Methods for removal of unwanted signals from gravity time-series : comparison using linear techniques complemented with analysis of system dynamics

We thanks the participants of the 35th General Assembly of the European Seismological Commission for comments on preliminary results. The authors are grateful to all IGETS contributors, particularly to the station operators and to ISDC/GFZ-Potsdam for providing the original gravity data used in this study. We also thank the developers of ATLANTIDA3.1 and UTide. Part of this work was performed using the ICSMB High Performance Computing Cluster, University of Aberdeen. We also thanks M. Thiel and A. Moura for reviewing a preliminary version and making comments on the methods section and M.A. Ara´ujo for comments on Lyapunov exponents. Funding: A. Valencio is supported by CNPq, Brazil [206246/2014-5]; and received a travel grant from the School of Natural and Computing Sciences, University of Aberdeen [PO2073498], for a presentation including preliminary results.Peer reviewedPostprintPublisher PD

Análisis de serie de datos de gravedad de larga duración en Europa : consecuencias para el estudio de señales de pequeña amplitud y baja frecuencia incluyendo los efectos de resonancia del núcleo terrestre

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Matemáticas, Sección Departamental de Astronomía y Geodesía, leída el 24-04-2015El registro de las variaciones temporales de la gravedad tiene una larga tradición en Europa. En Estrasburgo el primer gravímetro adquirido con el propósito de observar dichas variaciones fue instalado en 1954. Desde entonces 8 modelos diferentes de gravímetros gravímetros de muelle, superconductores SG y gravímetros absolutos han estado registrando en períodos consecutivos. Durante estas seis décadas, los sensores, los sistemas de adquisición y las técnicas de análisis han mejorado drásticamente. Utilizando todas las series del observatorio J9 de Estrasburgo para verificar estas mejoras, se concluye que la precisión ha sido incrementada más de 10 veces con respecto a los primeros modelos de gravímetros de muelle utilizados. Posteriormente se utilizan varias de las series de datos de gravedad de mayor longitud registradas en Europa con diferentes modelos de gravímetros de muelle en BFO Alemania, 1980,2012, Walferdange Luxemburgo, 1980,1995 y Potsdam Alemania, 1974,1998, y varias estaciones de SGs con al menos 9 años de registro pertenecientes todas ellas a la red mundial GGP, Bad Homburg, Bruselas, Medicina, Membach, Moxa, Viena, Wettzell y J9, para poder comparar la sensibilidad de los diferentes tipos de instrumentos a través de la evolución temporal de los factores gravimétricos amplitud y desfase de las principales ondas de mareas diurnas y semidiurnas O1, P1, K1, M2, S2 y K2 y de la relación entre las amplitudes M2 O1 esta relación al ser independiente de la calibración instrumental es un muy buen indicador de la estabilidad del instrumento. Aparecen diversas variaciones temporales en los factores de amplitud de todas las series, siendo muy similares para casi todas las series de SGs, con una estabilidad entre el 0,03 y el 0,3 por ciento. Los posibles orígenes de estas variaciones son estudiados, para ello se analizan las correlaciones con efectos geofísicos, con el ruido instrumental y ambiental, etc. Y se lleva a cabo un estudio detallado de la estabilidad del factor de calibración del SG instalado en J9, a través de los numerosos experimentos de calibración realizados mediante el registro en paralelo de un gravímetro absoluto.Teniendo en cuenta todos estos resultados, se profundiza en los principales beneficios obtenidos al utilizar registros muy largos de SGs a la hora de realizar un análisis de mareas o un análisis espectral. Utilizando la serie de 27 años de los SGs instalados en J9, que es la serie más larga a nivel mundial, su longitud nos permite separar las contribuciones correspondientes a ondas con frecuencias muy cercanas, detectar señales muy débiles especialmente ondas derivadas del potencial de marea de grado 3 y detectar efectos con frecuencia muy bajas.En la última parte, después de revisar la historia de las primeras observaciones de la resonancia de la FCN utilizando datos de gravedad, se estiman los valores de sus parámetros Q y TFCN utilizando todas las series de J9 y del resto de las estaciones de SGs. Los valores obtenidos son muy cercanos a los valores estimados tanto mediante cálculos teóricos como a partir de observaciones de VLBI. También se estudia otro modo de rotación, el denominado FICN cuyos efectos no han sido nunca observados en los datos de gravedad, y del que los cálculos teóricos predicen un efecto muy pequeño de resonancia en las mareas terrestres. Para intentar observar alguna consecuencia derivada del FICN se desarrolla una metodología que nos permite ir limitando el rango de frecuencias de estudio, a través del análisis de mareas detallado en la banda de frecuencia diurna, separando ondas de amplitudes tan pequeñas que nunca antes habían sido observadas y que pudieran estar lo suficientemente próximas a la frecuencia asociada a la FCIN como para verse afectadas en términos de amplitud. Un posible efecto de resonancia aparece próximo a 1,00198 cpd 0,99924 CpsD, que correspondería a un período de alrededor de 1.300 días sid.Unidad Deptal. de Astronomía y GeodesiaFac. de Ciencias MatemáticasTRUEunpu

Filtering methods to extract the tide height from Global Navigation Satellite Systems (GNSS) signals for Hydrographic applications

