Roger Bayer

Time‐lapse microgravity surveys reveal water storage heterogeneity of a karst aquifer

Time-lapse microgravity surveying combined with absolute gravity measurements is used to investigate water storage changes in a karst aquifer of similar to 100 km(2) area. The survey consists of 40 gravity stations measured with a relative gravimeter; absolute gravity is measured at three stations for each survey. In total, four gravity surveys are performed over a 2 year time period during consecutive wet and dry periods. Survey precisions range between 2.4 and 5 mu Gal, enabling statistically significant detection of 10 mu Gal change, i.e., similar to 0.25 m equivalent water level change. Observed gravity changes are coherent between consecutive survey periods, i.e., net water withdrawal and net water recharge is observed, reaching changes as high as 22 mu Gal. Observed gravity changes allow refining evapotranspiration estimates, which may serve to improve the water budget of the aquifer. High-and low-gravity amplitude zones characterize the karst system, demonstrating spatially variable storage behavior. Geomorphologic considerations are invoked to explain the location of preferential zones of water storage, and a conceptual model of water storage is discussed for the studied karst.

Fortran routines for linear inverse problems

A package of Fortran routines using linear programming techniques in linear inverse problems as gravimetry or geomagnetism is presented. These routines, with a complete description and examples of their applications, are available at the SEG Business Office. This paper presents a brief description of the package and the general features of the method; taking into account the nonuniqueness of the solution, the bounds of some parameters or some particular solutions, such as the “ideal body” [Parker (1975)], are computed. This package can be applied to inverse gravity or geomagnetic problems with a maximum data size on IBM 360/65 of the order such that the number of measurements ⩽ 250, number of unknowns ⩽ 600.

The transition between Makran subduction and the Zagros collision: recent advances in its structure and Active deformation

Abstract SE Iran is the site of a rare case of young transition between subduction and collision. We have synthesized recent results in geodesy, tectonics, seismology and magnetism to help understand the structure and kinematics of the Zagros–Makran transition. Surface observations (tectonics, magnetism and geodesy) indicate a transpressive discontinuity consisting of several faults striking obliquely to the convergent plate motion, whereas deeper observations (seismology) support a smooth transition across the fault system. No lithospheric transform fault has been created, although the transition already behaves like a major boundary in terms of tectonic style, seismic structure, lithology and magnetism. The Zendan–Minab–Palami fault system consists of several faults that accommodate a transpressive tectonic regime. It is the surface expression of a southward propagation of the north–south-trending right-lateral strike-slip fault system of Jiroft–Sabzevaran. Within each system the numerous faults will coalesce into a single, lithospheric, wrench fault.

A model for the evolution of the axial zone of mid-ocean ridges as suggested by icelandic tectonics*

Abstract In Iceland tectonic activity in the neovolcanic zone leads to the formation of three kinds of parallel structures: open fissures, emissive fissures, and normal faults. This observation is used to built a kinematic model which is based on the superposition of lava flows generated from an active central belt of finite width. The results are in good agreement with the recent results in magnetism and tectonic observations of both subaerial and underwater active ridges.

Une nouvelle carte gravimétrique des Alpes occidentales et ses conséquences structurales et tectoniques

Abstract Two new gravity surveys performed in the western Alps allow the drawing of a high-resolution gravity map. Moreover, a well-constrained Moho depth model was used to correct the initial map from the Moho depth variation gravity effect and to interpret the residual effect in terms of crustal heterogeneities. The main structures of the western Alps are pointed out, specially the Ivrea body parallel to the Penninic front, the crystalline massifs (Simplon-Tessin, Pelvoux, Mercantour) and the sedimentary basins and nappes of the southern Alps (Digne, Castellane, Valensole).

Results of the European Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2007

The second international comparison of absolute gravimeters was held in Walferdange, Grand Duchy of Luxembourg, in November 2007, in which twenty absolute gravimeters took part. A short description of the data processing and adjustments will be presented here and will be followed by the presentation of the results. Two different methods were applied to estimate the relative offsets between the gravimeters. We show that the results are equivalent as the uncertainties of both adjustments overlap. The absolute gravity meters agree with one another with a standard deviation of 2 μgal (1 gal = 1 cm/s2).

Final report on the Seventh International Comparison of Absolute Gravimeters (ICAG 2005)

The Bureau International des Poids et Mesures (BIPM), S?vres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005.ICAG 2005 was prepared and performed as a metrological pilot study, which aimed: To determine the gravity comparison reference values; To determine the offsets of the absolute gravimeters; and As a pilot study to accumulate experience for the CIPM Key Comparisons. This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG?RGC comparison was held at the BIPM in 1980?1981 and since then meetings have been organized every 4 years.In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports.Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.

An attempt to formulate well-posed questions in gravity: Application of linear inverse techniques to mining exploration

The aim of this paper is to use linear inverse theory to interpret gravity surveys in mining exploration by incorporating a priori information on the densities and data in terms of Gaussian or uniform probability laws. The Bayesian approach and linear programming techniques lead to the solution of well-posed questions resulting from the exploration process. In particular, we develop a method of measuring the possible heterogeneity within a given domain by using linear programming. These techniques are applied to gravity data taken over the massive sulfide deposit of Neves Corvo (Portugal). We show how crude constraints on the densities lead to a first estimation of the location of sources, while further geologic constraints allow us to estimate the heterogeneity and to put definite bounds on the ore masses.

Airborne gravity measurements over mountainous areas by using a LaCoste & Romberg air‐sea gravity meter

This paper introduces a new approach to airborne gravity data reduction well-suited for surveys flown at high altitude with respect to gravity sources (mountainous areas). Classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western French Alps by using a LaCoste & Romberg air-sea gravity meter. The part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra. Beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft. This finding is theoretically argued by solving the equation of the gravimetric system (gravity meter and stabilized platform). The transfer function of the system is derived, and a new formulation of airborne gravity data reduction, which takes care of the sensitive response of spring tension to observable gravity field wavelengths, is given. The resulting gravity signal exhibits a residual noise caused by electronic devices and short-wavelength Eotvos effects. The use of dedicated exponential filters gives us a way to eliminate these high-frequency effects. Examples of the resulting free-air anomaly at 5100-m altitude along one particular profile are given and compared with free-air anomaly deduced from the classical method for processing airborne gravity data, and with upward-continued ground gravity data. The well-known trade-off between accuracy and resolution is discussed in the context of a mountainous area.

Results of the Seventh International Comparison of Absolute Gravimeters ICAG-2005 at the Bureau International des Poids et Mesures, Sèvres

The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sevres, France in September 2005. The organization of ICAG-2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented.