Fluid circulation in the active emerged Asal rift (east Africa, Djibouti) inferred from self-potential and Telluric-Telluric prospecting

In 1996 Self-Potential (SP) and Telluric-Telluric (TT) surveys were perform ed in the Asal rift, in the Republic of Djibouti, in order to study water c irculation through the rift, from Ghoubbet Strait to Asal lake located 11 k m Northwest, and at the altitude of 155 in below sea level. SP surveys reve al two main positive anomalies, several kilometres wide, up to 150 mV in am plitude, parallel to the rift and separated by a relatively large (1 km) ne gative anomaly. The northern anomaly is well centred above the active rift. The central part of the anomaly is cut by a sharp and narrow negative anom aly (200 mV in amplitude, 150 in width) due to a major fault running from G houbbet Strait to Asal lake. The two positive anomalies are interpreted by ground seawater flowing toward Asal lake and warmed up during their advance by the strong geothermal flux existing beneath the rift. The negative anom alies are most probably due to large and rapid fluid transfer along faults and high permeable geologic beds bending toward Asal lake. The rapid flow w ould prevent any large heating of the ground water. The largest negative an omaly, which isolates the two positive ones, gives new valuable information on the tectonics. The extension of the northern positive anomaly to the So uthwest is stopped by a fault located about 1 km to the North of a normal f ault considered until now, as a major rift structure. The 16 Telluric-Tellu ric (TT) surveys point out three behaviours of the telluric field. Near Gho ubbet Strait and inland the telluric field is well polarised in a N90-100 d egreesE direction and the amplitude expresses a medium to a low resistivity beneath the recording sites. This pattern is the signature of the coast ef fect and the regional polarisation of the field. Along normal faults border ing the rift the SE-NW polarisation seems to be controlled by the rift tect onics. Inside the rift the telluric field becomes weaker, indicating a deca y of the resistivity. In the central part of the rift the telluric field am plitude becomes 1/3 of the generally recorded amplitude and the polarisatio n cannot be estimated. It is corresponding to very low resistive areas in a greement with high positive SP values. (C) 2001 Elsevier Science B.V. All r ights reserved.