Geophysical Study on Arta Geothermal Prospect, Djibouti
Haissama OSMAN ALI, Fahman HASSAN, Nasradin IBRAHIM
As part of the geothermal research program in the Republic of Djibouti, the Djiboutian Office for Geothermal Energy Development (ODDEG) conducted a surface study on the geothermal field Arta. Arta geothermal prospect is on of nearest active geothermal system to Djibouti town ( Situated about 40 km away). The purpose of this study is to identify and delineate geothermal resources in the Arta geothermal field. Several geoscientific studies have been carried out (geology, geophysics and geochemistry). In this study, my discussion is limited to geophysical surveys. Several integrated geophysical surveys have been carried out, namely: Gravity and Magnetic, over Arta geothermal field. The Arta geothermal prospect is one of the several geothermal prospects in Djibouti, situated on the southern shore of Tadjourah gulf. The Tadjourah gulf is a young oceanic rift whose geology and tectonic activities have been investigated by several researches. Some of the units observed in the field include the ‘Dalha’ basalt and the ‘Ribta’ rhyolite both have been strongly altered by geothermal fluids and could constitute a sealing impervious formation, a possible explanation for the scarcity of surface thermal manifestation in the area. The most active faults’ direction which is in the NW-SE and fumaroles manifestations is related to it on- shore, but more thermal output could be presented in the off-shore portion of the field. A total of 451 gravity stations were measured in two phases using a Scintrex CG-5 gravimeter and a differential global positioning system (DGPS) for an accurate elevation of each station. The first phase consists of a total 233 points spaced by 500 m covering the whole area of the prospect and a second phase consists of a total 218 points spaced by 250 m covering specifically the central part of the region including the alteration zone and fumaroles of Qiqleh zone. Ground magnetic survey (Magnetometer) was carried out in the Arta geothermal prospect. The objective of the survey was to take measurements of magnetic fields on 250 station using two GSM19 proton magnetometers with integrated GPS (Global position system) and a high sensitivity Overhauser effect measuring the Earth’s magnetic field with 0.01 nT resolutions and 0.1 nT absolute accuracy. The gravity survey has an important role in geothermal energy investigation. Gravity data were done to give a better imagery of the underground and also to locate a possible geothermal reservoir for geothermal energy production. Several corrections were done to the measured gravity values to obtain the final gravity anomaly called Bouguer anomaly including regional and local anomaly and also same filtering method. The analyzed results of the magnetic survey revealed many distinct magnetic lineament which can suspect the presence of fracture or/and fault with the NW-SE, NE-SW, E-W and N-S directions. These anomalies resulting from both gravity and magnetic surveys were expected to have a strong relationship to the thermal sources of the Arta geothermal system. This expectation was supported by geological observation of the field, and it was revealed that the basement Rocks areas are intensively altered and intruded by Dome and /or Dyke. From both horizontal derivative X and Y direction, it was revealed that these structural configurations can be associated to the different tectonic activity realized in the Arta zone.
|        Topic: Geophysics||Paper Number: 13030|