Using the magnetotelluric method to study the deep structure of the Sai Gon river fault in the vicinity of Ho Chi Minh City

Abstract

This paper presented the study of the two magnetotelluric profiles crossing the Saigon river fault. The first one was 13.5 km long consisting in 8 stations extending from Cu Chi district, Ho Chi Minh city to Ben Cat district, Binh Duong province, and the second was 11.7 km long including 7 stations in Nhon Trach district, Dong Nai province, Vietnam. The data were recorded in the period range 0.002‒259 s using the GEPARD-4 instrument. Because of the influence of the equatorial electrojet (EEJ) on magnetotelluric soundings in this region, only data in the period range shorter than about 1 s for the analysis were used. The resistivity data of both TE and TM modes were used for inversion using the MT2DinvMatlab software. Firstly, data were inverted down to a depth of 10 km to obtain the main electric features of the region. The results showed that on both profiles, the resistivity cross-sections showed a 3-layer geoelectric structure in which the top low resistivity layer had a thickness of about 2 km, the resistive intermediate layer was about 6‒7 km thick, and the underlying low resistivity layer. On these cross-sections, the Saigon river fault could not be observed. Then data were inverted down to 3 km to obtain more detailed information to determine the location and dip direction of the Saigon river fault. The geoelectric cross-sections of 3 km depth showed that for the Cu Chi‒Ben Cat profile, the Saigon river fault corresponded to the low resistivity region below the stations CB5 and CB6, and for the Nhon Trach profile, the river fault Saigon would be related to the low resistivity region below the stations NT3 and NT4. The obtained results confirmed the existence of the Saigon river fault, with the location and its continuous extension from the Cu Chi‒Ben Cat area, through Thu Duc city to Nhon Trach district of Đồng Nai province.