Three-dimensional Forward And Inversion Of Multi-channel Transient Electromagnetic Method Based On Footprint And Parallelization Technique

In recent years,with the rapid growth of China's demand for oil and gas resources,and shallow oil and gas exploration technology has become more mature,oil and gas exploration has moved from the eastern,shallow,continental,tectonic oil and gas reservoirs in the past to the west,deep,marine,and strata Transformation of oil and gas reservoirs.As the burial depth of the exploration target increases,the geological environment and structure become more complex,and the difficulty of detection also rises sharply.Therefore,there is an urgent need for a new electromagnetic exploration method that is more sensitive to high-resistivity bodies and has a large exploration depth.As Multi-channel Transient Electromagnetic(MTEM)has shown significant advantages in the exploration of offshore oil and gas reservoirs abroad,this method has gained widespread attention from domestic scholars.Compared with traditional electromagnetic methods,this method has the high-resistivity body has the characteristics of higher resolution and strong anti-noise level.In recent years,this method has made great progress in the simulation of three-dimensional finite difference,two-dimensional finite element and three-dimensional integral equation method.However,for the three-dimensional response characteristics of MTEM,the sensitivity of each position within the range of the transmitting and receiving working device is still lacking.In terms of MTEM data processing and interpretation,the focus is still on the one-dimensional inversion and the two-dimensional inversion of the late upper step wave approximate DC method,and the lack of a truly effective multi-channel transient electromagnetic threedimensional inversion method.In this paper,the research of CSEM three-dimensional forward modeling technology based on integral equation method is first carried out,and the layered half-space response calculation method and various approximation methods are derived,such as Born approximation,generalized Born approximation,quasi-linear approximation(QL),and quasi-analytical approximation(QA),etc.,using cosine transform to perform frequencytime transform to obtain the time domain response,verify the reliability of the program.In order to clarify the characteristics and detection effects of multi-channel transient electromagnetic response under complex three-dimensional conditions,this paper uses the forward algorithm to analyze the multi-channel transient electromagnetic three-dimensional response.First,the sensitivity of the Ex field to anomalies of different depths at different offsets is discussed.From the analysis,it can be seen that the optimal offset is 2 to 4 times the exploration depth for the target body with a buried depth of h.Subsequently,the impulse response characteristics of the recording point to the target body at different positions under different offsets are discussed.It is revealed that the time of the appearance of the pulse wave peak generated by the abnormal body moves backward as the depth of the abnormal body increases,and the peak value decreases as the depth increases.feature.Finally,the anomalous fields generated by anomalous bodies of the same burial depth and different lengths are compared and analyzed,proving that in the entire area between the transmitter and the receiver,the detection sensitivity of the part close to the transmitter or the receiver is stronger than the center point.After clarifying the response characteristics and detection effects of multi-channel transient electromagnetics,this paper uses the regularized conjugate gradient method to construct a three-dimensional inversion,focusing on deducing the calculation method of conjugate direction and iterative step size,and the calculation method of Fréchet derivative.Due to the huge model parameters and data scale involved in the multi-channel transient electromagnetic 3D inversion,this paper introduces the footprint technology commonly used in the aviation domain transient electromagnetic inversion,and the multi-channel transient electromagnetic method at different offsets The Fréchet derivative in the time domain is normalized,and the construction method of the footprint weight matrix of working devices with different offsets is summarized.To solve the problem of too long calculation time of the three-dimensional electromagnetic field inversion in the time domain,the three longest time-consuming calculation tasks are optimized: One is to calculate the background field generated by the different emission sources relative to the meshing unit in the forward response calculation Tasks are distributed to different cores for simultaneous calculations,and the Green's function calculations for different receivers relative to the meshing unit are also processed in parallel.The second is for the calculation of the Fréchet derivative in the frequency domain,which is decomposed into multiple calculation tasks according to the different transmission sources for parallel acceleration.Third,for the frequency-time transformation part of the Fréchet derivative,for each working device,the task of allocating the frequency domain derivatives of all the split units is performed,so that all cores simultaneously perform the cosine transform of the frequency domain Fréchet derivative,and finally merge into the time domain Fréchet derivative.By parallelizing the above computing tasks,a parallel inversion flow chart is constructed,and the multi-channel transient electromagnetic threedimensional inversion program construction is completed.At the end of this paper,two complex models are selected for three-dimensional inversion analysis.One is the SEG salt mound model.This model has high complexity and the anomalous body extends from the shallow part to the deep part.The MTEM inversion results are good.The better response highlights the advantages of MTEM in detecting depth.The second is the high-low resistivity combination model.Compared with the traditional large offset CSAMT method,the resistivity of MTEM to the high resistivity body is significantly better than the CSAMT method,which proves that the sensitivity of the MTEM method to the high resistivity body is significantly better than the traditional electromagnetic law.The inversion effect of these two models proves that the multi-channel transient electromagnetic conjugate gradient three-dimensional inversion algorithm constructed in this paper is effective,and the isochronous curve of the upper step wave shows that the data fit is better,prove that the three-dimensional inversion method in this paper is stable and reliable.