On the Use of Repeat Leveling for the Determination of Vertical Land Motion: Artifacts, Aliasing, and Extrapolation Errors

Abstract

Leveling remains the most precise technique for measuring changes in heights. However, for the purposes of determining vertical land motion (VLM), a time series of repeat leveling measurements is susceptible to artifacts and aliasing that may arise due to systematic errors, seasonal surface fluctuations, motions occurring during a survey, and any inconsistencies in the observation conditions among epochs. Using measurements from 10 repeat leveling surveys conducted twice yearly along a profile spanning ~40 km across the Perth Basin, Western Australia, we describe the observation, processing, and analysis methods required to mitigate these potential error sources. We also demonstrate how these issues may lead to misinterpretation of the VLM derived from repeat leveling and may contribute to discrepancies between geologically inferred rates of ground motion or those derived from other geodetic measurement techniques. Finally, we employ historical (~40‐year‐old) leveling data in order to highlight the errors that can arise when attempting to extrapolate VLM derived from a geodetic time series, particularly in cases where the long‐term motion may be nonlinear.

Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia

“…This includes the initial period of Sentinel-1 operations, during which imagery was acquired at 24-day intervals for a 9-month period between August 2015 and May 2016 [40]. The results of these datasets are saturated by atmospheric noise [39] or capture only seasonal displacements, which are shown from levelling to be cm-scale (e.g., [41]). An assessment of a longer InSAR time-series is required to determine the recent rate of longer-term (multi-year) subsidence in Perth [40], which from levelling surveys is expected to be <10 mm/yr [37,41].…”

“…The results of these datasets are saturated by atmospheric noise [39] or capture only seasonal displacements, which are shown from levelling to be cm-scale (e.g., [41]). An assessment of a longer InSAR time-series is required to determine the recent rate of longer-term (multi-year) subsidence in Perth [40], which from levelling surveys is expected to be <10 mm/yr [37,41]. These data are now available from Sentinel-1, which has provided imagery at 12-day intervals since 2016.…”

Potentially Misleading GPS Leveling–Based Assessment of Gravimetric Geoid or Quasigeoid Models due to Vertical Land Motion and Different GPS Processing Software

“…Prime candidates were the proximity to the coast, where altimeter-derived and ship-track gravity anomalies are poor (e.g., Vignudelli et al, 2011;Featherstone, 2009), and the large mass-density contrast at the near-vertical Darling Fault that generates a very steep gravity gradient (e.g., Middleton et al, 1993). However, another candidate explanation for the tilt could be land subsidence in the Perth Basin (Featherstone et al, 2015;Parker et al, 2017a;Lyon et al, 2018), which will be investigated here. We will also investigate the use of different GPS processing software, as different packages can give different results.…”

“…The digital astrogeodetic traverse was co-located with a repeat levelling traverse that has been used to quantify vertical land motion (VLM) associated with groundwater abstraction from the Perth Basin. The land is subsiding at approximately 3 mm/yr, with a superposed seasonal signal of a few millimetres (Lyon et al, 2018). This is compounded by non-linear subsidence when groundwater abstraction rates change (Featherstone et al, 2015) and localised larger rates of subsidence associated with wetlands (Parker et al, 2017a).…”

Geodynamics

“…al, 2006;Szczerbowski, 2009;Ji, & Herring, 2012]. Based on the repeated determination of benchmark heights along a 40 km long profile in Western Australia, it has been shown that seasonal oscillation information for benchmarks is necessary to obtain reliable data on the velocity of vertical movements of the Earth's surface, especially in the case of a short interval between repeated leveling [Lyon, et al, 2018]. Only 40% of the actual magnitude of the annual vertical oscillations obtained from continuous GPS observations can be explained by the influence of known factors such as ocean (tidal and non-tidal), atmospheric and hydrological loads, tidal pole motion [Dong et al, 2002].…”