Imaging of Lower-crustal Magma Chambers at an Ultraslow Spreading Ridge Segment using Elastic Waveform Inversion of a Sparse OBS Dataset
Abstract:The existence of axial magma chambers (AMC) is indicative of the magmatic crustal accretion at Mid-Ocean Ridges. They have been extensively imaged with seismic reflection data (e.g. multichannel seismic data), showing that the depth of the top reflector increases from 1 km to ~3 km below the seafloor, when the spreading rate decreases from fast to slow spreading. Under the ultraslow spreading environment, we have previously reported the discovery of a large lower-crustal low-velocity zone at the Southwest Indian Ridge at 50°28'E from 3-D travel time tomography of refraction data registered by an ocean bottom seismometer (OBS) array. These results suggest the presence of partial melt within the lower crust (>4 km bsf).
Here we further improve the resolution of the AMC image by employing a 2-D time-domain elastic full waveform inversion (FWI) method. The FWI gives a higher resolution than travel time tomography as it utilizes amplitude information and does not require the high-frequency approximation used in travel time tomography. The non-linearity of the FWI is overcome by using the tomographic results as a starting model. We have selected a 70-km long profile running across the ridge axis around the segment center, where 340 shots spaced at ~220 m were recorded on 3 OBSs. The small number of OBS poses serious challenge for the success of the full waveform inversionFWI. In order to examine the resolvability of this sparse OBS dataset, we first performed FWI over a sparse synthetic data set. We find that the FWI of these this sparse dataset is capable of retrieving an isolated lower-crustal AMC anomaly beneath the ridge axis, although the resulting velocity anomaly is smeared out, particularly along the lateral direction. For the real-data inversion, the starting model was built from the 3-D travel time tomography. The inverted results clearly show the sharp boundary of the top of the low velocity zone, suggesting that the low velocity zone indeed corresponds to the crustal AMC.