Method For TEM Defining Apparent Resistivity Of Arbitrary Transmitted Waveforms

Transient Electromagnetic(Transient Electromagnetic)has developed rapidly in recent years,more and more geological problems can be solved,and the scope of solving problems is becoming wider and wider.It has important applications in many aspects such as water resources exploration and engineering site engineering geological exploration.Transient electromagnetic method has the advantages of high exploration efficiency,wide coverage area and low detection cost,which makes transient electromagnetic method an important geophysical exploration method.In actual work,the central loop device is widely used.In general,the data measured by the instrument is the attenuation voltage,and the attenuation voltage is not obvious to the underground electrical interface information,but the apparent resistivity can be defined close to the underground electricity.In previous studies,step waves were used instead of real emission waveforms to solve the corresponding or attenuated voltages of uniform half-space or layered media,so that the true response was replaced,and then the apparent resistivity was defined later.The media is not reflected accurately.In this paper,the transient electromagnetic method under the central loop device is taken as the main research object,and the transient electromagnetic response and apparent resistivity definition results under different excitation waveform excitations are discussed.For the forward response calculation,the solution with waveform is solved by the convolution theorem and the relationship between the step response and the impulse response,so that the convolution formula of the electromagnetic response of any emission current in the time domain can be obtained.The forward response of any transmitted waveform.In order to make the transmitted waveform closer to the real waveform,the basic transmit waveform is classified.For the ground system,the transmit waveform of the transmitter is mostly bipolar square wave,the current cut-off time is about 0.1ms,There are also bipolar trapezoidal waves,or oblique step waves.For the aeronautical system,the waveform of the transmitter has triangular wave,half sine wave,square wave,trapezoidal wave and other waveforms.This paper mainly studies trapezoidal wave,half sine wave,triangle wave and pseudo wave.The random wave is used as the transient electromagnetic response of the transmitted waveform,and compared with the forward response of the rectangular wave respectively.The difference between the forward response of the rectangular wave and the different transmitted waveform is compared,and the transient electromagnetic which affects the excitation of different emitted waveforms response factor is discussed.For the time derivative of field value and magnetic induction,the typical waveform and the rectangular wave are significantly different in the early stage,and the response curve changes regularly when the flying height and apparent resistivity change.The apparent resistivity calculation method applied in this paper is based on the inverse function of apparent resistivity calculation method.The method has the characteristics of fast calculation,and can reflect the electrical information change of the model in a real and gradual manner.In the calculation process of apparent resistivity,Firstly,when the trapezoidal wave off time is different,the curve of the apparent resistance is defined by the rectangular wave,and the accuracy of the result of defining the apparent resistivity by using the step wave instead of the real emission waveform is numerically simulated.Secondly,the apparent resistivity of the transient electromagnetic response excited by different emission waveforms is defined,and the factors affecting the definition of apparent resistivity are discussed.The apparent resistivity curve tends to the true resistivity of the model in the early and late stages,and the curve is smooth.When the influencing factors change,the apparent resistivity curve shows a regular change,which is different for different model curves.In this paper,a comparative study of the forward response is carried out,and the definition method of apparent resistivity under different excitation waveform excitations is discussed and verified.The definition results of transient electromagnetic apparent resistivity of the central loop device excited by different emission waveforms are obtained.This provides a theoretical basis for the application of the apparent resistivity definition method in practical work in the future,and enriches the apparent resistivity definition method.