New observations of the shallow seismic structure of young oceanic crust

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Abstract
The results are presented of three new experiments carried out on the Mid‐Atlantic Ridge (MAR) near latitude 23°N using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust. The data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography. These 1‐ to 2‐km‐long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200–300 m of the young igneous crust. One experiment was carried out over the site of hole 648B of the Ocean Drilling Program on a small volcano within the median valley of the MAR. The seafloor velocity was observed to be 2.1 km s−1 underlain by an approximately linear velocity gradient of 4 s−1. Given that we know the crust at this location consists of fresh basalt lavas with laboratory‐measured velocities in excess of 5.8 km s−1, porosities at the seafloor of as high as 30–50% are inferred. The two other experiments were located over 7‐m.y.‐old crust near Deep Sea Drilling Project site 395 west of the MAR, separated by only 14 km laterally but by over 1400 m in water depth. The experiment positioned in the ∼100‐m‐thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable. The velocity of the uppermost basaltic basement was 4.1 km s−1, and the velocity gradient was less than 0.5 s−1. This ∼2 km s−1 difference in velocity between the two sites is assumed to be a consequence of age‐related modifications to the physical properties of the crust. If the primary change is in only the total porosity, then a 15–20% reduction is required to explain these observations. This seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data (e.g., off‐axis volcanism, secondary mineral deposition, tectonic compression). A combination of many processes acting in unison would be necessary to produce such a decrease in porosity. If the geometry of the cracks and voids were modified in just the right way, then no change in total porosity is required to explain the velocity difference. These on‐bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor's geologic features. They produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements.