MT and Gravity Surveys in Ambado-PK20 Geothermal Prospect, Djibouti
Moktar BILEH AWALEH, Nasradin-Ahmed Ibrahim, Hassan MOHAMED MAGAREH
Ambado-PK20 geothermal prospect is one of the nearest geothermal system to Djibouti town(situated about 20 km away). This area is located on the groundwater table which supplies water to the city of Djibouti. Several studies have been carried out on this site in the past especially on hydrogeological investigations. This site has two major advantages over other geothermal sites in the Republic of Djibouti. The first one is because of proximity to the national electrical grid and the second its location on the plain of Djibouti city which constitutes the main aquifer. However the site does not present any hydrothermal manifestations at surface which are generally useful in characterizing areas of geothermal potential. Only some water wells located in this area show a thermal anomaly of about 61.4°C. The main objective of work on this project was to locate eventual possible geothermal reservoir for geothermal energy production. To realize this, several geosciences studies, including geology, geochemistry and geophysics were carried out. Magnetotelluric (MT) and gravity were carried to delineate the boundaries of the geothermal system. 34 MT/TDEM soundings were acquired using the metronix equipment with period range between 0.001s and 800s. At the same time, to complement MT data, 66 Gravity measurements were collected with a Scintrex CG5 gravimeter. Combining MT and gravity data tends to reduce the usual intrinsic ambiguity on the conceptual model. A geoelectric structure model obtained by the 1-D MT surveys shows a resistive layer at the surface overlying a conductive layer and a deep resistive layer. Residual anomaly shows a central low gravity extending towards the south and delineated by a heavy body in the northern part of system.Based on interpretation of results from both 1D inversion MT and Bouguer map anomaly, we found that the conductive layer is defined by a low value of gravity. This correlation can be associated tothe cap rock of the system that is an impermeable layer affected by hydrothermal alterationwhich is responsible in reducing the resistivity and the mass of the rock.
|        Topic: Geophysics||Paper Number: 13058|