Experiments on ground-penetrating detecting coal-rock interface with radio pulse radar

Technologies such as transient electromagnetic,seismic wave,ultrasonic wave and ground penetrating radar, etc. have been utilized in the traditional coal and rock detection to transmit and receive signal frequencies below 1 GHz. However,utilizing these technologies cannot give consideration to both the detection depth and accuracy. As the coal-rock interface shall reach the detection accuracy of 20 mm according to requirements of the automatic horizon control of shearer in the fully mechanized coal mining face,the high-precision depth detection technology that penetrates the coal and rock seam with the radio pulse radar is proposed. Low-power and downsizing UWB ( Ultra-Wide Band) radar is designed with mono-radar chip+RF circuit. And it’s the PG ( Pulse Generator) that transmits a sub- nanosecond narrow pulsed electromagnetic waveform to output UWB signals at the center frequency ranging from 5. 3 to 8. 8 GHz,the bandwidth ranging from 1. 65 to 4. 40 GHz and the power ranging from -17. 2 to -10. 5 dBm. Mean- while,the input circuit collects the signal voltage of the peak-to-peak value ranging from 54 to 72 mV reflected from the coal-rock medium. The UWB radar is arranged closely through the dual Vivaldi antenna being perpendicular to the coal and rock seam. The transmitting antenna sends 7th order Gaussian pulse UWB band signal as low as -19 dBm, while the receiving antenna collects 512-depth return signals at a rate of 30 Gbps. In this way,the precise coal-rock in- terface location can be estimated through multiplying the propagation delay of pulse reflection and sending signals gen- erated by the apparent difference of dielectric constant in the coal and rock by the electromagnetic wave propagation rate calculated after calibrating the dielectric constant of at the coal-rock site. Moreover,based on changes in the depth of coal seam and dielectric constant,the relationship of coal-rock detection among pulse vertical resolution,detection depth and length of the signal sampling window are established to determine that the length of sampling window is 256 ns with the measuring resolution reaching 4 mm. A waveform gray-scale map is drawn with pulse signal data col- lected for intuitively distinguishing the coal-rock interface location. To be specific,dark black and bright white in the map are the valley and peak of the signals,respectively,representing the interface locations of two media of coal-rock with great differences in the dielectric constant. In the meantime,the thickness of the coal seam is calculated by the time difference from the valley or the peak to the starting point. In the fully mechanized coal mining face and roadway field test for roof coal mining,the thickness of the coal seam measured with the error less than 20 mm can detect the thickness of the roof coal and the floor coal of the working face in the virgin coal,providing accurate location reference for the automatic horizon control of the shearer.