VLBI data, acquisition, environmental effects

During this quadrennium, very long baseline interferometry (VLBI) data acquisition and system development has focused on improving the accuracy of the system sufficiently to allow the determination of reliable estimates of height variations. Associated with this aim has been improvements in the determinations of horizontal velocity fields, monitoring water vapor delay using interferometric methods, and improvements to Earth rotation measurements. The primary aims of the improvements to height measurement accuracy have been to directly measure the contemporary magnitudes of post glacial rebound, and to determine a height reference system for measuring global sea level rise. High frequency Earth rotation studies have been carried out to better define the transformation parameters from an inertial coordinate system to an Earth fixed one, and to better understand the coupling between the components of the atmosphere-ocean-solid Earth system. Two major VLBI campaigns were carried out in support of these studies: (1) Epoch-92 in July 1992 and (2) Cont-94 in January 1994. Each of these campaigns lasted approximately two weeks and involved multiple VLBI networks operating simultaneously in addition to other space geodetic systems operating during these periods. Two major compilations of the VLBI results (and results from other space geodetic systems) have been published during this quadrennium

The Algorithm Theoretical Basis Document for the Atmospheric Delay Correction to GLAS Laser Altimeter Ranges

NASA s Ice, Cloud, and Land Elevation Satellite (ICESat) mission will be launched late 2001. It s primary instrument is the Geoscience Laser Altimeter System (GLAS) instrument. The main purpose of this instrument is to measure elevation changes of the Greenland and Antarctic icesheets. To accurately measure the ranges it is necessary to correct for the atmospheric delay of the laser pulses. The atmospheric delay depends on the integral of the refractive index along the path that the laser pulse travels through the atmosphere. The refractive index of air at optical wavelengths is a function of density and molecular composition. For ray paths near zenith and closed form equations for the refractivity, the atmospheric delay can be shown to be directly related to surface pressure and total column precipitable water vapor. For ray paths off zenith a mapping function relates the delay to the zenith delay. The closed form equations for refractivity recommended by the International Union of Geodesy and Geophysics (IUGG) are optimized for ground based geodesy techniques and in the next section we will consider whether these equations are suitable for satellite laser altimetry

Surface Deformation Caused by Pressure Changes in the Fluid Core

Pressure load Love numbers are presented for the mantle deformation induced by the variation of the pressure field at the core mantle boundary (CNB). We find that the CMB geostrophic pressure fields, derived from 'frozen-flux' core surface flow estimates at epochs 1965 and 1975, produce a relative radial velocity (RRV) field in the range of 3mm/decade with uplift near the equator and subsidence near the poles. The contribution of this mechanism to the change in the length of day (l.o.d) is small --- about 2.3 x 10(exp -2) ms/decade. The contribution to the time variation of the ellipticity coefficient is more important --- -1.3 x 10(exp -11)/yr

Examination of the kinematic structures in İzmir (Western Anatolia) with repeated GPS observations (2009, 2010 and 2011)

