The potential of ground gravity measurements to validate GRACE data

New satellite missions are returning high precision, time-varying, satellite measurements of the Earth’s gravity field. The GRACE mission is now in its calibration/- validation phase and first results of the gravity field solutions are imminent. We consider here the possibility of external validation using data from the superconducting gravimeters in the European sub-array of the Global Geodynamics Project (GGP) as ‘ground truth’ for comparison with GRACE. This is a pilot study in which we use 14 months of 1-hour data from the beginning of GGP (1 July 1997) to 30 August 1998, when the Potsdam instrument was relocated to South Africa. There are 7 stations clustered in west central Europe, and one station, Metsahovi in Finland. We remove local tides, polar motion, local and global air pressure, and instrument drift and then decimate to 6-hour samples. We see large variations in the time series of 5–10<i>µ</i>gal between even some neighboring stations, but there are also common features that correlate well over the 427-day period. The 8 stations are used to interpolate a minimum curvature (gridded) surface that extends over the geographical region. This surface shows time and spatial coherency at the level of 2– 4<i>µ</i>gal over the first half of the data and 1–2<i>µ</i>gal over the latter half. The mean value of the surface clearly shows a rise in European gravity of about 3µgal over the first 150 days and a fairly constant value for the rest of the data. The accuracy of this mean is estimated at 1<i>µ</i>gal, which compares favorably with GRACE predictions for wavelengths of 500 km or less. Preliminary studies of hydrology loading over Western Europe shows the difficulty of correlating the local hydrology, which can be highly variable, with large-scale gravity variations.<br><br><b>Key words. </b>GRACE, satellite gravity, superconducting gravimeter, GGP, ground trut

Dosages par fluorescence X portable d’ateliers médiévaux de production des métaux non-ferreux

La difficulté liée à la détection des ateliers de métallurgie des non-ferreux a conduit à tester un nouveau mode de prospection par spectrométrie de fluorescence X portable. Après une phase de calibration qui a donné de très bons résultats pour les métaux recherchés dans cette étude, trois approches ont été tentées sur le site de Castel-Minier (site minier et pluri métallurgique du second Moyen Âge, Ariège, France). Grâce à un travail interdisciplinaire incluant à la géochimie, l’archéologie, l’histoire et la géophysique, les résultats ont été particulièrement positifs. Ils ont conduit à une meilleure perception chronologique du site, à la localisation d’ateliers de traitement de scorie et la sériation des anomalies géochimiques.Difficulties related to non-ferrous metal workshops detection led to test a new way of survey by a real portable x-ray spectrometer. After a calibration phase which yielded very good results for lead, zinc and cooper, three approaches were tried in south-west of France at the site of Castel-Minier, (mining and multi metallurgical site from the second Middle Ages). Thanks to an interdisciplinary work that include geochemistry, archeology, history and geophysic, the results were particularly positive. They led to a better understanding of chronological phases. They give the location of slag processing workshops and classification of geochemical anomalies

Reconstructing past terrace fields in the Pyrenees: Insights into land management and settlement from the Bronze Age to the Early Modern era at Vilalta (1650 masl, Cerdagne, France)

© Trustees of Boston University 2015. The building of a solar power station at Thémis, at 1650 masl on the south-facing slope of the Carlit massif in the eastern Pyrenees, led to an archaeological evaluation from April-June 2009. This evaluation covered a surface of 10 ha that included a medieval village as well as the surrounding agricultural land in terraces. Non-destructive archaeological methods were used for the village. A detailed study of the 6 ha of terraces began with a fieldwalking survey, mapping every visible feature, followed by systematic trial trenches. Fifty-five trenches, 11 in the village and 44 in the fields, were opened. The stratigraphies were then compared with a series of 22 radiocarbon dates and eight relative dates provided by ceramic typologies. This combination of surface and buried evidence supported our preliminary hypothesis about the dynamics of the slope. The results suggest the existence of agrarian features beginning in the Bronze Age and reveal that the field patterns were frequently transformed, both in the Medieval and Early Modern periods. The transformations in the terrace fields after the village was abandoned are as interesting as those during occupation because, contrary to the idea of a fixed, unchanging landscape after the end of the Middle Ages, they challenge the idea that mountain zones are marginal spaces by nature, or were marginalized later.Peer Reviewe

Crustal thickness of Antarctica estimated using data from gravimetric satellites

Computing a better crustal thickness model is still a necessary improvement in Antarctica. In this remote continent where almost all the bedrock is covered by the ice sheet, seismic investigations do not reach a sufficient spatial resolution for geological and geophysical purposes. Here, we present a global map of Antarctic crustal thickness computed from space gravity observations. The DIR5 gravity field model, built from GOCE and GRACE gravimetric data, is inverted with the Parker–Oldenburg iterative algorithm. The BEDMAP products are used to estimate the gravity effect of the ice and the rocky surface. Our result is compared to crustal thickness calculated from seismological studies and the CRUST1.0 and AN1 models. Although the CRUST1.0 model shows a very good agreement with ours, its spatial resolution is larger than the one we obtain with gravimetric data. Finally, we compute a model in which the crust–mantle density contrast is adjusted to fit the Moho depth from the CRUST1.0 model. In East Antarctica, the resulting density contrast clearly shows higher values than in West Antarctica

Detection of ancient mine voids by using geophysical methods: the case of Castel-Minier

International audienceThe mining operations of Ouels, also named Castel-Minier, close to the village of Aulus-les-Bains in the Pyrenees, was mainly effective between the 13th and 16th centuries. It was the most important mine in the French Kingdom during the 14th century, and was operated for silver, cupper and lead. Today, it is a site for archaeological studies and the archaeologists are specially interested in the cavities left by the exploitation. It is with the aim of retrieving the voids left in a remote area that geophysical facilities are requested, following the discovery of a 20th century plan attesting to the existence of a shaft and a vein in this area.Two geophysical methods have been implemented: Electrical Resistivity Tomography (ERT) and microgravimetry. The first has been used on two parallel profiles spaced 15 m apart, and allowed the detection of a very high resistive body that could fit a cavity at less than a depth of 5 meters. On the other side, microgravimetry revealed two anomalies. It seems to confirm the one seen by ERT, while a second anomaly may attest to a deeper cavity hypothetically lying at 15 to 20 m depth. These hypotheses are compatible with the archaeological expectations

Estimation of the Free Core Nutation parameters from SG data : sensitivity study and comparative analysis using linearized least-squares and Bayesian methods

The Free Core Nutation (FCN) is investigated with the help of its resonance effect on the tidal amplitudes in Superconducting Gravimeter (SG) records of the GGP network. The FCN resonance parameters are combined in a resonance equation involving the Earth's interior parameters. The sensitivity of the FCN parameters to the diurnal tidal waves demonstrates that the quality factor of the FCN is strongly dependent on the accuracy of the imaginary part estimates of the gravimetric factors close to the resonance. The weak amplitude of Psi(1) tidal wave on the Earth, which is the closest in frequency to the FCN, in addition to errors in ocean loading correction, explains the poor determination of the quality factor Q from surface gravimetric data. The inversion of tidal gravimetric factors leads to estimates of the period, Q and resonance strength of the FCN. We show that. by inverting log(Q) instead of Q the results using the least-squares method optimized using the Levenberg-Marquardt algorithm are in agreement with the Bayesian probabilistic results and agree with the results obtained from VLBI nutation data. Finally, a combined inversion of 7 GGP European SG data is performed giving T = 428 +/- 3 days and 7762 < Q < 31,989 (90% C.I.). An experimental estimate of the internal pressure Love number is also proposed