Seafloor Compliance Research Articles

Revisiting the OBS seafloor compliance signal removal with a stationarity and stacking-based approach: the BRUIT-FM toolbox

Summary This study focuses on improving the seafloor compliance noise removal method, which relies on estimates of the compliance transfer function frequency response (the deformation of the seafloor under long-period pressure waves). We first propose a new multi-scale deviation analysis of broadband ocean bottom seismometer data to evaluate stationarity properties that are key to the subsequent analysis. We then propose a new approach to removing the compliance noise from the vertical channel data, by stacking daily estimated transfer function frequency responses over a period of time. We also propose an automated transient event detection and data selection method based on a cross-correlation criterion. As an example, we apply the method to data from the Cascadia Initiative (network 7D2011). We find that the spectral extent of long-period forcing waves varies significantly over time so that standard daily transfer function calculation techniques poorly estimate the transfer function frequency response at the lowest frequencies, resulting in poor denoising performance. The proposed method more accurately removes noise at these lower frequencies, especially where coherence is low, reducing the mean deviation of the signal in our test case by 27 % or more. We also show that our calculated transfer functions can be transfered across time periods. The method should allow better estimates of seafloor compliance and help to remove compliance noise at stations with low pressure-acceleration coherence. Our results can be reproduced using the BRUIT-FM Python toolbox, available at https://gitlab.ifremer.fr/anr-bruitfm/bruit-fm-toolbox.

Shear‐Wave Velocity Structure of Sediments on Cascadia's Continental Margin From Probabilistic Inversion of Seafloor Compliance Data

AbstractSeveral seismic techniques, both passive and active, exist for estimating the shear‐wave velocity structure of shallow sedimentary structures. In particular, passive compliance signals recorded by broadband ocean‐bottom seismometers (OBSs) can be used to invert for structure. While compliance‐based imaging studies have been carried out at several locations across the Cascadia Subduction Zone, such an approach has not been extensively applied to OBSs deployed on the continental shelf and slope. In this study, we measure compliance and coherence signals at 13 broadband OBSs deployed along Cascadia's continental shelf and upper slope. We then use a recently developed technique to probabilistically invert compliance signals for shallow structure that makes use of mixture density neural networks. Finally, we compare and contrast our inverted profiles and derived properties obtained using this method with previous studies focused on the properties of basin sediments.

Probabilistic inversion of seafloor compliance for oceanic crustal shear velocity structure using mixture density neural networks

SUMMARY Measurements of various physical properties of oceanic sediment and crustal structures provide insight into a number of geological and geophysical processes. In particular, knowledge of the shear wave velocity (VS) structure of marine sediments and oceanic crust has wide ranging implications from geotechnical engineering projects to seismic mantle tomography studies. In this study, we propose a novel approach to nonlinearly invert compliance signals recorded by colocated ocean-bottom seismometers and high-sample-rate pressure gauges for shallow oceanic shear wave velocity structure. The inversion method is based on a type of machine learning neural network known as a mixture density neural network (MDN). We demonstrate the effectiveness of the MDN method on synthetic models with a fixed deployment depth of 2015 m and show that among 30 000 test models, the inverted shear wave velocity profiles achieve an average error of 0.025 km s−1. We then apply the method to observed data recorded by a broad-band ocean-bottom station in the Lau basin, for which a VS profile was estimated using Monte Carlo sampling methods. Using the mixture density network approach, we validate the method by showing that our VS profile is in excellent agreement with the previous result. Finally, we argue that the mixture density network approach to compliance inversion is advantageous over other compliance inversion methods because it is faster and allows for standardized measurements.

Melt-affected ocean crust and uppermost mantle near Hawaii—clues from ambient-noise phase velocity and seafloor compliance

