FISICA DE LA TIERRA

Servicio de Publicaciones de la Universidad Complutense de Madrid

Volumen 17 (2005)

--------------------------------------------------------------------------------

3.   Contents   

--------------------------------------------------------------------------------
 
5.   Topics in Geodesy  

--------------------------------------------------------------------------------
 
Robust estimation in geodetic networks     
BERNÉ VALERO, José Luis | BASELGA MORENO, Sergio
7 
Palabras Clave: Geodetic networks, robust estimation, global optimization, least-squares.
 
Resumen: The application of robust estimation to geodetic networks is analysed versus the classical least-squares approach. In case of gross or systematic errors appearance either in the mathematical model or in the observations to be adjusted, least-squares estimation along with detection statistical tests over the results present considerable problems for isolating them and avoiding their influence. Conversely, robust estimation provides a maximum-resistance solution and therefore the capability of identifying and quantifying them. Finally, we show the advantages of dealing with robust estimation as a global optimization problem rather than as an iteratively reweighted least squares scheme.
 

--------------------------------------------------------------------------------
 
Establishment of a non-permanent GPS network to monitor deformations in Zafarraya Fault and Sierra Tejeda Antiform (Spain)     
BORQUE ARANCÓN, María Jesús | GALINDO-ZALDÍVAR, Jesús | GIL CRUZ, Antonio J. | JABALOY SÁNCHEZ, Antonio | LACY PÉREZ DE LOS COBOS, M. Clara de | LÓPEZ GARRIDO, Ángel Carlos | RODRÍGUEZ CADEROT, Gracia | RUIZ ARMENTEROS, Antonio | SANZ DE GALDEANO EQUIZA, Carlos
23 
Palabras Clave: Active Tectonics, GPS Network, Zafarraya Fault, Sierra Tejeda Antiform, Betic Cordillera.
 
Resumen: The NW-SE to NNW-SSE convergence between the African and Eurasian plates in the western Mediterranean has developed the recent relief of the Betic-Rif Cordilleras. The central part of the Internal Zones of the Betic Cordilleras is deformed by large open folds and faults, mainly with normal character. The Zafarraya fault, located to the N of the Sierra Tejeda antiform, was active during the 1884 Andalusia earthquake of 6.7 estimated magnitude. In the framework of an interdisciplinary research project, a non-permanent GPS-network has been established at Zafarraya Fault and Sierra Tejeda antiform to monitor deformations related to these active tectonic structures.
 

--------------------------------------------------------------------------------
 
Calibration Altimeter Sites at Cape of Begur and Ibiza Island   
MARTÍNEZ BENJAMÍN, Juan José | MARTÍNEZ GARCÍA, Marina | ORTIZ CASTELLÓN, Miquel Ángel | RODRÍGUEZ VELASCO, Gema | MARTÍN DÁVILA, José | GARATE PASQUIN, Jorge | BONNEFOND, Pascal | PÉREZ GÓMEZ, Begoña | GARCÍA SILVA, Cristina
33 
Palabras Clave: Altimetry, Calibration, Geoid, GPS, Tide gauges, GPS Buoys, Catamaran.
 
Resumen: The three Begur experiments have been conducted on March, 16-19, 1999, which was the first altimeter calibration ever developed in Spain and the first Alt-B altimeter calibration made in the Mediterranean Sea; on July, 4-7, 2000, and on August, 25-28, 2002. Direct absolute altimeter calibration, estimating the TOPEX Alt-B bias, was made from direct overflights using GPS buoys. This method does not require any modelling of geoid and tidal error. Other main objective of the campaigns was to map with GPS buoys the Mean Sea Surface, MSS, along an ascending T/P groundtrack about 15-20 km from the coast, using coastal tide gauge measurements. This method requires geographical mapping of geoid and ocean tides which reduces the accuracy of the bias estimate by a factor of two. Indirect absolute altimeter calibration is possible for any satellite crossing the MSS, with the only requirement that tide gauges are operational during the overflight. In the framework of the JASON-1 CNES/NASA mission, a campaign was conducted on June 9-17, 2003, in the Absolute Calibration Site of the Island of Ibiza. The objective was to determine the local marine geoid slope under the ascending (187) and descending (248) Jason-1 ground tracks, in order to allow a better extrapolation of the open-ocean altimetric data with on-shore tide gauge locations, and thereby improve the overall precision of the calibration process. We present preliminary results on Jason-1 altimeter calibration using the derived marine geoid: from this analysis the altimeter bias is estimated to be 120 ±5 mm.
 

--------------------------------------------------------------------------------
 
Use of Gabor filters for texture classification of digital images    
RECIO RECIO, Jorge A. | RUIZ FERNÁNDEZ, Luis A. | FERNÁNDEZ-SARRIÁ, Alfonso
47 
Palabras Clave: Image classification, Gabor filters, multichannel filtering, texture analysis.
 
