| Transient electromagnetic observation voltage is the most original data obtained in field work,is the basis of reflecting geoelectric information of underground geological bodies,and is an important basis for inference and interpretation.Because the transmitting coil is a non-pure resistive load,it takes a certain time to turn off the actual transmitting current,which results in the doping of the primary field signal in the observed voltage.The inherent transition process of the receiving coil results in the distortion of the early observation voltage.In this paper,the influence of the basic parameters such as the length of the transmitting coil,the number of turns,the diameter of the cross section of the conductor,the resistivity of the conductor and the emission voltage on the inductance,the turn-off time and the observed voltage of the emitter coil are studied.The relationship between the three damping states of the receiving coil and the observed voltage is studied.The expressions of the observed voltage of the primary field before and after switching off when the receiving coil is under-damped or over-damped are derived,and the expressions of the observed voltage of the secondary field before and after switching off when the receiving coil is under-damped or over-damped are derived.The characteristics of primary field observation voltage,secondary field observation voltage and total observation voltage are studied when the receiving coil is in critical,under-damped and over-damped state.The influence of the parameters of the transmitting coil on the observed voltage is studied.That is,the effect of the primary field signal is not entirely determined by the turn-off time,but by the ratio of the magnitude of the transmitting current to the turn-off time(i.e.,the turn-off slope).The greater the absolute value of turn-off slope is,the more serious the effect of primary field on the total observed voltage is.The side length of transmitting coil,turn number,diameter of conductor cross section and transmitting voltage have great influence on the observed voltage of primary field.The resistivity of the transmitting coil conductor has little influence on the observed voltage in the primary field.The side length of the transmitting coil,the diameter of the cross section of the conductor,the resistivity of the conductor and the transmitting voltage have great influence on the observed voltage of the secondary field.In view of the influence of the transition process of the receiving coil on the observed voltage,the corresponding correction methods are proposed.In the first field correction,according to the relationship between the parameters of the transmitting coil and the observed voltage of the primary field,the magnitude of the observed voltage of the primary field is calculated quantitatively and then subtracted from the total observed voltage.Aiming at the transition process of secondary field,a numerical fitting method for calculating induced voltage of geological body is proposed,and the correction effect is verified.In the critical damping state,the maximum error between the observed voltage and the induced voltage of the geological body is 79% before correction,and the maximum error between the observed voltage and the induced voltage of the geological body after correction is 8.42%.Under the underdamped condition,the maximum error between the observed voltage and the induced voltage of the geological body is 95% before correction,and the maximum error between the observed voltage and the induced voltage of the geological body after correction is 0.45%.Finally,the LCR digital bridge instrument is used to measure the coil resistance and inductance through experiments,and a method of calculating the distributed capacitance by observing the voltage oscillation frequency is proposed. |
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