The Development Of Borehole Electromagnetic Receiver

With the mineral resources were exploited continually,shallow resources are almost exhausted,and exploration and development work is urgently needed for deep resources.The conventional ground electromagnetic method has the problem of shallow depth of detection and low resolution.The Borehole-tunnel-surface electromagnetic technology and equipment has developed a new method for three-dimensional electromagnetic observation in the ground surface,tunnel and borehole,aiming at increasing the depth of detection and improve deep resolution.This dissertation describes the development and current situation of electromagnetic detection in borehole,and introduces the design scheme of self-developed borehole electromagnetic receiver in detail.As a part of the system,the borehole electromagnetic receiver can work in many modes to acquire electromagnetic signals of natural field and artificial field,and complete the storage and transmission of signal data.Receiver consists of sensor,analog acquisition circuit,digital control circuit,power circuit,communication device,inclinometer and lithium batteries.The scheme adopts ARM+FPGA as the main control and 24 high precision AD design.When the external physical signal is converted into voltage signal by the sensor,it enters the analog acquisition circuit and is amplified by the front-end conditioning circuit,and the 24-bit high-precision AD can complete the analog-to-digital conversion.The analog acquisition circuit has four channels that can be used to acquire three-axis magnetic field and vertical electric field signals,A highaccuracy clock output is obtained from an OCXO,which cooperates with GPS signals to insure acquisition synchronization.The control circuit includes an ARM core board and an FPGA core board.The ARM core board implements the control of the acquisition process,data storage,transmission,access to peripheral circuits,and communication with the PC;the FPGA core board as a link between ARM and analog acquisition circuits,with ARM to complete the entire acquisition process control.The inclinometer can record the receiver's position in time,and obtain the angle,so as to take it into account in later data processing.The receiver converts the lithium battery voltage to power other circuits the through LDO and DC/DC circuits internally.FPGA program written by VHDL language,mainly for data read timing processing and clock frequency division,ARM_Linux driver implements SSC,SPI,input interrupt and DMA mapping,etc.ARM_Linux application to complete the acquisition control and data write disk.The borehole electromagnetic receiver uses a series of key technologies such as low noise,wide frequency bandwidth,and large dynamic range to acquire high signal to noise ratio electromagnetic signals,to collect signals of three-axis magnetic field and vertical electric field,self-storage and high-speed data transmission,and ultra-longdistance communication over 3000 m are realized.Finally,through a large number of indoor and field tests,the feasibility of the design of the borehole electromagnetic receiver was verified.