The gravity database for Belgium

Gravity changes associated with volcanic processes occur over a wide range of time scales, from minutes to years and with magnitudes between a few and a few hundred microGal. High-precision instruments are needed to detect such small signals and both time-lapse surveys along networks of stations, and continuous measurements at single points, are accomplished. Continuous volcano gravimetry is mostly carried out through relative gravimeters, either superconducting instruments, providing higher quality data, or the more widely used spring meters. On the other hand, time-lapse surveys can be carried out with relative (spring) gravimeters, that measure gravity differences between pairs of stations, or by absolute gravimeters, capable of measuring the absolute value of the gravitational acceleration at the observation point. Here we present the state-of-the-art of terrestrial gravity measurements to monitor and study active volcanoes and the possibilities of new gravimeters that are under development. In particular, we present data from a mini array of three iGrav superconducting gravimeters (SGs) at Mount Etna (the first network of SGs ever installed on an active volcano). A comparison between continuous gravity measurements recorded through the iGrav#016 superconducting gravimeter at Serra La Nave station (1730 m a.s.l.) and absolute gravity data collected with the Microg LaCoste FG5#238 gravimeter in the framework of repeated campaigns is also presented. Furthermore, we introduce the Horizon 2020 NEWTON-g project (New Tools for Terrain Gravimetry), funded under the FET-OPEN Research and Innovation Actions call, Work Programme 2016–2017 (Grant Agreement No 801221). In the framework of this project, we aim to develop a field-compatible gravity imager, including an array of low-costs Micro-Electro-Mechanical Systems (MEMS)-based relative gravimeters, anchored on an absolute quantum gravimeter. After the design and production phases, the gravity imager will be field-tested at Mt. Etna (Italy) during the last 2 years of the project.

<p>In this study, we investigate the optimal combination of local gravity observations and their contributions to the regional quasi-geoid model. The study area is located in Colorado, USA, with two types of regional data sets, namely terrestrial gravity data and airborne gravity data, available within the “1 cm geoid experiment”. The approach based on series expansions in terms of spherical radial basis functions (SRBF) is applied, which has been developed at DGFI-TUM in the last two decades. We use two different types of basis functions covering the same spectral domain separately for the terrestrial and the airborne measurements. The Shannon function is applied to the terrestrial data, and the Cubic Polynomial (CuP) function which has smoothing features is applied to the airborne data for filtering their high-frequency noise.</p><p>To assess the contributions of the regional terrestrial and airborne gravity data to the final quasi-geoid model, four solutions are compared, namely the combined solution, the terrestrial only, the airborne only, and finally the model only solution, i.e., only the global gravity model and the topographic model are used without any gravity data from regional measurements. By adding the terrestrial data to the GGM and the topographic model, the RMS error of the quasi-geoid model w.r.t the validation data (the mean solution of independent computations delivered by fourteen institutions from all over the world) drops from 4 to 1.8 cm, and it is further reduced to 1 cm by including the airborne data.</p>

Terrestrial mobile gravimetry using temperature-stabilized Inertial Measurement Units (IMUs) is emerging as an efficient technique for acquiring high-accuracy gravity data in dynamic environments. This study explores the integration of the iCORUS-2 IMU system with GNSS along the Cebeci-Elmadağ route in Ankara, Türkiye. The iTempStab technology was incorporated into the iCORUS-2 system to stabilize temperature variations to enhance the accuracy of gravity measurements. Notably, this research represents the results of vehicle-based terrestrial mobile gravimetry using a thermally stabilized IMU. An Extended Kalman Filter (EKF) was applied to integrate IMU and GNSS data. The advantage of terrestrial mobile gravimetry over airborne methods is the ability to make static updates for vertical gravity, zero gravity gradient and zero velocity measurements, which can be introduced into the EKF as measurement updates. The method was tested by analyzing the statistical impact of these updates at varying intervals: without any gravity measurement updates, and by applying updates at approximately 2 km, 3 km, 4 km, 5 km, 6 km, and 7 km intervals. Results indicated that without updates, a root mean square error (rmse) value between external terrestrial gravity data and mobile gravimetry estimates is 1.362 mGal. However, with updates at approximately 2 km intervals, the rmse value significantly decreased to 0.948 mGal, representing a %30.4 reduction in rmse and achieving sub-mGal accuracy. The inclusion of temperature stabilization and frequent gravity measurement updates demonstrated a marked improvement in data accuracy and consistency. These advancements not only validate the potential of mobile gravimetry for geophysical surveys but also broaden its applications in fields requiring precise gravity data acquisition.

