THE SENSITIVITY OF SEAFLOOR COMPLIANCE MEASUREMENTS TO SUB-BASALT SEDIMENTS

Abstract

We investigate the sensitivity of seafloor compliance - the transfer function between the seafloor stress and displacement under ocean waves - to sub-basalt sediments. Seafloor compliance is sensitive to the subsurface shear modulus as a function of depth and is particularly sensitive to low shear modulus regions such as fluid-saturated sediments. We generate synthetic seafloor compliance data for different models of sediments beneath basalts, run geophysical inversions on the data and compare the inversion results with the input models. Our input models are variations on a reference model in which sub-basalt sediments start 3.5 km beneath the seafloor and have an average shear modulus of $6.8 × 109 Pa$ (shear wave velocity $ca 1.7 km s-1 )$. The sensitivity of seafloor compliance to sub-basalt sediments depends mostly on the water depth and the sub-basalt sediment layer depth, thickness and shear modulus. If the water depth is 1 km or more, seafloor compliance measurements will detect a sub-basalt sediment layer 0.6 km or more thick and will constrain the depth to the top and bottom of a 2-km thick sub-basalt sediment layer to within 0.2 km. If the water depth is 0.25 km, the thinnest detectable layer will be 1.2 km and the depth uncertainty for a 2-km thick sub-basalt sediment layer will be 0.5-0.8 km. Neither interlayered sediments within the basalts nor sediments above the basalt layer have a strong effect on the measurement sensitivity.