15 research outputs found
Empleo de agrupaciones estadÃsticas en los estudios geodinámicos multiescala
El conocimiento de la cinemática de la corteza terrestre puede estudiarse con precisión mediante técnicas que determinen los desplazamientos superficiales, siempre con un periodo de tiempo mÃnimo de 5 años, lo que provoca trabajar con un número ingente de datos. La investigación desarrollada en esta Tesis Doctoral se ha centrado en buscar técnicas que ayuden a discriminar comportamientos geodinámicos en las diferentes escalas de estudio, presentando tres trabajos según:
i) Un enfoque global-regional de la región de la Macaronesia: empleando estaciones GNSS continuas en un periodo temporal de 15 años. Con la ayuda de las agrupaciones de datos y los mapas de esfuerzo/cizalla se identificaron claramente los lÃmites tectónicos de la zona denominada Macaronesia, validando su uso global, asà como apoyar con estos resultados, la posible existencia de una zona de corte intraplaca entre las islas Canarias y la PenÃnsula Ibérica.
ii) Una visión regional-local de la isla de Tenerife (Islas Canarias): durante 10 años de observaciones en estaciones GNSS continuas y episódicas por campañas, analizados nuevamente por las mismas metodologÃas mencionadas en i). El análisis de conglomerados identificó valores locales anómalos en una estación y aclaró, junto a los resultados del análisis de esfuerzos, la identificación de dos dinámicas regionales en la isla. Asimismo, la tendencia direccional general de los residuales, respecto a la estación de referencia de Gran Canaria, hizo posible ver, por primera vez, el efecto de la dinámica regional de una zona de corte con gran sismicidad entre ambas islas, que parece indicar el acercamiento de estas islas.
iii) Un último enfoque que emplea de nuevo la escala regional-local en Tenerife: pero centrado en esta ocasión en la dinámica vertical, empleando datos derivados de técnicas de interferometrÃa con Radar de Apertura Sintética (SAR) durante 6 años, obteniendo agrupaciones de anomalÃas cinemáticas en la isla según el estadÃstico geoespacial Gi* de Getis-Ord. Se obtuvieron delimitaciones locales con valores anómalos para la cinemática vertical que mostraron los efectos derivados de la sobreexplotación de los recursos hÃdricos subterráneos en toda la isla, sin encontrar ningún efecto regional relacionados con la actividad volcánica de la isla.
En todos los casos propuestos se han empleado distintas técnicas de agrupación de datos que han ayudado a mejorar la interpretación de los resultados derivados del tratamiento de grandes cantidades de datos de deformación terrestre, vinculados a las tecnologÃas espaciales GNSS y geoespaciales procedentes de la interferometrÃa radar multitemporal
Remote Sensing of the Aquatic Environments
The book highlights recent research efforts in the monitoring of aquatic districts with remote sensing observations and proximal sensing technology integrated with laboratory measurements. Optical satellite imagery gathered at spatial resolutions down to few meters has been used for quantitative estimations of harmful algal bloom extent and Chl-a mapping, as well as winds and currents from SAR acquisitions. The knowledge and understanding gained from this book can be used for the sustainable management of bodies of water across our planet
The gulf of cadiz as a natural laboratory for the validation and exploitation of coastal altimetry and model data
Hydrodynamic models and satellite altimetry can be complementary tools for the study of
sea level variations. Nowadays, users of these tools demand high quality products in coastal
zones. In this sense, this doctoral dissertation focusses on the validation of innovative
products that entail an advance in the understanding of sea level variation in coastal areas.
The study was carried out in the Gulf of Cadiz (GoC), an important region that connects
the Atlantic Ocean and the Mediterranean Sea, although other study areas were also used to
strengthen the analysis. The research was performed using: in-situ data, sea level altimetry
measurements from Sentinel-3 SRAL, and observations from a hydrodynamic model
implemented in the main study area. The in-situ data were used to validate the altimetry
measurements, as well as to calibrate and validate the numerical model.
Different specific objectives were proposed. The first was to determine the quality of
altimetric data in coastal areas from the new Sentinel-3 space mission. Altimetry data of
Sentinel-3A SRAL (S3A) were validated at the sampling frequency of 80 Hz. The data were
obtained from the European Space Agency (ESA) Grid Processing On Demand (GPOD)
service over three coastal sites in Spain: Huelva (GoC), Barcelona (Western Mediterranean
Sea), and Bilbao (Bay of Biscay). Two tracks were selected at each site: one ascending and
one descending. Data were validated using in-situ tide gauge (TG) data provided by the
Spanish Puertos del Estado. The altimetry Sea Level Anomaly (SLA) time series were
obtained using the corrections available in GPOD. The validation was performed using
two statistical parameters, the Pearson correlation coefficient (r) and the root mean square
error (rmse). In the 5–20 km segment with respect to the coastline, the results obtained
were 6–8 cm (rmse) and 0.7–0.8 (r) for all of the tracks. The 0–5 km segment was also
analysed in detail to study the effect of land on the quality of altimetry data. Results showed
that the track orientation, the angle of intersection with the coast, and the land topography
concur to determine the nearest distance to the coast at which the data retain a similar level
of accuracy than in the 5–20 km segment. This ‗distance of good quality‘ to shore reaches a
minimum of 3 km for the tracks at Huelva and the descending track at Barcelona.