The Western Anatolia and the Aegean Sea regions are one of the most significant seismically active and rapidly deforming fields in the world. Generally, seismic activities cause deformations and these deformations are monitored with Global Positioning System (GPS) /Global Navigation System (GNSS). In this context, GPS data were used to determine the deformation of İzmir and its surrounding to estimate the relative plate motions. In this study, the kinematic structures of the faults, which control the seismic hazard in İzmir and its surroundings, processing results of the three-year (2009, 2010 and 2011) episodic GPS observations and the estimation of displacements for 21 GPS stations were presented. The aim of this study is to examine interplate motion of the stations and their relations with the tectonic structures, seismicity and paleomagnetism and additionally, to interprete the motions of the study area relative to different block motions. Consequently, the mean motion of the study area was found approximately 25 mm/yr (towards the SSW) in the Eurasia fixed frame solution. The Aegean block fixed frame and the Anatolian block fixed frame solutions were computed relative to Euler vectors. In Aegean and Anatolian block solutions it was determined that the stations move separately, not as a group. In Euler pole solution, some stations are separated from each other and meanwhile some stations are grouped by considering the differences and similarities of the station motions. According to this solution three lines and two regions were described in the study area. The relations between seismicity and paleomagnetic studies and the kinematic structures determined in Anatolian block fixed frame and Euler pole solution were also investigated. When the Anatolian block fixed frame solution and the earthquakes occurred between the years 1973 and 2011 were evaluated together, it was found that in the high seismically active region especially near to Sığacık bay, the motions of GPS stations were different even though their locations were close to each other. As a result of this, the relationship between the vector directions and active tectonism was determined. Additionally, in the Euler pole solution directions of the motion were found to be coherent with the paleomagnetic results, particularly in Urla and its surroundings. Here, the block fixed frame and Euler pole solutions and additionally, relations of them with seismicity and tectonism were mentioned as difference from previous studies. Besides, in this study, high importance was given to locate each station in main geological formations of the study area. Keywords: GPS/GNSS; Western Anatolia; Izmir; Tectonic feature

Multiple aperture INSAR (MAI) with C-band and L-band data: Noise and precision

MAI is a technique to extract along-track (horizontal) phase-based displacements from InSAR data. MAI’s theoretical precision can be at the centimeter level, an order of magnitude improvement over amplitude-based pixel offset approaches. However, MAI has been challenging to implement with most academic InSAR processors, and the theoretical precision difficult to reach for low to medium coherence terrains. We implement MAI with the JPL/CALTECH InSAR processor ROI PAC. We study the MAI noise structure with Envisat, Radarsat-1, ERS, and ALOS data, and develop phase corrections and filtering based on the results. We study the MAI noise with ’zero’ signal, all noise data. We process 11 Envisat pairs presenting low to medium coherence with less than 2 cm along track displacements, taken over the larger Los Angeles basin/San Gabriel Mountains in California, US. The test data contain a variety of decorrelation sources and cover different types of terrain, including urban, mountainous, vegetated and sea surfaces, as well as variety in temporal and spatial baselines. To test the MAI filter we superimpose the MAI noise images with signal simulating coseismic displacements from the 1812 Mw 7 Wrightwood earthquake sequence. The results present a correlation dependent RMSE ranging from 8 cm in correlation coefficient 0.4 to 2 cm in correlation coefficient 0.75. In an actual signal case (Hawaii, L-band), the random component of the noise for correlation coefficient of 0.5 to 0.95 varies from 2 cm to 4 mm.Shell Oil Company (Postdoctoral Fellowship

Report of the panel on earth rotation and reference frames, section 7

Objectives and requirements for Earth rotation and reference frame studies in the 1990s are discussed. The objectives are to observe and understand interactions of air and water with the rotational dynamics of the Earth, the effects of the Earth's crust and mantle on the dynamics and excitation of Earth rotation variations over time scales of hours to centuries, and the effects of the Earth's core on the rotational dynamics and the excitation of Earth rotation variations over time scales of a year or longer. Another objective is to establish, refine and maintain terrestrial and celestrial reference frames. Requirements include improvements in observations and analysis, improvements in celestial and terrestrial reference frames and reference frame connections, and improved observations of crustal motion and mass redistribution on the Earth

La culture du vanillier

We use observations of ice sheet surface motion from a Global Positioning System network operating from 2006 to 2014 around North Lake in west Greenland to investigate the dynamical response of the Greenland Ice Sheet's ablation area to interannual variability in surface melting. We find no statistically significant relationship between runoff season characteristics and ice flow velocities within a given year or season. Over the 7 year time series, annual velocities at North Lake decrease at an average rate of −0.9 ± 1.1 m yr−2, consistent with the negative trend in annual velocities observed in neighboring regions over recent decades. We find that net runoff integrated over several preceding years has a negative correlation with annual velocities, similar to findings from the two other available decadal records of ice velocity in western Greenland. However, we argue that this correlation is not necessarily evidence for a direct hydrologic mechanism acting on the timescale of multiple years but could be a statistical construct. Finally, we stress that neither the decadal slowdown trend nor the negative correlation between velocity and integrated runoff is predicted by current ice-sheet models, underscoring that these models do not yet capture all the relevant feedbacks between runoff and ice dynamics needed to predict long-term trends in ice sheet flow

