Study On The Depth Of Coal Seam Floor Failure Detected By Direct Current Resistivity In Borehole

The safety of coal mining has always been concerned by all sectors of society,among which water inrush disaster is one of the major disasters in coal mine,which causes great economic and personnel losses,especially the floor pressure area.It will cause rocky damage of working face floor,the electrical property will be changed and the thickness of floor water-proof layer will be reduced when the working face of coal seam begins to be mined.So,if it cannot determine the thickness of water-proof layer whether can withstand the pressure of floor confined water,it will cause serious impact on the normal mining of working face.The dc resistivity method mainly reflects the electrical information of underground rocks and ores,while the borehole is generally close to the target body.In this paper,combined with the advantages of the dc resistivity method and borehole,a dc resistivity method in the borehole is proposed to detect the depth of coal seam failure.The supply and receiving electrodes are arranged in the borehole and a data is acquired before the working face mining and calculating the apparent resistivity.The apparent resistivity change rate is obtained by detecting the apparent resistivity which under the different degrees of mining and the initial apparent resistivity.Combine the apparent resistivity,apparent resistivity change rate and the change of current data which collected by each electrode to determine the depth of working face floor fracture zone.Firstly,through the numerical simulation,analysis the detection results characteristic by direct current method of high and low resistivity of working face floor fracture zone.The apparent resistivity change rate characteristic is got by the formula,and get the results: the apparent resistivity change rate form of high resistivity fracture zone of working face is relatively complex,change rate closed contour are formed in the front and bottom of fracture zone of working face with the mining working face advancing;the apparent resistivity change rate form of low resistivity fracture zone of working face is relatively simple,the maximum apparent resistivity change rate on the center of the fracture zone when the resistivity of floor beneath is consistent,the maximum apparent resistivity change rate in the bottom boundary of thinner fracture zone and the maximum apparent resistivity change rate in the center of thicker fracture zone when the resistivity under the floor is high-low-high-low state.Then,selecting a preparation of mining working face and acquiring current and potential data by network parallel electrical instrument.And changing the potential data to apparent resistivity and apparent resistivity change rate.Later,cross section profile of current,apparent resistivity and apparent resistivity change rate are drawn.Combining with the three kinds of data to comprehensive analysis the depth of working fracture zone.Comparing with the traditional DC method,the joint interpretation of current variation,apparent resistivity and apparent resistivity change rate which collected by DC method in the borehole has the higher resolution for the thickness of working face floor fracture zone.