Resumen: In this article various methodologies, based on the use of Gabor filters, are described and analysed for the extraction of texture features and the subsequent classification of aerial and satellite digital images. Images of urban, forest and agricultural areas were used, where the complexity of the terrain and the differences in vegetation density require the consideration of the existing texture features as a base for elaborating land use cartography. The use of Gabor filters is driven by the potential they have to isolate texture according to particular frequencies and orientations. The parameters that define a Gabor filter are its frequency, standard deviation and orientation. By varying these parameters, a filter bank is obtained that covers the frequency domain almost completely. Several alternatives have been studied for the application of Gabor filters: (a) the use of complete filter banks; (b) the sum of the filters of equal frequency; and (c) the selection of those filters that minimise, a priori, the classification error. From the application of filters in each of the three methods, a group of images is obtained that allow for the numeric quantification of textures in the image. The evaluation of the classification results shows that combining these textural variables with the multispectral information permits us to characterize the existing regions in the territory with more precision, using supervised digital image classification techniques.
 

--------------------------------------------------------------------------------
 
Analysis of Geoid and Sea Level in the Area of the JASON-1 Calibration Campaign, IBIZA 2003       RODRÍGUEZ VELASCO, G. | SEVILLA, M.J. | MARTÍNEZ BENJAMÍN, J.J.
61 
Palabras Clave: geoid, sea surface topography, GPS, satellite altimetry.
 
Resumen: Actual studies related to calibration of altimeters involves the use of GPS buoys and other systems for the determination of absolute bias in just purely geometric sense. Doing so it seems to be avoided the estimating a marine geoid or the mean sea surface. However, this is not at all true. On the one hand, we need the cross track sea level gradient in order to account the difference in the distance between the altimeter ground track and the position of the point to use in the comparison. On the second hand, an accurate estimation of the surface slope is also needed for linking offshore altimetric data and coastal tide gauges. This is the followed method used to process the Spanish/French JASON-1 calibration campaign, IBIZA 2003. This campaign took place in June 9th-17th, 2003. The area, close to a big island and in a singular place from a dynamic point of view, has a complex local geoid and mean sea surface around. For this reason, in this paper we present some comparisons and correlations between results from this campaign data and some previous results about geoid and mean sea surface in the area, completely independent (in time, and in the kind of data employed: gravimetry or satellite altimetry). The compared surfaces have been some mean sea surface models over the area and local marine geoids, built up from gravimetry and altimetry of ERS ESA satellite, with a higher spatial resolution.
 

--------------------------------------------------------------------------------
 
Non-linear processes in shallow waters. Influence on local observed mean sea level     
GIL, E. | DE TORO, C.
77 
Palabras Clave: Tide gauge observations, ocean tides, mean sea level, nonlinear interactions, shallow water constituents
 
Resumen: A combined theoretical and empirical method is applied to study the nonlinear interactions occurring in shallow waters among astronomical, overtide and compound constituents. Although the isolated nonlinear terms present a small amplitude, the global response of the oceans to the primary and secondary interactions can mean an important contribution to the sea level variability in continental shelf and coasts with a strong tidal signal. Thus, an accurate determination inside these areas is of considerably importance both in mean sea level studies and in tidal prediction. To this purpose, a formal theoretical development of the generating processes has been accomplished. At the same time, the tidal parameters of the most significant shallow water constituents have been empirically determined in six nearby stations located in the Cantabric Sea. The methodology developed for this determination and later elimination of over and compound tides is described in this work. Besides, the simultaneous determination allows us to study the dependence of these non primary waves on the properties of the emplacements. The effectivity of the method has been contrasted through an iterative process based on spectral analysis of the residuals. The application, component by component, of this technique, allows us to minimize the influence of tidal forces on mean sea level, as it is shown in the increased accuracy.
 

--------------------------------------------------------------------------------
 
Computation of surface displacements, tilt and gravity variations due to ocean tide loading     
BENAVENT, M. | ARNOSO, J. | MONTESINOS, F.G.
97 
Palabras Clave: Ocean tide loading, Green’s functions
 
Resumen: This paper deals with the computation of surface displacements, tilt and gravity variations at the Earth’s surface produced by ocean tide loading, through two different methods. The first method consists of the convolution between the ocean tide distribution and the corresponding Green’s functions, following the procedure of Farrell (1972). To avoid the Gibbs effect associated to the truncation of the infinite harmonic series, various asymptotic expressions of the load Love numbers are given. Besides, successive improvements of the Farrell’s method are also revised. The second method is based on the preliminary development of the ocean tides in spherical harmonics. Thereby, expressions for displacements, gravity and tilt variations in terms of the load Love numbers and the spectral amplitudes of the load are obtained.
 