Plate Boundary Observatory and related networks: GPS data analysis methods and geodetic products

The Geodesy Advancing Geosciences and EarthScope (GAGE) Facility Global Positioning System (GPS) Data Analysis Centers produce position time series, velocities, and other parameters for approximately 2000 continuously operating GPS receivers spanning a quadrant of Earth’s surface encompassing the high Arctic, North America, and Caribbean. The purpose of this review is to document the methodology for generating station positions and their evolution over time and to describe the requisite trade-offs involved with combination of results. GAGE GPS analysis involves formal merging within a Kalman filter of two independent, loosely constrained solutions: one is based on precise point positioning produced with the GIPSY/OASIS software at Central Washington University and the other is a network solution based on phase and range double-differencing produced with the GAMIT software at New Mexico Institute of Mining and Technology. The primary products generated are the position time series that show motions relative to a North America reference frame and secular motions of the stations represented in the velocity field. The position time series themselves contain a multitude of signals in addition to the secular motions. Coseismic and postseismic signals, seasonal signals from hydrology, and transient events, some understood and others not yet fully explained, are all evident in the time series and ready for further analysis and interpretation. We explore the impact of analysis assumptions on the reference frame realization and on the final solutions, and we compare within the GAGE solutions and with others

Reservoir characterization in an underground gas storage field using joint inversion of flow and geodetic data

Characterization of reservoir properties like porosity and permeability in reservoir models typically relies on history matching of production data, well pressure data, and possibly other fluid-dynamical data. Calibrated (history-matched) reservoir models are then used for forecasting production and designing effective strategies for improved oil and gas recovery. Here, we perform assimilation of both flow and deformation data for joint inversion of reservoir properties. Given the coupled nature of subsurface flow and deformation processes, joint inversion requires efficient simulation tools of coupled reservoir flow and mechanical deformation. We apply our coupled simulation tool to a real underground gas storage field in Italy. We simulate the initial gas production period and several decades of seasonal natural gas storage and production. We perform a probabilistic estimation of rock properties by joint inversion of ground deformation data from geodetic measurements and fluid flow data from wells. Using an efficient implementation of the ensemble smoother as the estimator and our coupled multiphase flow and geomechanics simulator as the forward model, we show that incorporating deformation data leads to a significant reduction of uncertainty in the prior distributions of rock properties such as porosity, permeability, and pore compressibility.Eni S.p.A. (Firm

py4DSTEM: a software package for multimodal analysis of four-dimensional scanning transmission electron microscopy datasets

Scanning transmission electron microscopy (STEM) allows for imaging, diffraction, and spectroscopy of materials on length scales ranging from microns to atoms. By using a high-speed, direct electron detector, it is now possible to record a full 2D image of the diffracted electron beam at each probe position, typically a 2D grid of probe positions. These 4D-STEM datasets are rich in information, including signatures of the local structure, orientation, deformation, electromagnetic fields and other sample-dependent properties. However, extracting this information requires complex analysis pipelines, from data wrangling to calibration to analysis to visualization, all while maintaining robustness against imaging distortions and artifacts. In this paper, we present py4DSTEM, an analysis toolkit for measuring material properties from 4D-STEM datasets, written in the Python language and released with an open source license. We describe the algorithmic steps for dataset calibration and various 4D-STEM property measurements in detail, and present results from several experimental datasets. We have also implemented a simple and universal file format appropriate for electron microscopy data in py4DSTEM, which uses the open source HDF5 standard. We hope this tool will benefit the research community, helps to move the developing standards for data and computational methods in electron microscopy, and invite the community to contribute to this ongoing, fully open-source project