SUMMARYWe present models of crustal and uppermost mantle structure beneath the Hawaiian Swell and surrounding region. The models were derived from ambient-noise intermediate-period Rayleigh-wave phase velocities and from seafloor compliance that were estimated from continuous seismic and pressure recordings collected during the Hawaiian Plume-Lithosphere Undersea Mantle Experiment (PLUME). We jointly inverted these data at the locations of over 50 ocean-bottom instruments, after accounting for variations in local bathymetry and sediment properties. Our results suggest that the crystalline crust is up to 15 km thick beneath the swell and up to 23 km thick closer to the islands. Anomalously thick crust extends towards the older seamounts, downstream of Hawaii. In a second region, anomalies immediately to the south of Hawaii may be associated with the leading edge of the shallow Hawaiian magma conduit. In a third region, thickened crust to the immediate west of Hawaii may be related to Cretaceous seamounts. Low seismic velocities identified in the uppermost mantle to the northeast of Hawaii may be linked to the Molokai fracture zone and may be manifest of complex non-vertical pathways of melt through the upper lithosphere. Velocity anomalies decrease in amplitude towards the surface, suggesting that melt becomes focused into conduits at depths between 20 and 40 km that escape the resolution capabilities of our data set.

Continuous evolution of oceanic crustal structure following an eruption at Axial Seamount, Juan de Fuca Ridge

Abstract We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.

Crustal shear velocity structure in the Southern Lau Basin constrained by seafloor compliance

AbstractSeafloor morphology and crustal structure vary significantly in the Lau back‐arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low‐velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5–0.8 km/s slower than crust produced at the present‐day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.

Infragravity waves and horizontal seafloor compliance

AbstractWe report the first consistent observation of horizontal seafloor compliance induced by infragravity (IG) waves. Long‐period IG ocean waves manifest themselves as broad, dominant features in ocean bottom pressure and vertical deformation spectra, but signals are rarely (if ever) identified on the horizontal components of traditional ocean bottom seismometers (OBS) due to low signal level and high current‐induced tilt noise at long periods. We examine two OBS stations with shallow‐buried seismometers: the Monterey Ocean Bottom Broadband site offshore California and the Ocean Seismic Network (OSN) pilot site OSN1B near Hawaii. We use nearby weather buoys to investigate the relationship between the presence of infragravity waves and environmental conditions. We find strong evidence that infragravity wave generation is primarily confined to the near‐coastal environment. Additional IG source information is found by examining the directionality of passing IG waves as a function of frequency, which we analyze using the coherence between pressure and the two horizontal components. Finally, we evaluate the implications for a joint vertical and horizontal compliance inversion.

Faulting and hydration of the upper crust of the SW Okinawa Trough during continental rifting: Evidence from seafloor compliance inversion

AbstractThe elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back‐arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.

A joint Monte Carlo analysis of seafloor compliance, Rayleigh wave dispersion and receiver functions at ocean bottom seismic stations offshore New Zealand

AbstractTeleseismic body‐wave imaging techniques such as receiver function analysis can be notoriously difficult to employ on ocean‐bottom seismic data due largely to multiple reverberations within the water and low‐velocity sediments. In lieu of suppressing this coherently scattered noise in ocean‐bottom receiver functions, these site effects can be modeled in conjunction with shear velocity information from seafloor compliance and surface wave dispersion measurements to discern crustal structure. A novel technique to estimate 1‐D crustal shear‐velocity profiles from these data using Monte Carlo sampling is presented here. We find that seafloor compliance inversions and P‐S conversions observed in the receiver functions provide complimentary constraints on sediment velocity and thickness. Incoherent noise in receiver functions from the MOANA ocean bottom seismic experiment limit the accuracy of the practical analysis at crustal scales, but synthetic recovery tests and comparison with independent unconstrained nonlinear optimization results affirm the utility of this technique in principle.

Spatial distribution and temporal evolution of crustal melt distribution beneath the East Pacific Rise at 9°–10°N inferred from 3‐D seafloor compliance modeling

AbstractDetermining the melt distribution in oceanic crust at mid‐ocean ridges is critical to understanding how magma is transported and emplaced in the crust. Seafloor compliance—deformation under ocean wave forcing—is primarily sensitive to regions of low shear velocity in the crust, making it a useful tool to probe melt distribution. Analysis of compliance data collected at East Pacific Rise between 9° and 10°N through 3‐D numerical modeling reveals strong along‐axis variations in the lower crustal shear velocities, as well as temporal variation of crustal shear velocity near 9°48′N between measurements spanning 8 years. Compliance measured across the rise axis at 9°48′N and 9°33′N suggest a deep crustal low‐velocity zone beneath the ridge axis, with a low Vs/Vp ratio consistent with melt in low aspect ratio cracks or sills. Changes in compliance measured at 9°48′N between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval, perhaps following the 2005–2006 seafloor eruption. This temporal variability provides direct evidence for short‐term variations of the magmatic system at a fast spreading ridge.