--------------------------------------------------------------------------------
 
New study of the local gravity field of El Hierro (Canary Islands)     
MONTESINOS, F.G. | ARNOSO, J. | BENAVENT, M.
113 
Palabras Clave: Canary Islands, gravity inversion, genetic algorithm, volcanism, optimisation method
 
Resumen: El Hierro island has been built-up at the from a classic triple armed rift system, being its subaerial morphology enhanced by large scale landsliding. The ridges are marked on the Island by frequently aligned faults and volcanic centres. Three volcanic cycles take part in its evolution: Old, Intermediate and Recent Series, which can be clearly separated. The Bouguer anomaly map in this island shows several gravity anomalies which can be associated to the different stages of its growth. Adetailed study, increasing the number of gravity stations and improving previous researches, has let us establish a better interpretation of the local gravity anomalies associated with the recent volcanism in the area. To remove the influence of the deepest sources of the gravity field in this local study, we identify a regional trend by a robust polynomial fit. The residual map is analyzed with statistical techniques. The observational noise is filtered distinguishing the signal corresponding to the local gravity field. The inversion of these data is based on a genetic algorithm which optimizes the model of distribution of gravity sources by emulating the Darwin’s principle about evolution. The geometry of these sources is determined upon a prismatic partition of the subsurface, and adopting a priori values of density contrasts. This local gravity study shows low-density bodies following the concentration of recent emission centres on El Hierro, and they conform the three-armed rift structure regularly developed in this island. The high density structures coincide where the oldest volcanism outcrops, just in the areas which suffered giant landslides.
 

--------------------------------------------------------------------------------
 
Modelling Gravity changes and crustal deformation in active volcanic areas    
CHARCO, Maria | F. TIAMPO, Kristy | LUZÓN, Francisco | FERNÁNDEZ, José
129 
Palabras Clave: Elastic-gravitational model, volcanic source, gravity, displacement and Genetic Algorithm
 
Resumen: Elastic half-space models are widely used to interpret displacements and gravity changes in active volcanic areas. Those models usually compute the displacement response to dilatational sources that simulate a change in pressure of the magma chamber. However, elastic-gravitational model allows one to compute gravity, deformation and potential changes due to pressurized cavities and intruded masses together. First, we interpret deformation and gravity change data in Long Valley caldera, California, by using both a classical elastic and an elastic-gravitational model. Our results show that intruded mass can not be neglected for interpretation of gravity changes while displacements are mainly caused by pressurization. Therefore, the intrusion mass together with the associated pressurization produces distinctive changes in gravity that could be used to interpret gravity changes without ground deformation or viceversa depending on what is the source playing the main role in modelling. Second, we model the source of inflation at Long Valley caldera using a Genetic Algorithm inversion technique and microgravity data (1982-1998). The results of the performed inversions fit gravity anomaly centered under the resurgent dome. The two source inversion suggest that the gravity change could be caused by a more spatially distributed source under the resurgent dome.
 

--------------------------------------------------------------------------------
 
Absolute Gravity Network in Spain    
RODRÍGUEZ PUJOL, E.
147 
Palabras Clave: Absolute Gravity, Absolute gravimeter, Gravity networks, ICAG, Gravity gradient.
 
Resumen: Since the first absolute gravity measurements in Spain (in 1882 by Joaquín Barraquer y Rovira) no absolute measurements were performed till 1989 by J. Makinen, at the Institute of Astronomy and Geodesy of Madrid (IAG), with a Jilag-5 absolute gravimeter. BKG (Bundesamt für Kartographie und Geodäsie) of Germany and IMGC (Metrological Institute G. Colonetti) of Italy have measured six and three sites in Spain in the last decade of twentieth century. The National Geographic Institute of Spain (IGN) purchased the well known FG5 (num. 211) and A-10 (num. 006) absolute gravimeters. In order to have a reference of measurements to other absolute gravimeters three International Comparison of Absolute Gravimeters (ICAG) have been performed: ICAG2001 at the Bureau International des Poids et Mesures (BIPM) in Sèvres, ICSPAB03 at San Pablo de los Montes (Toledo, Spain) between European Center for Geodynamics and Seismology (ECGS) from Luxembourg and IGN from Spain and, finally, Walferdange 2003 International Intercomparison in Luxembourg. Not only absolute but also vertical gradient and relative measurements were also executed at every station for processing and datum reduction on the floor, because nominal height of absolute gravimeters is placed around 1.30 and 0.70 m above ground level. Results of Zero and a First order absolute gravity network will be presented, which hopefully will better the actual accuracies provided by the IGSN71 and RGFE73 and will serve for multidisciplinary purposes.