Gravimetric Surveys Research Articles

Effect of Biomass Water Dynamics in Cosmic-Ray Neutron Sensor Observations: A Long-Term Analysis of Maize-Soybean Rotation in Nebraska.

Precise soil water content (SWC) measurement is crucial for effective water resource management. This study utilizes the Cosmic-Ray Neutron Sensor (CRNS) for area-averaged SWC measurements, emphasizing the need to consider all hydrogen sources, including time-variable plant biomass and water content. Near Mead, Nebraska, three field sites (CSP1, CSP2, and CSP3) growing a maize-soybean rotation were monitored for 5 (CSP1 and CSP2) and 13 (CSP3) years. Data collection included destructive biomass water equivalent (BWE) biweekly sampling, epithermal neutron counts, atmospheric meteorological variables, and point-scale SWC from a sparse time domain reflectometry (TDR) network (four locations and five depths). In 2023, dense gravimetric SWC surveys were collected eight (CSP1 and CSP2) and nine (CSP3) times over the growing season (April to October). The N0 parameter exhibited a linear relationship with BWE, suggesting that a straightforward vegetation correction factor may be suitable (fb). Results from the 2023 gravimetric surveys and long-term TDR data indicated a neutron count rate reduction of about 1% for every 1 kg m-2 (or mm of water) increase in BWE. This reduction factor aligns with existing shorter-term row crop studies but nearly doubles the value previously reported for forests. This long-term study contributes insights into the vegetation correction factor for CRNS, helping resolve a long-standing issue within the CRNS community.

GORDON LYALL PAVER (1913–1988) AND 42ND GEOLOGICAL SECTION, SOUTH AFRICAN ENGINEER CORPS: MILITARY GEOLOGY AND GEOPHYSICS IN WORLD WAR II SUPPORTING BRITISH ARMY OPERATIONS: PART 1, THE EAST AFRICAN CAMPAIGN 1940–1941

ABSTRACT 42nd Geological Section of the South African Engineer Corps was a unique unit that supported British armed forces during World War II. It was co-founded and led for most of the war by Gordon Lyall Paver (1913–1988), one of the few ‘British’ officers serving specifically as geologists during the war to achieve the rank of major. Born in South Africa at Johannesburg and in his early years educated there at St. John's College, from 1926 Paver was educated in England, at Charterhouse School until admitted in 1931 to Pembroke College in the University of Cambridge, where he studied chemistry, geology and mineralogy. He graduated in 1934 and returned to South Africa, being appointed to the Geological Survey of South Africa as one of its first geophysicists and contributing to magnetometric and gravimetric surveys in the Transvaal region, expertise used in 1938 to 1940 to draft his thesis for a PhD degree (awarded in 1942). Although married in 1939 and briefly employed as a consultant geophysicist, in August 1940 Paver was one of the first three geoscientists to be mobilized as officers to found 42nd Geological Section, at Zonderwater near Pretoria in South Africa. After only a month's military training, at the end of September the Section and its vehicles deployed by rail and sea to a base near Nairobi in Kenya for operational service in East Africa, with ‘Acting Captain’ Paver as its Second-in-Command. Detachments from the Section were widely deployed in Kenya and later in Italian and British Somaliland (present-day Somalia) and also in Abyssinia (present-day Ethiopia) for surveys by means primarily of electrical earth resistivity but also vertical force magnetometer. These guided drilling of wells by another unit of the South African Engineer Corps to abstract potable groundwater— thereby facilitating troop concentrations and forward movements in arid or semi-arid regions during the ‘British’ Army's East African Campaign. Members of the Section also compiled geological maps of Kenya at scales of 1:1,000,000 and 1:2,000,000 and pioneered a military geological unit created within the East African Engineers that supported British forces in the region from 1941 to 1945. The Campaign drew to a victorious close during 1941 and, from the end of August, the Section was re-deployed northwards to a base near Cairo in Egypt. It continued to serve within the British Army's Middle East Command but with leadership now by Paver, promoted ‘Acting Major’ from 31 August and in December ‘mentioned in despatches’ for his earlier distinguished service in East Africa.

GEOLOGICAL AND GEOPHYSICAL STUDIES AND EXPLORATORY DRILLING OF PALEOZOIC SEDIMENTS IN THE TASBULAK GRABEN

Despite the half-century history of prospecting, the geological and geophysical knowledge of the Shu-Sarysu basin does not exceed 50%. At the same time, the Tasbulak graben remains the most unstudied. From 1941 to 1969 Regional and exploratory aeromagnetic surveys using Z-aerial magnetometers and fluxgate magnetometers at scales of 1:500,000-1:100,000 covered the areas of the western, northwestern and partially central parts of the Shu-Sarysu depression. From 1969 to the present, up to 70% of the area of the Shu-Sarysu depression has been covered with high-precision surveys using proton, fluxgate and quantum aeromagnetometers on a scale of 1:50,000-1:25,000. In fact, the entire territory of the Shu-Sarysu depression is covered by areal gravimetric surveys on a scale of 1:200,000, scales of 1:50,000-1:100,000 - about 70-75% of the area. In the west and southeast of this depression, individual areas were worked out by detailed surveying on a scale of 1:25,000. Over the entire period of satellite surveys (from 1972 to the present), a significant archive of high, medium and low-resolution images has been accumulated. Seismic exploration of the territory of the Tasbulak graben using the CDP and CDP methods was carried out in the period from 1968 to 1993. Seismic exploration using the CDP - 3D method was not carried out. With a total area of the basin of more than 150 thousand square kilometers, the degree of its drilling exploration is extremely low (about 1 well per 375-475 square kilometers). The degree of study of the Tasbulak graben is even less. As a result of prospecting and exploration work, 12 gas and gas condensate deposits were identified, including: 6 in the Moyynkum graben, 3 in the Kokpansor graben and 3 on the Talas uplift (helium). The totality of geophysical work carried out within the Tasbulak graben indicates a premature cessation of oil and gas exploration in the region and the need for detailed work to verify the presence of more complex structures and their reservoir properties.

