Distributed acoustic sensing (DAS) offers a cost effective, nonintrusive method for high-resolution near-surface characterization in urban environments where conventional geophysical surveys are limited or nonexistent. However, passive imaging with DAS in urban settings presents challenges such as strong diurnal traffic noise, nonlinear array geometry, and poor fiber coupling to the ground. We repurposed a dark fiber in Melbourne, Australia, into a 25 km DAS array that traces busy arterial roads, tram routes, and orthogonal sections. By employing noise cross correlation and array beamforming, we calculated dispersion curves and successfully inverted for a near-surface shear-wave velocity model down to 100 meters. Stationary seismic sources are maximized by selecting daytime traffic signals, thereby recovering surface waves and reducing interference from acoustic waves from man-made structures in the subsurface. Poorly coupled channels, which are linked to fiber maintenance pits, are identified through cross-correlation amplitudes. The dispersion curve calculation further considers the channel orientation to avoid mixing Rayleigh and Love waves. Using a trans-dimensional Markov chain Monte Carlo sampling approach, we achieved effective model inversion without a prior reference model. The resulting near-surface profile aligns with mapped lithology and reveals previously undocumented lithological variation.

Abstract Space gravity missions have enabled the quantification of the mass component of sea‐level rise over the past two decades. Barystatic sea‐level rise is predominantly driven by melting polar ice sheets and mountain glaciers. However, continental hydrological processes also contribute to global sea level change at significant magnitudes. We show that for most coastal areas in low‐to‐mid latitudes, up to half of manometric sea‐level rise is due to changes in water storage in ice‐free continental regions. At other locations the direct attraction effect of anthropogenic pumping of groundwater over the duration of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GRACE‐FO) mission offsets sea‐level rise from ice sheet and glacier melt. If these trends in continental hydrological storage were to slow or stop, these regions would experience greatly accelerated sea‐level rise, posing a risk to coastal settlements and infrastructure, however, for most coastal communities current rates of sea‐level rise would be significantly reduced.

Models of the temporal gravity field derived from space gravity missions are typically produced with monthly temporal resolution and ~300 km spatial resolution. However, variations in instrument performance and altitude of the GRACE mission impact the spatial resolution that can be achieved month-to-month. As the altitude of the orbits of the twin spacecraft vary throughout the mission, so does the ability of the observations to recover certain components of the temporal gravity field. The spatial resolution of GRACE observations should increase as the altitude decreases throughout the mission because the reduced altitude intensifies the gravity signals acting on the satellites. Simulations using actual GRACE altitude and ground track coverage and realistic noise levels confirm this predicted influence of the altitude of the satellites on the accuracy of the estimated solutions. Solutions with larger mass concentration elements (mascons) are more numerically stable as the satellite altitude decreases but they suffer from greater error caused by the inability to properly represent spatial variations of signals within mascons, referred to as intra-mascon variability. Mascons as small as ~150 x 150 km (i.e. ~1.5 arc-degree) reduce the intra-mascon variability and, with appropriate regularisation, yield the most accurate solutions, especially during the low-altitude periods of the GRACE mission. Importantly, unlike spherical harmonic solutions, regularised mascon solutions are not degraded during resonant orbit months, and are of comparable quality to months with full ground track coverage.

The precise calculation of GRACE and GRACE-FO satellite orbits is reliant on knowledge of accurate non-gravitational accelerations acting on the spacecraft. These are measured by the on-board accelerometers that require a thermal environment stabilised to - +/- 0.1 degrees C per revolution. However, during periods of the GRACE mission with reduced thermal control, internal temperature variations reached up to 10 degrees C within a revolution, causing low-frequency and non-linear drifts in the accelerometer observations. Additionally, accelerometer bias drifts occurred throughout the GRACE mission as changes in the orientation of the orbital plane with respect to the Earth-to-Sun vector caused the satellites to absorb more or less solar energy. These temperature-induced drifts degrade the quality of mass change estimates, particularly during the latter half of the GRACE mission after thermal control of the satellites was terminated. We filter (in the frequency domain) the accelerometer observations to remove these low-frequency components (f < 0:045 mHz). The bias drift removed from the cross-track is then scaled to derive a thermally-based correction for the highly sensitive along-track observations. We then estimate temporal gravity fields using the ANU GRACE software, our filtered accelerometer observations and the range acceleration as the inter-satellite observation. The use of our thermally-corrected accelerometer measurements significantly improves the accuracy of both orbit modelling and gravity field estimation. (c) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

The estimation of mass anomalies using Gravity Recovery and Climate Experiment (GRACE) data involves parameterizing the temporal gravity field using basis functions. In this study, we show that the use of irregularly shaped mass concentration (mascon) tiles that follow land/ocean boundaries reduces the leakage of land signals into ocean regions and vice versa. Leakage of signal from continents to oceans in mascons that cross the coastline affect the integrated mass changes at a regional scale. For example, the calculated mass loss in 2016 is ∼5% greater for Greenland when using mascons that follow coastlines. We describe efficient algorithms for computing the accelerations acting on the satellites caused by mass changes on mascons, along with the partial derivatives relating the mass changes to the inter‐satellite observations. Through simulation, we quantify the impact of different mascon geometries, spatial resolution and regularization. The variations of mass change signals within mascons, which we call “intra‐mascon variability,” contribute to errors in estimates of mass variation from GRACE data. While this can be mitigated through the regularization of the inversions, it cannot be removed entirely. The use of irregularly shaped mascons that follow land/ocean boundaries reduces the “intra‐mascon leakage” of land signals into ocean regions and vice versa. This approach can also be applied to hydrological basins for calculating integrated mass changes on catchment scales.

