| Title | 1D Inversion Resistivity Structure of High-Temperature Geothermal System in Arta Prospect, Republic of Djibouti |
|---|---|
| Authors | Nasradin-Ahmed IBRAHIM AHMED, Fahman HASSAN ABDALLAH |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | High temperature, conductive cap, resistive core, 1D inversion |
| Abstract | The region of Arta (Figure 1) is considered as one of the most promising geothermal site in the Republic of Djibouti. A combined Magnetotelluric (MT) and Time Domain Electromagnetic (TDEM) surveys were carried out in February 2018 at the Arta geothermal prospect. On this field, 63 pairs of co-located MT/TDEM soundings were performed. The MT time series were acquired by using two sets of Metronix system. The TDEM data were collected by using a terraTEM system with a coincident loop configuration (100m x 100m). Each MT station was kept running overnight (about 12 to 18 hours each). The MT data from Metronix system were processed using the ProcMT program from Metronix. The TDEM data obtained from the field were processed by Geotools software from General Company of Geophysics (CGG). After processing the data, 1-D resistivity model was estimated from inversion MT data after we have applied a static shift correction to the MT data using TDEM data as reference. The resistivity models from the 1D inversion have been interpolated into vertical resistivity cross-sections (4 WE , 3 NS, 2 NW-SE and 2 SW-NE cross sections) and into resistivity maps as function of the depth. All the cross-sections in EW and NS from 1D inversion show a shallow low resistivity (5-10 Ωm) from a depth of 0 to 800m deep and a resistive body below the shallow low resistivity. It suggests that there is a possibility of the existence of a geothermal system. The low resistivity of the rocks has often been considered as a good indicator of the presence of a geothermal reservoir or its clay cap. The underlying resistant bodies at a depth of 2500 m may be more directly associated with the base of the reservoir. An intermediate resistivity zone (20-40 Ωm) is observed between the deep resistive body and the shallow conductive body. This intermediate resistivity is interpreted as the geothermal reservoir. The general increase in resistivity as a function of depth is interpreted as an indication of a typical high temperature geothermal system. This will mean that the resistivity values are controlled by the types of alteration, fluid content, and temperature. In this case the shallow low resistivity is due to alteration into smectite, highly conductive clay that forms at more than 50 degrees Celsius. The low resistivity layer forms the cap-rock for the geothermal system. the resistivity values below the fumaroles vary significantly laterally irrespective of depth. This suggests the existence of flow paths of the geothermal fluids close of fracture zones or a high permeability fault zones that extend from deep to below the fumaroles. |