Volcanosedimentary boron deposits are present within Tertiary lacustrine sediments and volcanic rocks in Xiongba, Tibet. Boron deposits are characterized by low density relative to country rocks; thus, it is possible to locate them by gravity measurements. We conducted a 1:50000 high-precision gravity survey in the Xiongba area, Tibet, and obtained the Bouguer and residual gravity anomalies. We analyzed fault systems and the distribution of sedimentary and volcanic rocks and their relation to the volcanosedimentary boron deposits. The processing of the gravity data revealed local gravity variations and fault structures. We applied preferential downward continuation and wavelet transform to the gravity data, and in conjunction with geological data, we predicted the distribution of volcanosedimentary boron deposits.

Eight magnetotelluric (MT) soundings were performed as a continuation of an earlier audiomagnetotelluric (AMT) survey conducted in the northeastern border of the Paraná basin, a large intracratonic basin located in central eastern South America and constituted by Silurian to Jurassic sedimentary rocks with Lower Cretaceous sill‐type magmatic intrusions and overlying volcanics. Two of the MT stations were carried out near two deep petroleum wells. The remaining MT soundings were done on a profile traversing two important gravity features: a positive anomaly near the border of the states of São Paulo and Minas Gerais and a strong (trending NW‐SE) linear gradient. Major findings of an integrated interpretation of the MT survey and of available gravity data are as follows: (1) inhomogeneities and/or strong magnetization of the superficial volcanics and diabase intrusions in the Paleozoic sedimentary rocks appearing to distort the MT results; and (2) identification of important structural discontinuities, including a possible different crustal structure beneath the Paraná basin compared to the region on the north, a thickening of the crust toward the NE, and the probable existence of a trough (graben?) within the basin, characterized by a thick accumulation of sediments and basalts.

The Earth's Gravitational Field and Its Exploitation

We describe a geophysical study of oceanic core complexes (OCC) and surrounding seafloor on the Mid‐Atlantic Ridge at 13°N–14°N and off‐axis to ∼1.9 Myr. Data include a detailed, deep‐towed side scan sonar, magnetic field and bathymetry survey, supplemented by concurrent sea‐surface bathymetry, magnetic field and gravity measurements. Using side scan and bathymetry, we infer areas and relative ages of seafloor volcanism, revealing a complex pattern of melt accretion across the median valley including close to its walls. We estimate tectonic and magmatic extension throughout the area, and find that average tectonic extension since chron 2 on plates containing OCCs is up to three times that on their conjugates. Deep‐towed magnetic data reveal asymmetric spreading (faster on OCC‐containing plates) and crustal magnetization that is highly heterogeneous on a scale of ∼5 km, suggesting that exhumed domes of OCCs have highly variable lithologies, perhaps comprising both serpentinized peridotite and gabbro. Improved fits to magnetic data are provided by models incorporating ∼45°of OCC footwall rotation. An axial zone of normal magnetization, of presumed Brunhes epoch, has highly variable width and amplitude, with parts of the ridge axis displaying very low or apparently reversed magnetization. Gravity requires that OCCs have dense cores capped by lower density zones several kilometers thick. Gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of OCC formation and show that waxing magmatism may terminate OCCs.

CESAR (Canadian Expedition to Study the Alpha Ridge), a program of the Department of Energy, Mines and Resources, was conducted from an ice station on the Arctic Ocean between March 30 and May 23, 1983. The base camp was established near 85 °52' north and 112 °34' west, 420 km north and west of Ellesmere Island and drifted in an irregular manner for the duration of the project. Preliminary disclosures were made at a one-day symposium (May 31, 1984) at the CMOS/COU meeting in Halifax. A digest of the geological and geophysical interpretations follows. In cooperation with the Canadian Hydrographic Service some 1300 individual depth soundings and gravity measurements were established over the Ellesmere Island Continental Shelf and Alpha Ridge to the 116 degrees west Meridian. In addition, about 120 kilometres of echo sounding and gravity recordings were established along the drift track of the CESAR station. A 100-m contour bathymetric map and a 10-m Gal contour free-air gravity anomaly map have been compiled and include data collected from Ice Island T-3 between 1969 and 1974 just before grounding for several months off the northern coast of Ellesmere Island. Unlike the Lomonosov Ridge, investigated in 1979 during Project LOREX, the topography of the Alpha Ridge is complex and rugged, showing ridges, grabens, seamounts, etc. in profusion. The minimum depth recorded over the Alpha Ridge is 1087 m. The Alpha Ridge joins the Continental Shelf at a depth of between 1800 and 2000 m. In general, the gravity free-air anomalies mirror the ridge topography and, over this portion of the Continental Shelf, the gravity map reveals similar positive elliptical free-air anomalies that have been mapped all along the Canadian Arctic and the Alaskan Continential Shelf; these are considered to be indicative of the transition between continental and oceanic crust. Based on gravity data alone, the Alpha Ridge might be interpreted as continental crust. During CESAR, sixteen Benthos piston cores and twelve gravity cores recovered as much as 6 m of sediment from the southeastern Alpha Ridge. In total to date, approximately 600 cores have been recovered and studied from the various ice stations in the Arctic Ocean. The cores provide the first information on the lithology and age of sediments covering the Alpha Ridge crest and adjacent grabens. Initial studies indicate a correlatable sequence of litho-facies that range in age from ate Cretaceous to Recent. The graben and basins on the flanks of the northern Alpha Ridge are filled with about 250 m of unconsolidated sediment of which the top 6 m is a continuous section of Lower Pliocene to Recent mud. Most facies of this mud can be correlated with U.S.G.S. cores from the Southern Ridge, western Alpha Ridge and Chukchi Rise. Planktonic and benthic foramanifera appear at intervals in the cores with coarse detrital material interspersed. Pollen has also been observed in the cores but is not as yet identified.