Effects of sea states on seafloor compliance studies

Gas hydrates affect the bulk physical properties of marine sediments, in particular, elastic parameters. Shear modulus is an important parameter for estimating the distribution of hydrates in the marine sediments. However, S-wave information is difficult to recover without proper datasets. Seafloor compliance, the transfer function between pressure induced by surface gravity waves and the associated seafloor deformation, is one of few techniques to study shear modulus in the marine sediments. The coherence between recorded time series of displacement and pressure provides a measure of the quality of the calculated transfer function, the seafloor compliance. Thus, it is important to understand how to collect high coherence datasets. Here we conducted a 10-month pilot experiment using broadband seismic sensors and differential pressure gauges. We found that data collected in shallow water depth and during rough seas gave high coherence. This study is the first time long-term data sets have been employed to investigate seafloor compliance data quality and its dependence on sea state. These results will help designing future large-scale compliance experiments to study anomalously high shear moduli associated with the presence of gas hydrate or cold vents, or alternatively anomalously low shear moduli, associated with partial melt and magma chamber.

Two-dimensional finite-difference model of seafloor compliance

SUMMARY Seafloor compliance is the measure of seafloor deformation under a pressure signal. Our new 2-D finite-difference compliance modelling algorithm presents several advantages over the existing compliance models, including the ability to handle any gridded subsurface structure with no limitations on the gradients of the material properties, as well as significantly improved performance. Applying this method to some of the problems inaccessible to previously existing methods, demonstrates that lateral variations in subsurface structure must be accounted for to adequately interpret compliance data. In areas with significant lateral variations, the utilization of 1-D modelling and inversion is likely to result in high interpretation errors, even when additional subsurface structure information is available. We find that flattened pure melt bodies have a significantly higher compliance than cylindrical melt bodies with the same cross-sectional area. The compliance created by such bodies often has side peaks over their edges, which are as strong as or stronger than the central peak, requiring a series of measurements to best constrain their size and shear velocity. Finally, we find that the compliance data are far and away most sensitive to the broad, thick, lower-crustal partial melt zone. Our simple data fitting model for the compliance measurements on the East Pacific Rise at 9°48′N required shear velocities as low as 700 m s−1 in the centre of this zone, far below the values previously estimated using 1-D model based inversions, suggesting higher melt percentages than those previously estimated, while small melt bodies in the upper part of the crust were found to have little or no effect on the measured compliance.

Seafloor compliance imaging of marine gas hydrate deposits and cold vent structures

Seafloor compliance, the transfer function between pressure induced by surface gravity waves and the associated seafloor deformation, can address outstanding questions about the nature of seismic blank zones, identified as cold vents offshore Vancouver Island, near Ocean Drilling Program (ODP) Site 889 and the recent Integrated Ocean Drilling Program (IODP) Expedition 311. Despite extensive seismic studies, it is not known whether free gas or gas hydrate is primarily responsible for the seismic blanking, nor are the concentration and distribution of either component conclusively determined. Here we use the formation's shear modulus to distinguish between gas hydrate, which can strongly alter the shear modulus, and free gas, which cannot. The depth‐dependent gas hydrate distribution and pore‐space saturation are estimated from the frequency variation of seafloor compliance measurements sensitive to the formation's shear modulus. Compliance data are consistent with pervasive moderate hydrate concentration over a broad region and significantly higher cylindrical concentrations associated with the cold vents, perhaps as much as 9 × 106 m3 in the 400 m diameter Bullseye vent, in agreement with controlled‐source electromagnetic data. Compliance data can help distinguish between different models for the structure of such gas hydrate‐bearing cold vents.

