1995 年 104 巻 7 号 p. 952-971
The history of developments in electrical and electromagnetic exploration techniquesis briefly summarized. In electrical and electromagnetic exploration, subsurface resistivity distributionis obtained from measured data by inversion. The physical meaning of resistivityand the relationship between ground resistivity and other properties of the ground arediscussed.
Many kinds of electrical and electromagnetic exploration techniques are classified withrespect to basic physical phenomena, the dimensions of the structure being and observationgeometries. Some essential parts of acquisition technique are also given.
The author describes the problem of nonlinearity in electrical and electromagnetic exploration, and offers as a solution an iterative modification technique. Another difficulty in theinversion of electrical and electromagnetic data is complex sensitivity of data. To deal withthis problem, analysis is extended to an area of the ground beyond that from which datahas been measured. In addition, both in-line and cross-line data are used in crossholeexploration.
Two practical inversion schemes are described. The conventional method for outliningsubsurface structures and making an initial model for inversion is resistivity back projection. Iterative inversion based on Bayes's theory can deal with both information from measured dataand prior in formation, and maximizes the likelihood of the inverted model. A numericalexperiment is presented to assess the reliability and resolution of the inverted model.
The characteristics and uses of the following five examples of electrical and electromagneticexploration techniques are given:
1. 2D surface electrical exploration
2. 3D resistivity tomography
3. 2D CSAMT
4. 2D electromagnetic tomography
5. 2D radar tomography These examples show that the inversion technique is widely accepted in practical electricaland electromagnetic exploration activities and that inverted detailed resistivity images provideuseful information in many kinds of geological and geotechnical situations.