Application research of geophysical exploration technology in the investigation of shallow geological structure in Chengdu

This paper is the result of the geological survey engineering.

Objective There is great significance to find out the characteristics of urban shallow geological structure for urban underground space planning, development zone construction, geological disaster evaluation. While the nondestructive and strong anti-interference geophysical exploration technology can play a key role in providing urban underground geological information.

Method In order to support the three-dimensional geological mapping of Chengdu City, 11 kinds of new and old geophysical exploration techniques are used to study the shallow geological structure of Chengdu City.

Results The results show that: (1) GPR has a high resolution for the formation structure imaging and human structure identification near the surface; (2) Transient surface wave, mixed source surface wave and three-component resonance method have a strong recognition ability for the geological structure with a shallow depth of 50m, although the resolution is very high, but the detection depth is limited; (3) Microtremor survey method and high- density resistivity method have a better resolution for the geological structure with a shallow depth of 50 m, but under the deep of 50 m, the resolution is general, especially the high-density electrical method is greatly disturbed by humanity, and the detection effect is not stable; (4) Opossing coils transient electromagnetic method anti-interference ability is strong, the construction efficiency is high, the resolution is better in the range of 0-15 m, and general in the range of 50-200 m, which has a good identification ability for gypsum mudstone; (5) The semi-airborne TEM method can effectively identify the base form of Chang'an landfill and make up for the lack of conventional geophysical methods that the special site for detection; (6) The shallow seismic exploration has a good effect on the variation characteristics and fault structure characteristics of the sand shale interbedding within the depth range of 50- 300 m underground, and can effectively identify the lithology interface of the sand shale in combination with the natural gamma curve; (7) The audio-frequency magnetotelluric method can effectively identify the deep fracture in Chengdu Plain.

Conclusions Based on the merits and demerits of various geophysical methods, a combined geophysical exploration scheme of shallow geological structure in Chengdu is established under conditions of multiple geological structures and complex working conditions, which is expected to provide a reference for the investigation of urban shallow geological structure in the future.