion will provide the resistivity profile in the region, and analyzing the resistivity profile can locate the water-rich areas, which is the basis for the coal mine drainage and grouting engineering.

The mine in the North-Anhui for the test, its 1018 main seam is 10 coal seam, which is in the middle of Shanxi Formation. The seam is 75.5 cm below the Al mudstones, 50.79 cm average above Taiyuan Formation first Limestone. Its floor thickness is 4.65 - 5.36 cm, 4.76 cm in average, variation coefficient γ = 24%, workability index K = 1, dip 5˚ - 15˚° (average dip12˚). It is estimated that the floor is threaten by limestone water of coal mine floor, so the hydrogeology condition should be researched before the extraction. This operation detects the strata aquosity under the floor of working face using the two-gateways parallel 3-D electrical technology to determine the water-rich areas of coal face floor accurately, to guide drilling surveying, releasing water and grouting work, and to ensure the safety of the working face extraction.

The data of the two-gateways parallel 3-D electrical technology method are decoded by AGI inversion software to invert the resistivity profile (Figure 4). The diagram indicates 6 low resistivity abnormal areas in the working

face floor. The rest are all above 40. In the low resistivity areas, 4 stretch widely to the floor and face except that 1# and 2# are low and outskirt of roadways. 40 is the dividing line of high and low resistivity: 30 - 40 indicates water-bearing area; less than 30 indicates water-rich area. As a result, the low resistivity is a big threat to working floor extraction.

To cope with the 6 low resistivity abnormal areas, mine workers implemented lots of drilling grouting, The water hole position, water yield, and grout amount of the detection area shows that achieves great effects.

Figure 5 is drilling water yield and grouting consumption statistics distribution map. The resistivity picture before grouting shows that the resistivity of abnormal area 1, 2, 3 is ρ_s1 = 35, ρ_s2 = 25, ρ_s3 = 25respectively; the inflow of abnormal area 1, 2, 3 is Q_i1 = 14 m³/h, Q_i2 = 44 m³/h, Q_i3 = 10 m³/h respectively; and the grouting amount of abnormal area 1, 2, 3 is Q_g1 = 100 m³, Q_g2 = 600 m³, Q_g3 = 200 m³ respectively. As can be seen, the areas of low resistivity before grouting are consistent with the results of the water inflow and the grouting. That is low resistivity abnormal area in the detection indicates rich water in the calcareous rock floor, so the grouting quantity should be high values too.