Magnetotelluric Imaging of the Zhangzhou Basin Geothermal Zone, Southeastern China

Chaofeng Wu,Shuang Liu,Wenjing Lin,Yufei Xi,Xiangyun Hu,Jianchao Cai,Guiling Wang,Bo Yang

doi:10.3390/en11082170

Abstract

The geothermal zone of southeast China, which is one of the country’s known geothermal zones, contains significant natural geothermal resources. To understand the formation of geothermal resources, a magnetotelluric (MT) investigation with a site spacing of 1–2 km was carried out around the Zhangzhou Basin. The recorded MT data were processed by robust time series and remote reference processing techniques. The data analysis results revealed that two-dimensional (2-D) modeling can be used to approximately determine the electrical structure. The joint inversions of TE and TM modes have been performed after distortion decomposition. In the inversion models, a low resistivity cap of 200–800 m thickness was observed, which represented the blanketing sediments composed of Quaternary and volcanic rocks of the late Jurassic period. The presence of high resistivity above a depth of 20 km indicates the granites are widely developed in the upper and middle crust. MT measurements have revealed some deep-seated high conductive zones, which were inferred to be partially melting at depth of 8–17 km, which is likely to be reason behind the formation of higher-temperature hot springs. The results also show that there is a shallower Moho, which indicates that the heat from the upper mantle may have a big contribution to the surface heat flow. Fractures-controlled meteoric fluid circulation is the most likely explanation for the hot springs.

Highlights

Six main geothermal zones can be recognized in China (Figure 1), including the Taiwan geothermal area, Southeast coast geothermal zone, Sichuan-Yunnan N-S geothermal area, Tancheng-Lujiang geothermal area, Xizang-Yunnan geothermal area and Qilian-Luliang arc-shaped geothermal area [1].The southeast coast geothermal zone mainly includes the provinces of Guangdong, Fujian, southernHunan and eastern Jiangxi
We describe the electrical structures in Zhangzhou Basin geothermal zone with the purpose of showing the subsurface anomalous structures related to geothermal resources
The Zhangzhou Basin geothermal zone, which is located in the southeast coast of China (Figure 2), is geologically special as it was created from the interactions of Eurasia Plate, Philippine Plate and Pacific Plate [7]

Summary

Introduction

Six main geothermal zones can be recognized in China (Figure 1), including the Taiwan geothermal area, Southeast coast geothermal zone, Sichuan-Yunnan N-S geothermal area, Tancheng-Lujiang geothermal area, Xizang-Yunnan geothermal area and Qilian-Luliang arc-shaped geothermal area [1]. The geothermal field of Zhangzhou (red star in Figure 2) has a temperature of 106 ◦ C at the orifice and 121.5 ◦ C at the bottom of the borehole with the depth of 90 m. A low-velocity layer is found at the depths of 12–16 km by deep seismic wide-angle reflection and refraction-profile. This layer was inferred to be melting or partial melting, and is likely to be the crustal internal heat source of Zhangzhou geothermal zone [3,4,5]. Seismic tomography shows that the low-velocity anomalies extend to deep depths near Zhangzhou Basin with high convective heat flow [6]. Our results will help to determine the heat source and underlying magmatic system supplying the geothermal fields

Geological Setting

Multi-frequency

Two-Dimensional Model

Results and Discussion

Two-dimensional

Conclusions