Geophysical structure of the Troodos Ophiolite from downhole logging

Velocity (Vp and Vs), density, porosity, natural gamma, resistivity, total magnetic field and magnetic susceptibility logs demonstrate that the Troodos ophiolite complex is geophysically similar to the oceanic crust. Insitu velocities (Vp) range from 3.2-3.5 km/s in the pillow basalts (Layer 2A) and then increase gradually through the pillow basalt/dike transition and the sheeted dikes (Layer 2B) to classic Layer 3 velocities (6.7-6.9 km/s) in the gabbros while Vs averages 2.1 km/s in the pillow basalts and rises to values of 3.2-3.5 km/s in the gabbros. In the pyroxenites, Vp increases only slightly (to 7.0 km/s) but Vs rises to 4.0 km/s. The porosity and natural gamma radiation decrease from averages of 20% and 30 API units, respectively, in the pillow basalts to negligible values in the gabbros, while the formation density and resistivity increase from 2.2 to 2.95 glcm3 and from 10 to 20K-50K ohm-m over the same interval, suggesting that seawater penetrates no deeper than the base of the dikes. Aside from Vs, the only property which changes at the gabbro/pyroxenite boundary is density, which increases abruptly to 3.1 glcm3 The magnetic susceptibility increases steadily from 0.5 x 10-3S.J. units in the pillow basalts to 4 x 10-3units at the 50% dike/50% gabbro transition, below which it decreases abruptly to 0.5 x 10-3units. Since resistivity increases by an order of magnitude at the same depth and Vp increases rapidly from 5.75 to 6.4 km/s at this depth and then increases steadily to 6.8 km/s within the next few hundred meters, we interpret the dike/gabbro transition as the Layer 2/3 boundary of the oceanic crust.