CHARACTERISTICS OF THE SPATIAL VARIATION OF GEOELECTRIC FIELD SIGNALS RECORDED AT THE STATIONS IN HUADONG AREA (SOUTH‐EAST CHINA) FROM 4 HEAVY CURRENT SOURCES

The relation between the flowing characteristics of electric current and the geological structure is one of very important issues in the study of geoelectric field in seismo-electromagnetics. This paper deals with heavy currents from four different sources injected into the underground and their consequences in the observations from the network of geoelectric field stations associated with the currents, which provides us with a good opportunity to study the issues. The sources are the earth electrodes of some substations of HVDC transmission line systems in East-south China around Shanghai area (Huadong area) with the currents of 1200 to 4780A injected. The recordings of the additional signals of geoelectric field corresponding to the currents have been collected at eight geoelectric field stations in the same area. The following features could be seen in the study. ① Some stations show the selectivity effect. The magnitude of the additional geoelectric field recorded at different stations varied from one source to another, for example Pudong station is near all the heavy current sources, but only the clear signals originated from Nanqiao earth electrode were recorded while no signals from other sources (Tongli, Huaxin, and Fengxian), which shows so-called selectivity effect of the site; ② Station-source distance effect. The maximum observation distance is 350 km, beyond that no additional geoelectric signals from the sources could be recorded; ③ The source azimuth determination has complicated characteristics. For example, at Chongming station the errors are minimum, only 0.2° and 0.8°, respectively, between the actual and calculated source azimuths determined by the signals from Tongli and Xinhua earth electrodes. However, Qingpu station is the nearest station from the heavy current sources, these errors are larger than that obtained from Chongming station due to some unknown reasons; ④ The effect of non-uniformity of the geoelectric field. At the stations in the studied area, the ratios of signals from long dipole over that of short dipoles do not follow the regular patterns which appear in the cases of homogeneous medium structure underneath the observation stations, which illustrates a kind of non-uniformity effect, which is discussed in detail in this paper and proved by the results from a 2-D high density geoelectrical prospecting in the studied area. ⑤ The peculiar reverse effect of polarization direction. For the additional geoelectric field signals recorded at Qingpu and Chongming stations the polarization directions on the long diploes and short diploes are reverse, which need to be further studied in the future to explain the phenomenon. The influence of the three factors, i.e., the configuration of a point source and the electrode distribution of station, the heterogeneity of medium in a large area, as well as the fine structure of the underground medium at a station, is discussed in this paper which might give a relatively clear explanation for the features mentioned above.