Evidence for a higher porosity upper crust in the North Atlantic Ocean

The most common way to estimate the porosity of the mature oceanic crust, and hence its contribution to geochemical exchange, is from the inversion of active source seismic data. Previous studies have suggested that hydrothermal activity effectively ceases in crust older than 10 Ma. This is based on observations that the seismic velocity of the uppermost crust increases rapidly within 10 Ma but changes little beyond that. The velocity increase is widely explained by reduced porosity and permeability due to hydrothermal mineral precipitation and fracture closure due to sediment blanketing. However, a potential problem with conventional wide-angle Ocean Bottom Seismometer (OBS) modelling over mature oceanic crust is the imaging geometry, where the water wave obscures the onset of the crustal refractions for tomographic inversion needed to resolve the velocity of the upper hundred meters of the igneous crust.

 

To investigate the issue of imaging geometry and accurately extract the physical properties of the upper crust, we applied downward continuation on conventional OBS records across 65 Ma Atlantic Ocean crust. The method eliminates the effect of the thick water column by locating shots on a datum close to the seabed. This enables refractions from the uppermost 100-200 m of igneous crust to be viewed as first arrivals, and hence significantly improves the accuracy of the velocity inversion of the upper layers. Using travel time picks from downward continued and original OBS records, we applied tomographic inversion with three starting models taken from the latest compilation of crustal velocity-depth (VZ) models. The three models have a velocity variation of ±10% for the uppermost crust, representing a low, mean, and high bound for crustal VZ relations. By comparing the results, we show that with downward continued data the inverted velocity of the uppermost crust is less dependent on the starting model and converges to the same trend closer to the low bound of previous VZ relations. The average velocity of the uppermost crust inverted with downward continued data is ~0.3 km/s lower than that inverted with original data. These results would translate into a porosity of 14%, compared to 10% for the non-downward continued analysis. We also resolve stronger along-strike variation in the inverted velocity of the uppermost crust (4.2 km/s to 4.8 km/s) using downward continued data compared to original data, which may correspond to porosity as large as 18%, much higher than previously suggested porosity of 8 % for mature oceanic crust. This implies that open cracks may be still present and thick sediments may seal hydrologically but not close fractures that affect the seismic refraction data. Moreover, our results reconcile with the recent work in the South Atlantic performed with full-waveform inversion (FWI) on streamer data. Therefore, downward continuation, also FWI may need to be incorporated into the workflow of conventional wide-angle OBS modelling, especially for mature oceanic crust. This will improve the resolution and accuracy of the physical properties of the uppermost crust and may shed new light on its evolution and role in off-axis hydrothermal circulation and seawater chemistry.