Analysis And Inversion Of MTEM 3D Response Characteristics Under Complex Geological Conditions

With the further exploration of oil and gas resources,the development of oil and gas resources in China has also changed from the shallow layer in the east,continental oil and gas to the deep layer in the west,and the oil and gas in marine strata.The difficulty of exploration and exploitation has also increased dramatically.Therefore,it is urgent to find a new electromagnetic exploration method that is more sensitive to high resistivity body exploration and has a greater exploration depth.David Wright proposed the concept of Multi-channel Transient Electromagnetic(MTEM)for the first time in 2002.It mainly uses pseudo-random current to transmit electromagnetic signals to the underground.At the same time,by measuring the change of the receiving terminal voltage and the transmitting terminal current,the earth impulse response is extracted by deconvolution.Compared with the traditional electromagnetic method,this method has the characteristics of high resolution and strong anti-noise level.At present,the response characteristics and detection effects of the MTEM method under complex three-dimensional conditions are still unclear,and the inversion and interpretation work is still concentrated in one-dimensional and similar to DC two-dimensional inversion methods.Therefore,it is urgent to improve the MTEM method for complex geological conditions.Threedimensional response and inversion research.In order to solve the above problems,this article has carried out related research.This paper first carried out the research on the MTEM three-dimensional forward modeling technology based on the integral equation method,deduced the calculation method of Green's function and a variety of methods to approximate the integral equation,and used the cosine transform to perform the time-frequency transformation in the frequency domain forward calculation.,The frequency domain response is converted to the time domain response,and the reliability of the related algorithm is verified through the model.In order to further clarify the response characteristics and detection effects of the multichannel transient electromagnetic method to the three-dimensional complex model,this paper uses the forward algorithm to analyze the multi-channel transient electromagnetic response characteristics of the three-dimensional complex geological body.Firstly,the sensitivity of the Ex field to anomalous bodies at different depths is discussed at different offsets.Through analysis,the relationship between the offset and the exploration depth is obtained.Then it discusses the response characteristics of recording points to targets at different locations under different offsets.Finally,a comparative analysis of the anomalous fields generated by anomalous objects with the same buried depth and different survey lines proves that there is a difference between the transmitter and the receiver.In the area,the detection sensitivity of the part close to the transmitter or receiver is stronger than that of the center point.After clarifying the response characteristics and detection effects of multi-channel transient electromagnetics,this paper uses the Gauss-Newton method to construct a three-dimensional inversion,focusing on the derivation of the sensitivity matrix calculation method,and introduces an improved regularization factor to enhance understanding The stability of the system and the introduction of footprint technology improve the efficiency of inversion.Finally,this paper carried out a three-dimensional inversion analysis of two complex models.One is the SEG salt dome model.The model is highly complex and the anomaly extends from shallow to deep.The MTEM inversion results are good,and the shape of the anomaly is depicted.Better,highlighting the advantages of MTEM in detection accuracy.The second is the reservoir water injection model,in which different water injection states of the same model are inverted.Analysis of the inversion results shows that the MTEM method has better detection sensitivity for high and low resistance anomalies.Through the inversion effects of these two complex models,it is proved that the multi-channel transient electromagnetic three-dimensional inversion algorithm constructed in this paper is stable and effective.