Clear evidence for the sign-reversal of the pressure admittance to gravity near 3mHz

A geophysical investigation of the Peru-Chile Trench

SUMMARY A new algorithm is presented for the integrated 2-D inversion of seismic traveltime and gravity data. The algorithm adopts the ‘maximum likelihood’ regularization scheme. We construct a ‘probability density function’ which includes three kinds of information: information derived from gravity measurements; information derived from the seismic traveltime inversion procedure applied to the model; and information on the physical correlation among the density and the velocity parameters. We assume a linear relation between density and velocity, which can be node-dependent; that is, we can choose diVerent relationships for diVerent parts of the velocity‐density grid. In addition, our procedure allows us to consider a covariance matrix related to the error propagation in linking density to velocity. We use seismic data to estimate starting velocity values and the position of boundary nodes. Subsequently, the sequential integrated inversion (SII) optimizes the layer velocities and densities for our models. The procedure is applicable, as an additional step, to any type of seismic tomographic inversion. We illustrate the method by comparing the velocity models recovered from a standard seismic traveltime inversion with those retrieved using our algorithm. The inversion of synthetic data calculated for a 2-D isotropic, laterally inhomogeneous model shows the stability and accuracy of this procedure, demonstrates the improvements to the recovery of true velocity anomalies, and proves that this technique can eYciently overcome some of the limitations of both gravity and seismic traveltime inversions, when they are used independently. An interpretation of field data from the 1994 Vesuvius test experiment is also presented. At depths down to 4.5 km, the model retrieved after a SII shows a more detailed structure than the model obtained from an interpretation of seismic traveltime only, and yields additional information for a further study of the area.

The magnetic data and gravity data have been recovered from magnetic tapes and earlier paper maps for Iraq. The magnetic data belonging to the State Company of Geology and Mining of Iraq (GEOSURV) were recovered by Frontier Processing Company and the U.S. Geological Survey (USGS) from the tapes originally flown by the French firm Compagnie General de Geophysique (CGG) in 1974. GETECH reprocessed the magnetic data using Geosoft and GETECH in‐house micro‐levelling software to its full glory which displays as a comprehensive and high quality dataset and shows fine detail along the profiles. GETECH recovered the legacy gravity data from the original field data sheets archived by IPC (Iraq Petroleum Company). These gravity data have been compiled with more recent GEOSURV sponsored surveys thus allowing for the first time a comprehensive digital and unified national gravity database to be constructed with full principal facts. Figure 1 shows the final aeromagnetic and gravity data coverage of Iraq. The only part of Iraq lacking gravity and aeromagnetic data coverage is the mountainous areas of the Kurdish Region of northeastern Iraq. Joint interpretation of the magnetic and gravity data will help to guide future geophysical investigations by GEOSURV, whose ultimate aim is to discover economical mineral and energy resources.