Several different basis functions have been used to represent the Earth's gravity field in order to generate estimates of mass variations on Earth from the analysis of data of the Gravity Recovery and Climate Experiment ( grace ) and its successor grace Follow‐On missions, including spherical harmonics, mass concentration elements (mascons) and slepian functions. Each approach depends inherently upon accurate modeling of the orbits of the pair of satellites as they revolve around the Earth, so that the observations of inter‐satellite changes in range (or, more specifically, range rate) can be exploited to identify mass variations. We have developed software using a classical orbit modeling approach, mascons and 24‐hr orbit integration, to estimate simultaneously corrections to orbital parameters and the temporal gravity field from grace data. Rather than using the range rate, we use the range acceleration as the inter‐satellite observable as it aids in localizing the mass variations. Level‐1 B range acceleration observations contain high levels of high‐frequency noise that inhibits their usefulness for this purpose. Instead, we generate range acceleration observations by numerical differentiation of the Level‐1B range rate prefit residuals. Simulations show that the gravity signal is not attenuated in this process. Our monthly estimates of mass anomalies from grace data (2003–2016) agree well with previous studies, both spatially and temporally. When converted to spherical harmonics our time series of C 2,0 , derived from grace data alone, are close to the independent estimates from satellite laser ranging, but the overall solution is improved by substituting the SLR C 2,0 .

The GRACE Follow-On mission is the first twin-satellite mission equipped with a laser ranging interferometer (LRI) to measure the inter-satellite distance between the pair of satellites. The LRI operates independently of the K/Ka-band interferometer (KBR) and uses wavelengths 104 times shorter than the K-band system. Released at the end of July 2019, the LRI range data is therefore expected to be of higher accuracy than the KBR and offers the possibility of a better spatial resolution. We compare the LRI and KBR observations of the GRACE-FO mission, from launch to December 2019, to assess the quality of the new LRI system. Spectral analysis of the level1B data shows that the noise level of the LRI is 3 orders of magnitude smaller than the KBR and that the gravity signal can be detected in the spectral band up to 30mHz in the LRI data compared to 20mHz in the KBR data. We compare gravity fields estimated using LRI and KBR and show which parts of the spherical harmonic spectrum are affected by the improved accuracy of the LRI observations.

Papua New Guinea is one of the most active tectonic regions in the world. It comprises several microplates and deforming zones trapped in the collision of the Australian and Pacific Plates. GPS observations have been used to estimate plate velocities across a network of sites spanning most of the country. We present new velocites in the northwestern region of New Guinea, and look in detail at the strain accumulation region between the South Bismarck and Pacific Plates in the New Ireland/New Britain region.

We use seismic tomography, exploiting group velocities derived from ambient noise, to delineate the crustal structure beneath Mt Isa and the surrounding blocks and basins. The depth extent of the blocks can be traced into the mid-crust and the spatial extent of the associated velocity anomalies mapped over an area of approximately 500km by 500km. The Proterozoic Mt Isa block is imaged as a region of elevated seismic velocities comparable to the Yilgarn craton in Western Australia, while the surrounding basins have relatively low velocities. Seismic velocity anomalies display correlations with the regional Bouguer gravity data and with high crustal temperatures in the region. There are a number of isolated low-velocity anomalies under the Millungera basin that suggest either previously unknown thermal anomalies or zones with high permeability, which can also produce lowered velocities.

Tilted block basins develop as a consequence of crustal or lithospheric rotations during imbrication under horizontal in-plane compression. We develop the basic mechanics of the block interactions, consider the consequences of elastic flexure and outline how more realistic elastic-plastic rheologies may explain decoupling and deformation at depth.

