During February 1998 a field trip to the Oman ophiolite was carried out by Doug Toomey, Will Willcock, Benoit Ildefonse, and Steven Constable to assess the seismic and electrical properties of the peridotite (mantle) and gabbro (lower crust) rocks. A combination of hammer seismograph, thumper seismography, electrical resistitivty, transient EM, and limited magnetotelluric measurements were carried out in three locations, Wadi Hilti, Wadi Abyad, and Wadi Mahran. (The EM component of this work was supported under NSF grant INT-9512864.)

Schlumberger resistivity soundings were conducted in the Wadi Hilti area to assess (i) if there is any conductivity anisotropy in the mantle peridotites and (ii) if there is a consistent difference in conductivity between gabbros and peridotites. At the time the proposal was written, it was assumed that the peridotites were more porous than the gabbros, and that this was why oases were found preferentially at the paleo-moho.

The plot below summarizes the results of two soundings taken at right angles in the peridotites. The soundings look very different, but they can both be fit with the same rock resistivities by varying only the thickness of the units. Most of the difference between the two soundings comes from the shallow part of the section, which is presumably sediments in the wadi itself.

The drop in resistivity at depth is puzzling. It might be real, but might also be associated with working inside the wadi, which is clearly not one-dimensional and might produce distortions at large spacings caused by either the rocks on the wadi walls or the limited lateral extent of the stream sediments.

The bottom line is that the bulk resistivity of the peridotites is 1000 Ohm.m or slightly higher, and that any difference in resistivity with direction is minimal (less than a factor of 2).

The second plot shows a comparison of the above two peridotite soundings (red data) with two soundings carried out on the adjacent gabbros (green). One gabbro sounding was again carried out within the wadi, but this time the second sounding was conducted on the road above. The agreement between the two gabbro sites is encouraging and suggests that the shallow structure is not biasing the whole sounding. Interestingly, the wadi sounding has the most resistive near-surface rocks. This is probably because the washed surface gravel has a lower clay content than the road material.

Again, there is a downturn in the gabbro curves at largest elecrode spacing. The downturn is poorly developed in the road sounding, so it could indeed be an artifact of the wadi geometry.

The bottom line to this story is that the bulk resistivity of the peridotites is at least twice that of the gabbros. When we started the experiment we were expecting the opposite. It appears that fracturing is better developed in the more brittle gabbros.

The wadi water was measured directly as between 25 and 50 Ohm.m. Assuming well-connected cracks, this would place the bulk porosity of a 1000 Ohm.m rock at around 4%.

Four magnetotelluric (MT) soundings were carried out to assess deeper resistivity and determine the feasibility of mapping the thickness of the nappe using electrical methods. One must be very careful in interpreting single MT sites, but site 4, for example, in the Mahram area exhibits largely one-dimensional structure (phases in the N-S and E-W directions are similar and resistivities vary only by a static shift) and shows a clear drop in resistivity bewtween 2 and 3 km depth, which could be associated with underlying sediments.