In addition, altimetry sea level data of Sentinel-3A and Sentinel-3B SRAL (S3A and S3B)
were also validated at the sampling frequency of 80 Hz. Two tracks of S3A and two of S3B
were selected at seven different coasts around the Iberian Peninsula. The altimetry SLA
time series obtained were compared with TG in-situ data using the standard deviation of the difference (sdd) and the normalized sdd (sdd_n). Two tidal models were used: TPXO8
and TPXO9. They were previously validated with in-situ data and then used in the S3
assessment. Contrary to expectations, a more current version of the tide model did not
always lead to improvements for all of the coasts studied. The same data availability and
accuracy results (mean sdd <9cm) were obtained for both satellites, as the radar altimeter
on-board S3A and S3B are identical instruments. The sdd_n results were generally ranged
between 0.1 and 0.25 cm, higher values were obtained in coastal areas with a complex
hydrodynamic.
The second specific objective was to implement the Delft3D model in the estuary of the
Guadalquivir River and part of the GoC continental shelf with the aim of studying the
influence of its discharges on the sea level variability. Details of the Delft3D FLOW
module implementation are given in the manuscript. The model was calibrated and
validated along the river estuary comparing the output with in-situ observations of water
level and current velocities during normal and high-discharge events. In order to obtain
the best possible adjustment, the friction coefficient and bathymetry were used as
adjustment parameters. The determination coefficients attained mean values of R2= 0.9
and R2=0.8, for calibration and validation, respectively. Moreover, the model was
calibrated for the continental shelf during normal discharge conditions using data from
three current meters, obtaining good correlation results (rmse= 3.0 cm·s
-1
and R2=0.7, on
average). The model simulations were able to reproduce the low salinity plume-induced
over-elevations at the mouth of the estuary and its adjacent inner shelf during periods of
high river discharge from the head dam (> 400 m3
·s
-1
). These over-elevations were also
identified in a qualitative comparison with altimetry data. Despite the good results
obtained, there are some improvements that could be made for future work: including
wind, coupling the wave module, updating the bathymetry, etc.
The aim of this thesis last section was to apply the new Fully Focused SAR (FF SAR)
processing technique for the Sentinel-3 altimetry signal. The accuracy and precision of this
novel product were analysed in order to provide the best quality product close to the coast
(0-5 km track segment). FF SAR processing is similar to SAR altimetry, but with an
unprecedented high along-track resolution which goes up to the theoretical limit equal to
half the antenna length (~0.5 m). Two FF SAR algorithms still in development were used
in this work: FF SAR Back Projection (BP) (S3 prototype version of Kleinherenbrink et al., 2020), and FF SAR Omega-Kappa (WK) (Guccione et al., 2018), as well as different
retracking algorithms. Two tracks from Sentinel-3A and two from Sentinel-3B were
processed, at 80 Hz. The products were validated by comparing time series of SLA with
those obtained from a tide gauge in the Gulf of Cadiz. The accuracy was analysed using the
Percentage of Cycles for High Correlation (PCHC) and the standard deviation of the
difference (sdd); and the precision was determined by calculating the along-track noise. FF
SAR and unfocused SAR products were compared. The results showed improvements
using Adaptative Leading Edge Subwaveform (ALES+) retracker for unfocused SAR,
although FF SAR BP with ALES+ was the most precise product for all the tracks. In
addition, highly accurate SLA measurements were also obtained with FF SAR products.
The greatest advantage of FF SAR is that it produces good quality data closer to the coast
(1-2 km) than unfocused SAR (3-4 km).
Finally, these results highlight the potential of the implementation of validated altimetry
data and hydrodynamic models in sea level studies. Furthermore, the methodology
described here can be useful to validate altimetry data, as well as to implement the Delft3D
model in other coastal areas
Validation of CryoSat-2 SIRAL sea level data in the eastern continental shelf of the Gulf of Cadiz (Spain)
We present the validation of sea level measurements taken over the eastern shelf of the Gulf of Cadiz (southwestern Iberian Peninsula) by the European Space Agency CryoSat-2 satellite’s SIRAL altimeter in SAR mode. Time series of sea level anomaly (SLA) at 20-Hz posting rate (corresponding to approximately 350 m along the satellite track) are compared against sea level measurements from two tide gauge stations located along the Spanish coast of the study area. Due to the long repeat cycle of CryoSat-2 (369 days), data selection is performed by including all the tracks falling in a radius of 50 km from the location of the tide gauge stations. Then, time series of SLA are formed during the observational time period (August 2010–December 2014) by taking the altimeter measurements located at growing distances from the coast (1, 3, 5, 10, 15 and 20 km) as well as around four isobaths (10, 25, 50 and 75 m). The validation of AltiKa’s SARAL altimeter 20-Hz data, using its two 35-day-repeat ground tracks available in the area is also presented. Overall, CryoSat-2 20-Hz SLAs selected in the coastal strip from 3 to 20 km agree well with data from the western station (Huelva) with rmse ranging from 11.4 to 6.4 cm. The analysis at the eastern station (Bonanza) yields rmse ranging from 16.8 to 14.6 cm. The comparisons made with CryoSat-2 20-Hz SLAs are in agreement with the rmse estimated using AltiKa 20-Hz SLAs at distances to the coast higher than 5 km, and much better in the 1–5 coastal band. The differences in quantity of SAR mode data observed at the two stations and in their agreement with the tide gauges can be explained by (1) the more complex coastal morphology around Bonanza station with the altimetry tracks almost parallel to the coast; (2) the noisier hydrodynamics around Bonanza station due to the presence of the estuary mouth of the Guadalquivir River; and (3) residual (uncorrected) tidal signals in the Bonanza gauge record. We conclude that the quality of SAR altimetry is comparable or even slightly better than conventional altimetry, thus qualifying SAR data for exploitation in oceanographic studies and sea level monitoring. An example is the analysis made of the surface signal due to a heavy Guadalquivir River discharge event