Sediment shear properties from seafloor compliance measurements: Faroes‐Shetland basin case study

ABSTRACTShear properties provide important information about the lithology, fluid content and stability of sediments but are difficult to measure using conventional seismics in the marine environment. Seafloor compliance measurements are sensitive to subsurface shear properties but have only been used in the Pacific Ocean and on shallow coastal shelves, where the source wave energy is known to be strong. We show here that seafloor compliance measurements can provide useful information about shear properties of marine sediments in less energetic settings and under high noise conditions caused by strong seafloor currents. We measured compliance at three sites in the Faroes‐Shetland sedimentary basin north of the Atlantic ocean. The sites have 1000 times higher noise levels than quiet seafloor sites and the source wave power is highly variable, but the data still reveal significant differences in sediment properties between two sites down to 2 kilometres beneath the seafloor. The first site, at the northern end of the basin, has an average shear velocity of 400 m/s in the upper 0.6 kilometres beneath the seafloor, increasing to approximately 2100 m/s at 2 kilometres beneath the seafloor. The second site, further south and to the west of the basin axis, has an average shear velocity of 150 m/s in the upper 0.6 kilometres beneath the seafloor, increasing to 1400 m/s at 2 kilometres beneath the seafloor. The sediments are probably unconsolidated in the upper 0.6 kilometres beneath the seafloor at both sites, with a mean grain size of 1 μm at the southern site and 20 μm at the northern site. The southern site has higher porosity at all depths and a higher risk of borehole collapse during drilling.

東太平洋海膨のオフリッジ火山はモホ遷移帯起源か？

A large number of intraplate volcanoes erupted two to several hundred kilometers off the fast-spreading East Pacific Rise (EPR). These volcanoes consist of large lava fields, monogenetic volcanoes, and linear chains of monogenetic volcanoes and volcanic ridges. Large lava fields of 7-26 km3 in volume are known at 8°N, 14°S, and 16°S within 2-19 km from the rise axis and from the top 75-100 m of ODP Site 1256 on the 15 Ma Cocos plate. Monogenetic volcanoes form within ∼20 km from the rise axis or on the basement < 200 kyr, and are evenly distributed over the rise axis. Linearly aligned volcanoes and volcanic ridges occur farther from the rise axis than large lava fields and monogenetic volcanoes, and run subparallel to the direction of the Pacific plate motion. The Sojourn Ridge, the largest volcanic ridge, extends up to 440 km in length and is several hundred cubic kilometers in volume. Eruptive ages along a volcanic ridge and a volcano chain contradict the hot-spot origin of these volcanic features. Negative free-air and residual mantle Bouguer anomalies correlate well with the linearly aligned volcanoes and volcanic ridges, suggesting excess magma supply beneath the volcanic edifices. Seismic experiments show volcanic ridges have no keel below the Moho, indicating compensation of surface loading by plate flexure and underplating. Whole rock compositions of off-ridge volcanoes have a much wider spectrum than the adjacent axial lavas, spanning from depleted NMORB through TMORB to isotopically fertile EMORB. Some off-ridge lavas could be produced by the fractional crystallization of the same parent magma as the adjacent axial lavas. However, most off-ridge lavas originate from different parent magmas than the neighboring axial lavas. Some TMORB magmas including the 14°S large lava field are the mixing product of the NMORB and EMORB magmas. Copious differentiated lavas of the large lava fields require a large magma chamber as a the site for crystallization differentiation and magma mixing. The lava geochemistry of off-ridge volcanoes strongly suggests the presence of a magma source that is independent of the axial magma plumbing system. Seismic tomography and seafloor compliance measurements beneath the northern EPR indicate that the presence of melt across the rise axis is restricted in a narrow zone ∼4 km in width through the crust, but has a 10-14 km wide distribution in the uppermost mantle. Broad distribution, volume, and geochemistry of off-ridge monogenetic volcanoes and large lava fields strongly suggest that the off-ridge volcanoes originated from the Moho transition zone (MTZ). The MTZ is formed by a reaction between the uprising magma and the host mantle peridotite, leaving replacive dunite that experienced variable depletion and enrichment processes. Passive asthenospheric upwelling beneath the fast-spreading ridges produces a broad partial melt zone, through which magma ascends and accumulates beneath the off-ridge lithosphere. More depleted off-ridge magmas than axial magmas differentiate and mix with residual magmas in the MTZ, and react with variably enriched, impregnated dunite, resulting in variety of off-ridge lava compositions. Small clusters of volcanoes and linear volcano chains are created by partial melting in asthenospheric return flows or local instability of the thermal boundary layer beneath the cooling lithosphere. Linear volcano chains will develop into long and robust volcanic ridges extending several hundred kilometers in length.

