| Electromagnetic(EM)method is based on the electrical difference of underground medium,based on this principle,Subsurface resistivity information,which relates to geology,can be determined from the EM data.EM method has become one of the most important geophysical means of subsurface structure exploration because of its advantages of small environmental damage and high measurement efficiency.The controlled source electromagnetic(CSEM)method is an important branch of EM method,such surveys involve transmitting a controlled signal at a suite of frequencies into the ground from one source site and measuring the received electric and magnetic fields over many receiver sites,either along a line or over a grid,for that one source.In recent years,because the signals of the CSEM method are stronger than for the audio-frequency magnetotelluric(AMT)and magnetotelluric(MT)methods,which allows for easier processing and interpretation,and because data acquisition for the CSEM method is much faster,which can reduce overall survey costs,the CSEM method has been widely used for resource exploration,groundwater exploration,geothermal exploration,as well as hydrological investigation and general surveying of geological structure.However,because the EM fields in the ground are sensitive to large volumes of the subsurface,EM methods have a relatively low resolution of deep local geo-electric anomalies.Consequently,the boundaries,and hence the location and size of an anomalous body are challenging to discern.Therefore,studying how to improve the identification of the boundaries of conductivity anomalies in the subsurface,both the horizontal extents and vertical extents,is worthwhile and has the potential for producing useful new interpretation procedures for the EM method.Measurement of gradients of the field plays an important role in improving the resolution of gravity and magnetic methods.Drawing lessons from this,the frequency-domain,controlled-source electromagnetic gradients measurement(CSFEMG)method which can effectively improve the identification of the boundaries of anomalies in the subsurface is proposed in this paper.The theoretical formula of electromagnetic gradients data of a model with a conductive sphere hosted in a uniform half-space is firstly derived,then the response characteristics of the spatial gradient and the frequency gradient of 3D earth models are analyzed through the 3D forward-modeling results.By the comparative analysis of the electromagnetic field and its gradients data of these models,it is found that the CSFEMG method has obvious advantage in the reflection of the boundaries of anomalies.Because of this advantage,a rapid and high-resolution imaging method based on the gradients of the fields measured in frequency-domain controlled-source EM surveys is proposed,the new imaging method is introduced and the formulas is derived,by applying this imaging method to data-sets from several typical models with different complexity,the correctness of this new imaging method is verified.In order to further assess its performance in practical survey,instrumentation for the CSFEMG method is developed,and its effectiveness is demonstrated using a real-data example from the Xi'an coal mining district in Liaoyuan,Jilin Province,China.The data from the CSFEMG survey,and the apparent resistivity pseudo-section derived from the data,were compared with data and the inverted model from a DC resistivity survey done along the same survey lines.This comparison,and comparison with what is known about the geology of the area,showed that the CSFEMG method was capable of detecting the subsurface conductive features assumed to be water-filled abandoned tunnels and goafs from historical mine workings.The identification of the target's shape(boundaries both in horizontal direction and vertical direction)from the gradient data was good.It is therefore clear that the CSFEMG method is a useful and viable technique for geotechnical and other investigations of the subsurface of the Earth.The main contents of this paper are organized as follows:(1)3D Forward-modeling of the frequency-domain controlled source electromagnetic fields.Based on the response characteristics analysis requirements of CSFEMG method,the 3D forward-modeling of controlled-source electromagnetic problems using vector finite-element method and Comsol Multiphysics Software are studied and introduced,respectively.The electromagnetic fields of models with different complexity(including layered earth model,2D earth model and 3D earth model)are calculated by the above forward-modeling methods and results from these two methods are compared.Results show that the results of the vector finite-element method corresponded well with the results of Comsol,the correctness and effectiveness of the 3D forward-modeling of controlled-source electromagnetic problems using vector finite-element method are thus verified which lays a foundation for the characteristics analysis of the CSFEMG responses.(2)Characteristics analysis of the spatial gradient data from the frequency-domain controlled-source electromagnetic method.A survey geometry that has one transmitter and many receiver-locations,either along a line or over a grid,is used to obtain the electromagnetic field components.This kind of relatively dense coverage of receivers is needed in order to be able to determine the gradients.Then the spatial gradient can be obtained by the distance difference quotient of the electromagnetic field values measured from the adjacent survey sites and the same frequency.In order to analyze the response characteristics of spatial gradient,a theoretical model with a conductive sphere hosted in a uniform half-space is first considered,the original electromagnetic field components and the spatial gradient field are calculated and their response characteristics are compared,then forward analysis is carried out on a number of 3-D earth models with different complexity to assess the capabilities and usefulness of the spatial gradient data.Results of the above analysis show that the spatial gradients are able to indicate the horizontal extents of the target,with the horizontal boundary identification ability of the spatial gradients being stronger than that of the original electromagnetic field components.(3)The characteristics analysis of frequency gradient data from the frequency-domain controlled-source electromagnetic method.A frequency-domain controlled source electromagnetic frequency gradient measurement method which can effectively improve the identifying ability of the vertical boundaries of anomalies is proposed.The response characteristics of the frequency gradient is analyzed and its ability of reflecting the vertical boundaries of anomalies are evaluated through the comparative analysis of forward-modeling results from homogeneous earth model and 3D earth models.The influence of physical parameters and measurement parameters(such as the source–receiver distance,and the burial depth,resistivity and thickness of target,etc)of the model on the frequency gradient response is also analyzed.Results of the above analysis show that the frequency gradient is sensitive to the vertical extents of a target and is able to provide a quick and reasonably accurate indication of the vertical location of a target,with the vertical boundary identification ability of the frequency gradient being stronger than that of the original electromagnetic field components.(4)Rapid and high-resolution imaging method based on the gradients of frequency-domain controlled-source electromagnetic fields.In order to improve the recognition ability of boundaries,a new rapid and high-resolution imaging method based on the gradients of the fields measured in frequency-domain controlled-source EM surveys is proposed.The methodology of this imaging method is presented first.Then the performance of this imaging method is demonstrated using a number of models with different complexity by comparing the pseudo-sections generated by the proposed method and those generated by a traditional method.Relationship between the imaging performance and the send-receiver distance,and the sensitivity of the imaging method to the top and bottom boundaries of targets,are discussed and evaluated.It is found that the performance of this apparent resistivity imaging method is closely related to the transmitter-receiver distance: when a proper value of transmitter-receiver distance is used,good results can be obtained in which the horizontal locations of vertical boundaries and the positions of top and bottom boundaries can be identified clearly.Such methods are useful when we are testing instrumentation or assessing data quality during a survey,or when we need to get a general understanding of the geological structure during a field survey.(5)Development and application of the controlled-source frequency-domain electromagnetic gradient(CSFEMG)system.In order to give full play to the advantages of the CSFEMG method in the identification of the boundaries of anomalies in the subsurface,and to test the availability and effectiveness of this method in practical survey,the key technology and main technical specifications of the CSFEMG system is introduced in this paper based on the above theoretical research results of the CSFEMG method.A survey in the coal mining area of Xi'an District,Liaoyuan City,Jilin Province,China,which has many abandoned,water-filled workings,was conducted to test the effectiveness of the CSFEMG mehtod in a field test.Results show that the resistivity imaging results derived from the CSFEMG data correspond well with the result of inverting a high-density DC resistivity data-set acquired in the same area and with what is known about the geology of the region,the validity and practicability of the CSFEMG method is illustrated. |
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