Study On Wave Field Transformation And Migration Imaging Of MTEM Data

Multi-channel transient electromagnetic method(MTEM) is an electromagnetic prospecting method which has attracted significant attention recently. With a grounded wire source emitting pseudo-randomly coded electromagnetic signal, MTEM responses can be observed and recorded in an array of multiple channels, which enabling observed data in MTEM to be processed liked the way we do in seismic data processing; therefore, MTEM is able to detect underground targets with a depth of several thousand meters. Presently, the MTEM method is still a developing method, and has many work to do in widespread aspects, such as system design and data processing. Based on the work carried out by our group and those described in literature, the following problems was studied in this paper:1. According to the spectrum characteristics of emission waveforms and MTEM responses, its detection depth and anti-noise ability was analyzed, and the selecting principle of MTEM emission waveforms and system parameters was also obtained.2. The basic MTEM data processing method was studied. For the singularity of deconvolution, the MTEM response’s stable extraction method was derived. Based on the analysis of the existing MTEM apparent resistivity definition, the domain apparent resistivity, which is more suitable for MTEM data processing, was also defined.3. In order to improve the resolution of MTEM interpretation, the wave field transformation is introduced into MTEM data processing procedure, and the algorithm used in transforming diffusion field into wave field was also researched.4. After the wave field data transformation, the depth migration profile can be obtained by using Kirchhoff integral migration, which realizes a rapid imaging of underground electrical distribution. Combining with the characteristics of MTEM emission waveforms, the pre-stack migration imaging method was investigated.Unlike traditional TEM methods, specific emission waveforms(such as pseudorandom binary sequences) are generally used in MTEM. Both the emission signal and the whole period of response are recorded, and the impulse response is obtained by using the deconvolution method. It doesn’t need to wait for the shutting off of the transmitting source, which increased the efficiency greatly. Different emission waveforms have different energy distributions and different noise suppression abilities. Numerical filtering and convolution technique was used in this paper to calculate MTEM response of layered earth models. Consequently, the detection depth and ability to resist noise was analyzed according to the emission spectrum characteristics of the waveform, and then the selecting principle of MTEM system parameters was given.Certain basic processing techniques of MTEM data were also introduced in this paper. Traditional MTEM data processing techniques generally define the apparent resistivity with the peak time, and finally achieve the apparent resistivity-offset profile as the basic achievement of exploration. However, the apparent resistivity defined in the above described way can hardly reflect the changes in deep areas, and wastes a large amount of collected data. A full domain apparent resistivity definition method according to inverse function methods was presented in this paper, and based on which, a new method used in improving MTEM system was described.Transforming the MTEM data to wave field enables us using mature seismic data processing techniques, which can greatly improve the resolution of the electromagnetic prospecting. Wave field transformation, however, is an ill-posed process, because wave field transformation matrix has a very large condition number. Certain techniques used in reducing the condition number, such as the balance method, were discussed together with some further discussions about the regularization and singular value decomposition methods used in wave field transformation. A novel and stable wave field transformation method, sweep time wave field transformation, was presented based on former researches described in the literature.After transforming MTEM data to the wave field, the migration imaging method can be introduced to image the underground medium. The inverse time ’place’ which is based on Kirchhoff’s integral was introduced firstly, based on this method, a migration method used in obtaining the geometry of underground interfaces is presented. Several model tests were also conducted to verify the described method. Then, more complex models were designed to verify the correctness and stability of the wave field transform and migration imaging techniques introduced into MTEM data processing in this paper. At last, the implementation of the prestack migration imaging method was investigated, and comparisons of migration imaging results obtained with above mentioned migration method were made.