Seabed property estimation from ambient-noise recordings: Part I — Compliance and Scholte wave phase-velocity measurements

The ability to derive a near-surface shear-velocity profile from ambient-noise records is useful for seismic applications such as shear-wave statics estimation and geohazard prediction. Measurements of seafloor compliance and Scholte wave velocity and amplitude are all related to the near-surface shear-velocity profile. I analyzed a data set of [Formula: see text] of continuous noise records recorded by an ocean bottom cable deployed in [Formula: see text] deep water for seafloor compliance and Scholte waves. I failed to observe seafloor compliance because of limitations in the record length. I have detected Scholte waves on the inline and vertical component geophones and Love waves on the crossline component using [Formula: see text] spectra. Both the Scholte and Love wave phase-velocities can be explained by a simple 1D isotropic near-surface model. The Scholte waves may have been excited by acoustic energy from the recording vessel, while no satisfactory excitation mechanism has been found for the Love waves.

The sensitivity of seafloor compliance to two-dimensional low-velocity anomalies

SUMMARY Seafloor compliance (the deformation of the seafloor under pressure forcing by long wavelength ocean surface waves) is a passive source geophysical measurement that is particularly sensitive to subsurface S-wave velocity. Existing inversions of seafloor compliance data for S-wave structure assume a 1-D velocity model, whereas in reality there may be significant lateral velocity variation. The effect of 2-D structure on compliance measurements has previously been investigated using finite differences, but computational constraints limit the number and type of models that can be tested. We present a quasi-analytical calculation of seafloor compliance over a cylinder that allows us to efficiently investigate the effect of an important subset of subsurface models, including pure fluid inclusions. The compliance displays the same sensitivity to regions of low shear modulus as is seen for 1-D situations, generating a strong peak over the low-velocity zone. The spatial width of this peak, which is a measure of the horizontal resolution of seafloor compliance, is approximately proportional to the depth of the cylinder, while its amplitude is controlled by the radius and depth of the cylinder and the S-wave speed of the cylinder and the background. Minimum structure 1-D inversions of the compliance measured above a fluid cylinder recover distinct low-velocity zones, although the recovered S-wave speed is much greater than zero, and the low-velocity zone is smeared out over a greater depth range than that occupied by the cylinder.

The sensitivity of seafloor compliance measurements to sub-basalt sediments

SUMMARY 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 × 10 9 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.

Shear wave structure from joint analysis of seismic and seafloor compliance data

SUMMARY Determining shear wave structure is the key to identifying the amount and location of fluid within the crust. Seismic and seafloor compliance methods provide independent estimates of shear wave structure, and a joint analysis of the two data sets should provide better constraints on the properties of the uppermost oceanic crust. We consider an example from 9 ◦ 33 � N on the East Pacific Rise. Seismic data from an on-axis expanding spread profile have been reanalysed to determine a shear wave structure for layer 2B; pS arrivals require a high shear wave velocity within this layer (Poisson ratio in the range 0.22‐0.25). Compliance data from the same location are seemingly inconsistent with this result, requiring that layer 2B is a region of low shear wave velocity (Poisson ratio in the range 0.33‐0.44). The quantitative differences between the two results can be explained by anelasticity and anisotropy; conversely, a knowledge of this discrepancy can be used to constrain the attenuation structure.

Broadband marine MT exploration of the East Pacific Rise at 9°50′N

We present the first use at a mid‐ocean ridge of a recently developed broadband marine magnetotelluric (MT) instrument. The extended high frequency performance of the instrument allows resolution of electrical resistivity structure at shallower depths than traditional marine MT sensors. Our two‐dimensional inversion model from data collected at four MT sites on the East Pacific Rise (EPR) at 9°50′N demonstrates the viability of the method to image electrical resistivity structure in both the crust and shallow mantle. While our pilot experiment falls far short of the coverage needed to provide rigorous constraints on structure, a low resistivity zone in the crustal portion of the inversion model agrees well with seismic tomography and seafloor compliance results. Resistivities beneath the ridge imply a crustal partial melt fraction of 1–20%. A total melt volume of about 0.75 km3 per kilometer of ridge implies an average melt residence time of about 1000 years.