A Study On The Apparent Resistivity Calculating For Small Loop Transient Electromagnetic Method

In order to solve the problem of the traditional large-loop transient electromagnetic(TEM)method being difficult to use in the construction of mountainous areas,water areas,or narrow spaces under coal mines,the small-loop TEM method was developed.The small-loop transient electromagnetic method is generally several orders of magnitude lower than the actual resistivity calculation,which is not only inconsistent with the actual situation but also easy to cause misjudgment of the actual measurement data.The main reasons for the low apparent resistivity of the small loop transient electromagnetic method are analyzed in detail and can be attributed to the influence of the loop inductance,the influence of the turn-off current,and the inapplicability of the apparent resistivity calculation formula.The main research results obtained are:(1)It is found that there is a multiplier in the transient electromagnetic apparent resistivity calculation formula for the small and large loop.Using the large loop apparent resistivity formula to calculate the full-time apparent resistivity,followed by correction using coefficients,enables a more realistic full-time apparent resistivity for the small loop device to be obtained;and it increases the difference between high and low resistance,making the anomaly more obvious.(2)The turn-off time has a large effect on the early transient EM data,causing the transient electromagnetic decay curve to deviate downwards,and the degree of deviation increases with the turn-off time and gradually decreases in the later stages.(3)A method for calculating the apparent resistivity under non-stepping shutdown is given.(4)The presence of inductance between the transmitter coil and the receiver coil during the turn-off period will lead to a significantly lower apparent resistivity in the early stage.(5)The apparent resistivity calculated by the method in this paper is significantly greater than that calculated by the traditional method and is close to the real resistivity.The plotted apparent resistivity contour map can better reflect the location and morphology of the anomaly.