Hydrographic surveys have traditionally relied on the availability of tide information for the reduction of sounding observations to a common (chart) datum usually related to a specific tide level. In most cases, tide information is obtained from tide gauge observations and/or tide predictions using local, regional or global tide models. An emerging method that is increasingly being used is based on Global Navigation Satellite System (GNSS) positioning of the sea surface. This study assesses the suitability of two types of filtering methods to extract the tide signal from GNSS height observations of the water level. The Savitzky-Golay and Gaussian low-pass filters were implemented to eliminate high-frequency components due to waves, dynamic draft and measurement uncertainties. Over a 30-day period, GNSS heights were estimated from GPS and GLONASS carrier phase data collected by a Fugro Starpack GNSS receiver installed on a floating pontoon at Hillarys Boat Harbor, located in Western Australia. Sea level heights recorded by a traditional tide gauge at Hillarys were used as a reference to evaluate the effectiveness of using GNSS water level heights in extracting the tide signal. To assess the filters performance for a range of window lengths, amplitudes and phases of the four major tidal harmonic constituents (M2, S2, K1, and O1) were determined and compared for the unfiltered and filtered water level signals. The study found a high degree of agreement between the unfiltered tide information obtained by GNSS and the tide gauge documented by almost identical amplitudes of the tidal harmonic constituents. There was a correlation coefficient of up to 0.98 and an RMS value of 0.0325 m for the differences satisfying specifications of the International Hydrographic Organization (IHO) for Special Order hydrographic surveys. As a main finding regarding the filter performance, this study shows that the Savitzky-Golay filter suffers considerably less from signal loss (damping of the tidal harmonic constituents) when compared to the Gaussian filter

On the reliability of the geomagnetic quake as a short time earthquake's precursor for the Sofia region

International audienceThe local "when" for earthquake prediction is based on the connection between geomagnetic "quakes" and the next incoming minimum or maximum of tidal gravitational potential. The probability time window for the predicted earthquake is for the tidal minimum approximately ±1 day and for the maximum ±2 days. The preliminary statistic estimation on the basis of distribution of the time difference between occurred and predicted earthquakes for the period 2002-2003 for the Sofia region is given. The possibility for creating a local "when, where" earthquake research and prediction NETWORK is based on the accurate monitoring of the electromagnetic field with special space and time scales under, on and over the Earth's surface. The periodically upgraded information from seismic hazard maps and other standard geodetic information, as well as other precursory information, is essential

Volcanic signatures in time gravity variations during the volcanic unrest on El Hierro (Canary Islands)

Gravity changes occurring during the initial stage of the 2011–2012 El Hierro submarine eruption are interpreted in terms of the pre-eruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LCR gPhone-054. On September 15, 2011, an observed gravity decrease of 45 μGal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on October 10, 2011. High-frequency gravity signals also appeared on October 6–11, 2011, point to an interaction between a magmatic intrusion and the ocean floor was occurring. These important gravity changes, with amplitudes varying from 10 to −90 μGal, during the first three days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.Projects CGL2011-25494 and CGL2011-16144-E of the Spanish Ministry of Economy and Competitiveness and European Commission VULMAC-MAC/2.3/A7 (INTERREG) and VUELCO (FT7 Theme: ENV.2011.1.3.3-1; Grant 282759)Peer reviewe

Analysis of long-term gravity records in Europe. Consequences for the retrieval of small amplitude and low frequency signals including the Earth's core resonance effects

Tesis defendida el 24 de Abril de 2015 en la Universidad Complutense de Madrid.Temporal gravity variation measurements have been a long historical tradition in Central Europe, with some stations recording for decades. From the 80s, time varying gravity is permanently recorded at the Earth’s surface by a worldwide network of superconducting relative gravimeters within the Global Geodynamics Project of the International Association of Geodesy. In one of these stations, located in Strasbourg since the 1970s, the three main gravimeter types (relative spring gravimeter, relative superconducting gravimeter, and absolute gravimeter) have been set up. We use all these series to review the instrumental betterments. Studying the different improvements on gravimeters in the last years, mainly in terms of long term stability and instrumental drift, we show that the superconducting gravimeters can uniquely contribute to the study of the low frequency Earth's tides and small amplitudes waves. Also, the stability of the scale factor of the superconducting gravimeters is studied with the help of numerous calibration experiments carried out by collocated absolute measurements at Strasbourg Observatory. Finally, after estimating the values of the Free Core Nutation parameters, we search for the rotational normal mode called Free Inner Core Nutation (FICN), the gravity effect of which has never been observed before. For this purpose we develop a methodology to constrain the possible frequency range, through the detailed tidal analysis in the diurnal frequency band, using the 27-year superconducting gravity series recorded at J9 observatory, to separate small amplitude waves that have never been studied before, and which could be close enough to the frequency range of the FICN to be affected in terms of resonant amplitude. This work contributes to show the importance of not only the length, but also the quality of the data series to improve our knowledge of the Earth’s dynamics.Peer reviewe

Development and application of operational techniques for the inventory and monitoring of resources and uses for the Texas coastal zone

The author has identified the following significant results. Four LANDSAT scenes were analyzed for the Harbor Island area test sites to produce land cover and land use maps using both image interpretation and computer-assisted techniques. When evaluated against aerial photography, the mean accuracy for three scenes was 84% for the image interpretation product and 62% for the computer-assisted classification maps. Analysis of the fourth scene was not completed using the image interpretation technique, because of poor quality, false color composite, but was available from the computer technique. Preliminary results indicate that these LANDSAT products can be applied to a variety of planning and management activities in the Texas coastal zone