Groundwater exploration on the mountain plains of Dhamar and Rada in the Yemen Arabic Republic

Between 1977 and 1982 a series of short term geophysical surveys for groundwater exploration was conducted within a joint integrated development project. The Dhamar and Rada districts are situated at an elevation between 2000 and 2400 m. The geology of the area is complicated and predominantly volcanic. The main formations are Precambrian metamorphic rocks, Tertiary volcanics, Cretaceous sandstone and Quaternary volcanics.

Groundwater occurs in the metamorphic formations mostly in large faults and to a lesser degree in relatively thin alluvial overburden. The Tertiary volcanics consist of alternating layers of more or less permeable lavas, weathered or fractured volcanics and gravel, sand and clay layers. The Quaternary volcanics consist mostly of tuffs (ashes) and basic lavas with a vesicular nature, resulting in a good porosity and permeability. The Tawilah sandstones have a considerable variation in a quifer properties, but form generally an acceptable aquifer.

Site location surveys were carried out in a large number of villages using resistivity sounding and at a later stage EM profiling. The main practical problem during surveying was often caused by limitations in space for electrode expansion.

A limited number of cases is presented as examples of the results obtained in the various geological formations.

In the Precambrian metamorphic formations resistivity sounding was successfully used to locate groundwater in a major faulting system. Yet it must be concluded that EM profiling should be used in advance of resistivity sounding to obtain good results.

The surveys in the Tertiary volcanics were also successful, although a good correlation could not always be made between interpretation and borehole logs. The main reason must be found in the generally low formation resistivities of even dry formations, which makes it difficult to indicate water bearing strata. Water bearing formations occur at various depths, presenting an inconsistent groundwater occurrence.

A survey near the northwestern edge of the sandstone formations near Rada led to the discovery of a major local aquifer, possibly related to a large fault. The question is raised whether recent volcanic activity may have removed (part of) the sandstones in this area.

Resistivity soundings at the foot of the caldera Jebel Isbil indicated groundwater potentials at a depth greater than 200 m. Drilling in this location was successful, while previous boreholes were abandoned as dry, proving that geophysics can improve the rate of success considerably. These results, combined with those obtained near the boundary between the Young Volcanics and the sandstones southeast of the caldera, give good hopes of finding groundwater for villages situated on the Young Volcanics in between.

The successful results of the surveys over this period has led to a firm establishment of this activity within the project, where it now plays a permanent and prominent role in groundwater exploration.