A new method for detecting gypsum mineral salt enriched layer of urban underground space in red bed area: A case study of Chengdu International Biological City

Gypsum mineral salt enriched layers are common bad geological body in the process of utilizing urban underground space in the red beds area. The detection of the burial depth and spatial distribution of gypsum-salt enriched strata in advance is of great significance for the development of urban underground space resources,the determination of foundation bearing capacity,bearing stratum and foundation type selection. Taking the International Biological City of Tianfu New District of Chengdu as the experimental area,the authors chose a fast,non-destructive and anti-jamming new transient electromagnetic method,opposing coils transient electromagnetic method (OCTEM),to detect and study the gypsum-salt enriched layer in red mudstone of Cretaceous Guankou Formation. Firstly,according to the resistivity characteristics of different strata in the study area,the authors established geoelectric models such as three-layer model,four-layer model and five-layer model,and carried out OCTEM forward response analysis for these models,respectively. The forwarding calculation shows that,with OCTEM method,the gypsum salt enriched layer can be effectively identified from surrounding rocks with obvious resistivity difference. Then,the authors took the practical measurement in the test area covered about two square kilometers,and swept the field with a network of 100m×10 m. Finally,the authors obtained the inversion results and established the three-dimensional electrical structure of strata in the depth greater than 200 m in the study area. With the comprehensive verification and analysis of drilling results,the spatial distribution pattern of gypsum mineral salt enriched layer was successfully inferred,which proves that this method is effective for the detection of gypsum-salt enrichment layer in urban underground space of the red bed area.