We present results of repeated absolute gravity and GPS measurements, carried out at Mt. Etna volcano between 2009 and 2018. Absolute gravity measurements are rarely performed along arrays of stations on active volcanoes and, through our unprecedented dataset, we highlight the possibilities of this method to track underground mass changes over long time-scales.Analysis of the residual absolute gravity data and ground deformation reveals a cycle of gravity increase and uplift during 2009 to 2011, followed by gravity decrease and subsidence during 2011 to 2014.Data inversion points to a common mass and pressure source, lying beneath the summit area of the volcano, at depth of ~5 km b.s.l. The bulk volume change inferred by the inversion of the deformation data can account for only a small portion of the mass change needed to explain the correspondent gravity variations. We propose that the observed relationship between gravity and vertical deformation was mostly due to the compressibility of the magma in the inferred reservoir, which, in turn, was enhanced by the presence of exsolved gas.Overall, the gravity and deformation data we present reveal a cycle of magma recharge (2009 – 2011) and discharge (2011 – 2014) to/from the inferred storage zone. During the recharge phase only degassing occurred from the summit craters of Mt. Etna. During the following phase of discharge, the magma lost from the reservoir at ~5 km b.s.l. fed the exceptional phase of volcanic activity during 2011 to 2014, when tens of lava fountaining episodes took place.

Over the years since the Apollo era, the current lunar interior structure has been investigated using seismic, gravity, and magnetic field data. The Apollo seismic network recorded about 1,800 meteoroid impacts, 28 energetic shallow moonquakes (with body wave magnitudes up to five and hypocenters about 100 km below the surface), and about 7,000 extremely weak deep moonquakes that were located about halfway to the center of the Moon (e.g., Wieczorek 2009). The deep moonquakes are very enigmatic in that their occurrences are correlated with the tides raised by the Earth, they involve very low stress drops (less than 1 bar), they appear to originate from about 300 “nests” that are repeatedly activated, and almost all of these nests are located on the Moon’s nearside hemisphere (Nakamura 2003; Bulow et al. 2007; Qin et al. 2012). The nearside distribution of the deep moonquakes could perhaps indicate that the farside hemisphere is seismically inactive. Such a hypothesis is possible, especially if one considers that almost all of the Moon’s mare basalts erupted on its nearside hemisphere. Alternatively, it is possible that the deepest interior of the Moon attenuates seismic signals originating from farside events. In support of this hypothesis, seismic arrivals along ray paths passing through regions deeper than 700 km radius are not detected, suggesting mantle in this region hot enough to be seismically attenuating and possibly partially molten (Nakamura 2005). One scenario that could be consistent with the above observations is that numerous magma-filled fractures currently located in the deep mantle are capable of relieving the small stresses induced by Earth-raised tides. The Lunar Laser Ranging mission (Williams et al. 2001) and recent Gravity Recovery and Interior Laboratory (GRAIL) mission (Zuber et al. 2012) provide us the gravity measurements. Especially the Love numbers from the GRAIL mission are unprecedentedly accurate. These Love numbers have been used to invert the bulk solid-body “dissipation factor” Q (Williams et al. 2001) as well as the tidal tomography (Zhong et al. 2012). The Q number is around 30, which is compatible to that of the Earth andMars. This Q number infers a core–mantle boundary (CMB) temperature 1,400 C (Nimmo et al. 2012). This temperature is below the harzburgite mantle solidus while higher than ilmenite-mixed harzburgite solidus (Zhang et al. 2013a, see Fig. 1). Joint inversion of seismic and gravitational data (Khan et al. 2006) suggests a CMB temperature range from 1,170 C to 1,500 C, consistent with that inferred from the Q facter. To gain a partial melt, the lunar deep mantle has to be chemically heterogeneous (either ilmenite rich or water rich (Evans et al. 2014)). The surface and orbit magnetic records have also been used to explore the present-day mantle temperature (Sonett 1982). The electrical conductivity profile of the Moon has been estimated by using electromagnetic-sounding techniques, whereby the relationship between simultaneous time variations in the ambient magnetic field as measured from orbit and the resulting field measured on the surface was analyzed (Wieczorek 2009; Hood et al. 1982). This electrical conductivity profile suggests the lunar mantle temperature from 1,100 C to 1,250 C (see Fig. 1). The lunar mantle is energetically interacting with its core. A number of seismic and gravity studies identify the lunar core with a radius of about 250–430 km (Wieczorek 2009; Weber et al. 2011; Garcia et al. 2011). The core is believed to have liquid outer and solid inner regions based on seismic

At the invitation of the Swedish National Committee of Geodesy and Geophysics, the International Association of Geodesy will hold a symposium on ‘Terrestrial Electromagnetic Distance Measurements and Atmospheric Effects on Angular Measurements’ at the Royal Institute of Technology, Stockholm, August 19–24, 1974.The scientific program will be built up around a number of invited papers. The proceedings of the symposium will be published.