The integration of core and log data has become increasingly important to understand the parameters controlling the seismic nature of the oceanic crust. For this reason, we constructed VP and VS synthetic seismograms from the Leg 176 logging data (Fig. F12) and compared the results with previously published synthetic seismograms constructed with core data and VSP results (Iturrino and Christensen, 1991; Swift et al., 1991).
For refraction and reflection studies of the oceanic crust, the frequencies of air gun and explosive sources generally range from 5 to 30 Hz. Synthetic seismograms were calculated using compressional wave and shear wave data obtained with the DSI, the Leg 176 density log, and 30-Hz Ricker wavelets (Ricker, 1953). When calculating both synthetic seismograms using one-dimensional modeling, we assumed that our layered models persisted laterally over distances of a Fresnel zone. Assuming an average velocity of 6520 m/s and a 30-Hz source at a depth of 300 mbsf, the radius of a Fresnel zone would be 117 m. This value falls within the calculated range for the radius of a Fresnel zone obtained with VSP experiments in Hole 735B (Swift et al., 1991). Also, field observations of the well-explored Samail ophiolite plutonic section show that some layer sets can be traced along the strike in northern Oman for distances of ~10 km (Pallister and Hopson, 1981). These values and observations, in combinations with wavelengths of 217 and 117 m for the VP and VS synthetic seismograms, respectively, suggest the possibility of having laterally continuous igneous and metamorphic units in Hole 735B that can be imaged seismically.
The synthetic seismograms produced with compressional wave velocities show several prominent reflections at approximately 94, 213, 265, and 555 mbsf and several other low-amplitude events located at approximately 137, 343, 380, 425, 462, and 502 mbsf (Fig. F12). The reflection at 94 mbsf is associated with an ~20-m-thick interval characterized by changes in porosity, density, velocity, and composition (Fig. F13). This zone is bounded by high-density and -porosity Fe-Ti oxide intervals with low velocities and SiO2, MgO, and Al2O3 contents (Fig. F11). The reflections at 213 and 265 mbsf are also produced by a combination of compositional changes associated with Fe-Ti oxides and variations in density and velocity (Fig. F11). The event at 555 mbsf is associated with a fractured zone where two faults have been identified. The lower-amplitude reflections are related to compositional changes in SiO2, CaO, and Al2O3 content and porosity that seem to affect the compressional wave velocities. Compressional wave synthetic seismograms were also constructed using lower frequency Ricker (10 Hz) and Ormsby (low cut = 2 Hz, low pass = 5 Hz, high pass = 15 Hz, and high cut = 20 Hz) wavelets. The results showed smaller amplitude reflections that correlate with the results from Units II, IV, V, and VII (Fig. F12). The shear wave synthetic seismogram (Fig. F10) shows prominent reflections located at the same depths as those observed in the compressional wave reflection profile, but overall, a more detailed profile was produced due to the shorter wavelengths. This synthetic seismogram seems to be more sensitive to smaller variations in the velocity and density profiles.
A comparison between the compressional wave synthetic seismograms constructed with the Leg 176 logs and those records from the Leg 118 VSP experiment (Robinson, Von Herzen, et al., 1989; Swift et al., 1991) and laboratory data-derived synthetics seismograms (Iturrino and Christensen, 1991) shows very good agreement between the different types of data (Fig. F14). The normal-polarity log-derived synthetic seismogram correlates well with the top five VSP stacks, especially at depths of 94, 213, 265, and 555 mbsf. The correlation between the weaker reflectors and the VSP data is not as clear because the amplitudes of some reflections in the VSP profiles are not coherent for all receiver depths. It has been suggested that the difficulties in tracing events across the VSP sections may be due in part to phase shifting caused by clipping of the air gun data, interference from the ship, pipe noises when the receiver was clamped close to the bottom-hole assembly, waveform distortion from the applied filters, or differences in power received at different borehole depths (Swift et al., 1991).
The similarities between the core and log synthetic seismograms are remarkable (Fig. F14) considering that the core-derived synthetic seismogram was produced for a 40-layer model with ~100 discrete density and velocity measurements over a 500-m interval. All the prominent reflectors observed in the upper 500 m of Hole 735B of the reversed-polarity log-derived synthetic seismogram correlate with the core data. In addition, a reflector in the core synthetic seismogram at a depth of 55 mbsf seems to correlate with the VSP data. This reflection may indicate the transition between the mylonites and porphyroclastic metagabbros of Unit I into the olivine and olivine-bearing gabbros of Unit II.