José Arnoso

VAV: a program for tidal data processing

The paper deals with a new computer program, named VAV, for the analysis of any kind of tidal data. VAV can be applied on unevenly spaced data without interpolation. The basic algorithm consists in a transformation of the data from the time domain into a time/frequency domain. This is done through filtration of data intervals without overlapping, whose operation eliminates a rather flexible model of the drift. The filters used are orthogonal and in the situation of unevenly spaced data, they are adaptable in the time domain. After the transformation, the tidal parameters are estimated through the method of the least squares that is applied in the time/frequency domain. Since the noise of the data is correlated (colored noise), VAV provides frequency-dependent estimates of the precision. The program is specially orientated towards the search of various anomalies of the data, which may be useful, eventually, for earthquake and volcano monitoring.

Interpretation of tidal gravity anomalies in Lanzarote, Canary Islands

Since 1987, gravity tide observations have been conducted on the island of Lanzarote, with different gravimeters at different locations. Very anomalous values of heat flow data have also been collected in the Timanfaya National Park (southwestern part of the island). Seismic experiments provide us a crustal model of the island, with an average depth of 11.5 km. The gravity tide observations show that, after correction of the indirect oceanic effect using global and local ocean charts, the cosine component of the final residual vector for station Cueva de los Verdes (northeastern of the island) depends on the ocean model considered. On the other side, the final results change greatly if we take into account the porosity effects of the lava rock at this station. Then, the cosine component of the final residual is closer to, or greater than, zero. In Timanfaya station (located in the Timanfaya National Park), we observe that both M2 and O1 residuals are equally large. Their phases are more consistent with a body tide effect than with an ocean loading effect. Furthermore, the negative sign of the residuals looks consistent with the response of a porous or cavity-filled, local, upper crust under the influence of tidal strain. Therefore, we found a similar explanation for the existence of negative values of the final residuals at both stations. Our study seems to suggests the lack of correlation between heat flow and tidal gravity anomalies. Nevertheless, we must point out again the existing necessity of theoretical studies and highest quality observations to answer the different questions regarding the significance of the tidal gravity anomaly-heat flow trends. Also, the effect of tidal strain in a porous or cavity-filled, local, upper crust should be considered.

Comparison of the performances of different spring and superconducting gravimeters and STS-2 seismometer at the Gravimetric Observatory of Strasbourg, France

Since 1973, the Gravimetric Observatory of Strasbourg (France) is located in an old fort named J9 and has been the place for various gravity experiments. We present a comparison of the noise levels of various instruments that are or were continuously recording at J9, including the LaCoste&Romberg Earth-Tide Meter ET-5 (1973–1985), the GWR Superconducting Gravimeter TT-T005 (1987–1996), the Superconducting Gravimeter C026 (since 1996), the STS-2 seismometer (since 2010) and the LaCoste&Romberg ET-11 (continuously since October 2010). Besides these instruments, the J9 Observatory has hosted temporary gravity experiments with the Micro-g LaCoste Inc. gPhone-054 (May–December 2008 and May–September 2009) and the Micro-g LaCoste Inc. Graviton-EG1194 (June–October 2011). We include also in the comparison the absolute gravimeter Micro-g FG5 #206 which is regularly performing absolute gravity measurements at J9 since 1997 and a spring gravimeter Scintrex CG5 which recorded at J9 between March 2009 and February 2010. We present the performances of these various instruments in terms of noise levels using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The different responses to atmospheric pressure changes of all the instruments are also investigated. A final part is devoted to the instrumental self-noise of the SG C026, STS-2 and L&R ET-11 using the three channel correlation analysis method applied to 1-Hz data.

El Hierro Island Model–Canary–On a Basis of Joint Interpretation of Microseismic Sounding and Gravity Inversions

To study the deep structure of El Hierro Island, Canary Archipelago, we used a new microseismic sounding method based on the fact that Earths crust heterogeneities disturb in their vicinity the spectrum of low-frequency microseismic field. At the Earth’s surface above the high-velocity heterogeneities the spectral amplitudes of definite frequency f are decreasing, and above the low-velocity ones they are increasing. The frequency f is connected with depth of heterogeneity deposition H and velocity of fundamental Rayleigh mode VR(f) by the following relation H = K*VR(f) /f, here К- is numerical factor close to 0.4. From microseismic data two large intrusive bodies were revealed beneath El Hierro Island. Joint interpretation of microseismic and gravimetric data and their comparison with previously obtained geological and geochemical data by other authors enables supposing that the eastern intrusive body relates to the early stage of the island formation. In the western body at the depths ~15 km the area with lowest seismic velocities could be revealed. We suggest that a modern magmatic chamber is located there.

Aeromagnetic anomalies reveal the link between magmatism and tectonics during the early formation of the Canary Islands

The 3-D inverse modelling of a magnetic anomaly measured over the NW submarine edifice of the volcanic island of Gran Canaria revealed a large, reversely-magnetized, elongated structure following an ENE-WSW direction, which we interpreted as a sill-like magmatic intrusion emplaced during the submarine growth of this volcanic island, with a volume that could represent up to about 20% of the whole island. The elongated shape of this body suggests the existence of a major crustal fracture in the central part of the Canary Archipelago which would have favoured the rapid ascent and emplacement of magmas during a time span from 0.5 to 1.9 My during a reverse polarity chron of the Earth’s magnetic field prior to 16 Ma. The agreement of our results with those of previous gravimetric, seismological and geodynamical studies strongly supports the idea that the genesis of the Canary Islands was conditioned by a strike-slip tectonic framework probably related to Atlas tectonic features in Africa. These results do not contradict the hotspot theory for the origin of the Canary magmatism, but they do introduce the essential role of regional crustal tectonics to explain where and how those magmas both reached the surface and built the volcanic edifices.

Characterization of volcanic structures using ground penetrating radar and additional inverse modelling: Multidisciplinary geophysical investigation in the Timanfaya National Park (Spain)

A ground penetrating radar survey is presented over a recent lava flow at the volcanic area of Timanfaya National Park (Canary Island, Spain). The purpose was to locate lava tubes into the lava flow through the combination of field and simulated data. Different modelling strategies were used for the analysis and knowledge of the signal behaviour. Finite-difference time-domain algorithm was considered for simulations, and the pattern of reflections generated from previously known volcanic structures were characterized. After the characterization of the radar-wave response, the interpretation achieved was applied over the field data acquired at other non-studied area in which different lava tubes were recognized.