An observation scheme, which is designed several frequencies suited over the natural audio electric field frequency range, is used to qualitatively understand the electrical structure of underground geological body. Weak signal detection technology has been used to amplify and filter the weak natural electric field signal, and read out them after selecting corresponding frequencies and integrating. This instrument can observe eight natural electric field signal at frequency of 8Hz, 16Hz, 32Hz, 64Hz, 128Hz, 256Hz, 512Hz, 1024Hz. Several reliable technologies have been used in the hardware design and software development to improve both the anti-interferences capability and system stability. Compared with field work instance, we conclude that natural audio electric field frequency selecting instrument can get the underground geological information within a certain depth, and therefore it can be used for fast survey in geology and mineral resources, and in engineering geophysical and water resources.Key words: Natural audio electric field, Frequency selecting instrument, magnetotelluric sounding, Observe system
PrefaceGeophysical techniques and theory have developed rapidly for last century [1,2,3] . New electrical device was also emerging, and especially the geophysical equipment of electromagnetic developed faster. Advanced electromagnetic instrument pursue the perfect combination of hardware and software and wish reach the following aims: ①High resolution, high Signal noise ratio, ②Broadband, high dynamic range, ③Highly integrated, multi-functional, low-power, ④Easy to operation, light and flexible, on-site displays the results. For example, EH-4、GDP-32Ⅱ、MT-24、 V5-2000、V6、V8. When these equipments run in field, the broadband therefore can probe greater depth, and automatic operation, intelligent, high precision, and even for some equipment can be used pre-processing in real time and do 1D retrieval. So these equipments were very important for solving deep geology problems [4][5][6][7] . But such equipments are large-scale and the price is very high, and general departments cannot afford these devices.When we do some scan larger area survey with purpose of finding abnormal, we mainly require fast and high efficient completion of the field work, and can accurately delineate the abnormal location [8][9][10][11] . In some cases it is not necessary to use the large-scale electromagnetic devices to find low resistivity fractured zone with not too deep buried and the steep orientation. Author：Liao Xiuying, Female, Born in 1981，Liuyang,
