Gravity,Mean Sea Surface,and Bathymetry Models Comparison in the Canary Islands

In geodetic and oceanographic studies generally, some reference surfaces are needed. These surfaces must represent as much as possible the gravity field of the Earth and the height/bathymetry systems. In the last years, several gravimetric, bathymetric, and mean sea surface models have appeared. Analyzing them it is possible to see that there are significant discrepancies between the models provided by different authors or organizations; there are also differences between the models and data obtained by independent measurements. We present the analysis of such differences and determine the most representative choice of models, in our opinion, for the Canary Islands region.     

Geodetic and Structural Research in La Palma, Canary Islands, Spain: 1992–2007 Results

We review the results of the geodetic and structural studies carried out on La Palma Island using geodetic and geophysical data during the period 1992–2007. InSAR and GPS observation techniques were applied to study the existence of deformation on the island and gravity observations were carried out for structural studies. Gravity data were inverted using a nonlinear three-dimensional gravity inversion approach to obtain the geometry of the anomalous bodies constructed in a random growth process with respect to an exponentially stratified background. The main structural feature is a large central body (under the Caldera de Taburiente) with high density, which was interpreted as the Pliocene-age uplifted seamount and a relatively dense intrusive plutonic complex/magma body. The Cumbre Vieja series is characterized by elongated minima distributed according to the rift structure. InSAR results show a clear subsidence located on the Teneguía volcano, where the last eruption took place in 1971. A thermal source is the most probable origin for this deformation. A GPS network composed of 26 stations covering the total island surface was set up. Vertical displacements determined comparing the GPS coordinates obtained in 2007 with coordinates determined in 1994 are consistent with the InSAR results obtained in the southern part of the island. This is not the case for the northern part. From the comparison of 2006 and 2007 coordinates it is clear that more time is needed to obtain significant displacements, but observed trends are also consistent with InSAR results. All the observed significant displacements are in stations located outside of the large high-density central body.     

GPS Monitoring in the N-W Part of the Volcanic Island of Tenerife,Canaries, Spain: Strategy and Results

This paper describes design, observation methodology, results and interpretation of the GPS surveys conducted in the areas of the N-W of Tenerife where deformation was detected using InSAR. To avoid undesirable antenna positioning errors in the stations built using nails, we designed and used calibrated, fixed-length metal poles, allowing us to guarantee that the GPS antenna was stationed with a height repeatability of the order of 1 mm and of less than 3 millimeters on the horizontal plane. The results demonstrate that this system is ideal for field observation, especially to detect small displacements that might be masked by accidental errors in height measurements or centering when observed with a tripod. When observations were processed, we found that using different antenna models in the same session sometimes causes errors that can lead to rather inaccurate results. We also found that it is advisable to observe one or two stations in all the sessions. The results have reconfirmed the displacement in the Chío deformation zone for the period 1995–2000 and indicate a vertical rebound from 2000 to 2002. They also confirm that the subsidence detected by InSAR to the south of the Garachico village has continued since 2000, although the magnitude of the vertical deformation has increased from around 1 cm to more than 3 cm a year. Detected displacements could be due to groundwater level variation throughout the island. A first attempt of modelling has been made using a simple model. The results indicate that the observed deformation and the groundwater level variation are related in some way. The obtained results are very important because they might affect the design of the geodetic monitoring of volcanic reactivation on the island, which will only be actually useful if it is capable of distinguishing between displacements that might be linked to volcanic activity and those produced by other causes. Even though the study was limited to a given area of Tenerife, in the Canary Islands, some conclusions apply to, and are of general interest in similar geodynamic studies.     

Dependence of Mean Sea Surfaces from Altimeter Data on the Reference Model Used

A mean sea surface model is used as the frame of reference in processing altimeter data. This article focuses on ascertaining the extent to which results depend on the different mean sea surface models used. In particular, we have analyzed the results from the OSU95MSS and the CLS_SHOMv.98.2 models in an area in the North Atlantic Ocean comprising the Canary and Azores Islands. Special attention has been paid to data editing and several criteria were proposed. The amount of detected data is quite small because we used a well corrected data set. However, it was enough to show important relations between the applied criteria and the kind of area. Therefore we analyzed the best way to apply these criteria according to the areas where the points have been found. Singular areas related to several factors have been detected by all the suggested criteria. In particular, rough sea bottom features, dynamic circulation, and amphidromic points of the tidal waves, among others. As a result of this analysis, we have not considered it appropriate to remove all the detected points. Two time-averaged and corrected mean sea surfaces were determined in the test area.     

Time Evolution of Deformation Using Time Series of Differential Interferograms: Application to La Palma Island (Canary Islands)

Differential interferometry is a very powerful tool for detecting changes in the Earth’s crust where coherence conditions are good, but is difficult to employ in some volcanic areas due to dense vegetation. We apply two differential InSAR methods using the time series associated with the interferograms to perform a phase analysis on a data set for La Palma island (Canary Islands) from the ERS-1 and ERS-2 European Space Agency (ESA) satellites for the time period 1992 to 2000. Both methods involve choosing a master image from the database and creating a series of interferograms with respect to this image. The “Coherent Pixel Time Series” (CPTS) technique chooses pixels with good average coherence, aligns the unwrapped interferograms with a stable area and then performs an inversion to calculate the linear velocity to quantify the deformation. The Coherent Target Modeling (CTM) method calculates the temporal coherence of each pixel to identify stable targets and then determines the best velocity for each pixel by using a linear fit that maximizes the temporal coherence. Using these two methods we have been able to detect deformation on La Palma Island that has been previously undetectable by conventional InSAR methods. There is a roughly circular region on the Southern part of the island that is actively deforming at ~ −4 to −8 mm/yr. This region is located near the Teneguia valcano, the host of the last known eruption on La Palma in 1971. A thorough investigation of the possible sources for this deformation revealed that it was most likely created by a subsurface thermal source.     

Gravity, Mean Sea Surface, and Bathymetry Models Comparison in the Canary Islands

In geodetic and oceanographic studies generally, some reference surfaces are needed. These surfaces must represent as much as possible the gravity field of the Earth and the height/bathymetry systems. In the last years, several gravimetric, bathymetric, and mean sea surface models have appeared. Analyzing them it is possible to see that there are significant discrepancies between the models provided by different authors or organizations; there are also differences between the models and data obtained by independent measurements. We present the analysis of such differences and determine the most representative choice of models, in our opinion, for the Canary Islands region.     

Dependence of Mean Sea Surfaces from Altimeter Data on the Reference Model Used

A mean sea surface model is used as the frame of reference in processing altimeter data. This article focuses on ascertaining the extent to which results depend on the different mean sea surface models used. In particular, we have analyzed the results from the OSU95MSS and the CLS_SHOMv.98.2 models in an area in the North Atlantic Ocean comprising the Canary and Azores Islands. Special attention has been paid to data editing and several criteria were proposed. The amount of detected data is quite small because we used a well corrected data set. However, it was enough to show important relations between the applied criteria and the kind of area. Therefore we analyzed the best way to apply these criteria according to the areas where the points have been found. Singular areas related to several factors have been detected by all the suggested criteria. In particular, rough sea bottom features, dynamic circulation, and amphidromic points of the tidal waves, among others. As a result of this analysis, we have not considered it appropriate to remove all the detected points. Two time-averaged and corrected mean sea surfaces were determined in the test area.