The 21st Century has seen the occurrence of 17 great earthquakes (Mw >8), including some of the largest earthquakes ever recorded. Numerical modeling of the earthquakes shows that nearly half of the Earth's surface has undergone horizontal coseismic deformation >1 mm, with the 2004 Sumatra‐Andaman earthquake dominating the global deformation field. This has important implications for both the realization of a terrestrial reference frame and in the interpretation of regional tectonic studies based on GPS velocities. We show that far‐field coseismic deformations from great earthquakes will, if unaccounted for, introduce errors in estimates of linear site velocities of at least 0.1‐0.3 mm/yr across most of the surface of the Earth. The accumulated global deformation field shows that two regions, Australia and the north Atlantic/Arctic Ocean, have been largely undeformed by these great earthquakes, with accumulated deformations generally <0.5 mm. Using GPS estimates of surface deformation, we show that the majority of the Australian continent is deforming at <0.2 mm/yr, the northern part of New Zealand is rotating clockwise relative to the Australian Plate with relative horizontal velocities of ∼2 mm/yr, while the southeastern coast of Australia is undergoing post‐seismic relaxation caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake. The presence of ongoing post‐seismic relaxation thousands of kilometers from plate margins violates the secular/linear assumption made in current terrestrial reference frame definitions. These effects have significant ramifications for regional tectonic interpretations and global studies such as sea level rise that require reference frame accuracy greater than this level.

Changes in relative sea-level (RSL) are generally caused by variations in sea surface heights from steric effects (thermal expansion and salinity changes) and the mechanical response of the Earth to past and current redistributions of ice and water between land and oceans. This paper focuses on the latter, where we present scenario calculations of the spatial variability in present-day RSL change around the Australian coastline resulting from melting land-based ice masses. Three scenarios are investigated: (1) the ongoing effect of glacial isostatic adjustment (GIA) arising from ice- and water-load redistribution during the last glacial–interglacial transition; (2) the effect of present-day changes in the Greenland and West and East Antarctic ice sheets (GIS, WAIS and EAIS, respectively) and two regions of major mountain glaciation, Alaska and Patagonia; and (3) a hypothetical complete melting of the GIS, WAIS and EAIS occurring over 5000 years. The first scenario shows falling RSL around Australia of the order of 0.4 to 1.2 times the average value around the coast (equivalent to a RSL fall of between 0.2 and 0.6 mm/a). For the second scenario, the spatial variability is strongly dependent upon the location of each ice mass relative to Australia. For Greenland and Patagonia, the resulting changes to the Earth's rotation strongly affect the spatial variability, while the direct gravitational effect is more important when considering the Antarctic ice sheets. The variability associated with the first two scenarios becomes clearer when examining RSL change estimates for the locations of tide-gauge stations around the Australian coast, especially for the ongoing GIA (a south-to-north increase in the simulated rate of RSL change), the WAIS (east-to-west increase) and the EAIS (south-to-north increase), with the melting of the EAIS potentially having the greatest influence on the variability of the melting land-based ice contribution to RSL change around Australia. The spatial variability associated with the third scenario is strongly influenced over century-length time-scales by the resulting changes in the Earth's rotation and the direct gravitational attraction of the ice masses, while after several thousand years the uplift of the continent by mantle material displaced towards it by increased ocean loading becomes more prominent. It must, however, be kept in mind that the spatial variability associated with these scenarios is generally a small proportion of the total RSL change, and that the steric (especially thermosteric) contribution is not included in these results.

Hydrological processes cause variations in gravitational potential and surface deformations, both of which are detectable using space geodetic techniques. We computed elastic deformation using continental water load estimates derived from the Gravity Recovery and Climate Experiment and compared to 3D deformation estimated from GPS observations. The agreement is very good in areas where large hydrologic signals occur over broad spatial scales, with correlation in horizontal components as high as 0.9. Agreement is also observed at smaller scales, including across Europe. This suggests that: a) both techniques are perhaps more accurate than previously thought and b) a large percentage of the non‐linear variations seen in our GPS time series are most likely related to geophysical processes rather than analysis error. Low correlation at some sites suggests that local processes or site specific analysis errors dominate the GPS deformation estimates rather than the broad‐scale hydrologic signals detected by GRACE.

Changes in hydrologic surface loads, glacier mass balance, and glacial isostatic adjustment (GIA) have been observed using data from the Gravity Recovery and Climate Experiment (GRACE) mission. In some cases, the estimates have been made by calculating a combination of the linear rate of change of the time series and periodic seasonal variations of GRACE estimates, yet the geophysical phenomena are often not stationary in nature or are dominated by other nonstationary signals. We investigate the variation in linear rate estimates that arise when selecting different time intervals of GRACE solutions and show that more accurate estimates of stationary signals such as GIA can be obtained after the removal of model‐based hydrologic effects. We focus on North America, where numerical hydrological models exist, and East Antarctica, where such models are not readily available. The root mean square of vertical velocities in North America are reduced by ∼20% in a comparison of GRACE‐ and GPS‐derived uplift rates when the GRACE products are corrected for hydrological effects using the GLDAS model. The correlation between the rate estimates of the two techniques increases from 0.58 to 0.73. While acknowledging that the GLDAS model does not model all aspects of the hydrological cycle, it is sufficiently accurate to demonstrate the importance of accounting for hydrological effects before estimating linear trends from GRACE signals. We also show from a comparison of predicted GIA models and observed GPS uplift rates that the positive anomaly seen in Enderby Land, East Antarctica, is not a stationary signal related to GIA.