Overdeepenings in the Swiss plateau: U-shaped geometries underlain by inner gorges

We investigated the mechanisms leading to the formation of tunnel valleys in the Swiss foreland near Bern. We proceeded through producing 3D maps of the bedrock topography based on drillhole information and a new gravimetric survey combined with modelling. In this context, the combination of information about the densities of the sedimentary fill and of the bedrock, together with published borehole data and the results of gravity surveys along 11 profiles across the valleys, served as input for the application of our 3D gravity modelling software referred to as PRISMA. This ultimately allowed us to model the gravity effect of the Quaternary fill of the overdeepenings and to produce cross-sectional geometries of these troughs. The results show that 2–3 km upstream of the city of Bern, the overdeepenings are approximately 3 km wide. They are characterized by steep to oversteepened lateral flanks and a wide flat base, which we consider as a U-shaped cross-sectional geometry. There, the maximum residual gravity anomaly ranges between − 3 to − 4 mGal for the Aare valley, which is the main overdeepening of the region. Modelling shows that this corresponds to a depression, which reaches a depth of c. 300 m a.s.l. Farther downstream approaching Bern, the erosional trough narrows by c. 1 km, and the base gets shallower by c. 100 m as revealed by drillings. This is supported by the results of our gravity surveys, which disclose a lower maximum gravity effect of c. − 0.8 to − 1.3 mGal. Interestingly, in the Bern city area, these shallow troughs with maximum gravity anomalies ranging from − 1.4 to − 1.8 mGal are underlain by one or multiple inner gorges, which are at least 100 m deep (based on drilling information) and only a few tens of meters wide (disclosed by gravity modelling). At the downstream end of the Bern area, we observe that the trough widens from 2 km at the northern border of Bern to c. 4 km approximately 2 km farther downstream, while the bottom still reaches c. 300 to 200 m a.s.l. Our gravity survey implies that this change is associated with an increase in the maximum residual anomaly, reaching values of − 2.5 mGal. Interestingly, the overdeepening’s cross-sectional geometry in this area has steeply dipping flanks converging to a narrow base, which we consider as V-shaped. We attribute this shape to erosion by water either underneath or at the snout of a glacier, forming a gorge. This narrow bedrock depression was subsequently widened by glacial carving. In this context, strong glacial erosion upstream of the Bern area appears to have overprinted these traces. In contrast, beneath the city of Bern and farther downstream these V-shaped features have been preserved. Available chronological data suggest that the formation of this gorge occurred prior to MIS 8 and possibly during the aftermath of one of the largest glaciations when large fluxes of meltwater resulted in the fluvial carving into the bedrock.

Predicted resource assessment of Central Kazakhsta ore districts based on airborne geophysical methods

Purpose. The research is aimed at creating a high-quality geophysical basis for the additional geological study of Central Kazakhstan in determining the geological structure of the study area, identifying promising areas for further geological exploration, as well as analyzing the anomalous distribution of various minerals. Methodology. The research uses the methods of aeromagnetic, airborne gamma-ray spectrometry and gravimetric (ground) surveys. The obtained new airborne geophysical data are used for additional study of the geological structure of the district and the creation of a model of the geological structure of the study area. Gamma-ray spectrometry data analysis is conducted for detailed mapping of intrusive complexes and study of their lithological heterogeneity. Findings. The processing and interpretation of the materials of the conducted field studies in combination with the results of the geological-geophysical data analysis made it possible to refine the geological structure and to present a model of ore-prospecting complexes in the study area. Areas with an anomalous distribution of potassium, uranium, and thorium have been identified, which make it possible to assess the structural heterogeneity of hidden magmatic massifs and to reveal a connection with gold, copper-polymetallic and rare-metal mineralization. Recommendations for further detailed geological exploration, including prospecting-exploration drilling, are given. Originality. Research has shown the high efficiency of airborne geophysical methods, such as aeromagnetic and airborne gamma-ray spectrometry surveys to study the geological structure of ore areas, and in combination with detailed gravimetric exploration to identify lithological heterogeneity of igneous and sedimentary rocks in the study area. New scientific-methodological techniques and the geological structure model based on them gave an opportunity to assess the prospect of the study area for ore mineralization. Practical value. The identified promising areas for mining of minerals are of interest for further research and exploration. The integrated use of geological-geophysical data will make it possible to determine more precisely the nature of radioactive anomalies and to reveal their relationship with ore-prospecting horizons. In general, the research results contribute to increasing the efficiency and reducing the geological survey costs.

Results of geophysical research on the Zmiyinyy island

The article presents the results of geophysical observations by the Dnepropetrovsk geophysical expedition «Dneprogeofizika» on Zmiyinyy Island, which performed a full cycle of work from prospecting and surveying studies to providing the island with drinking water.
 In structural and tectonic conditions, the island is considered as the Zmiyinyy insular block of the Danubian horst megablock. The geodynamics of the region is determined by the position and inclination of the collision sutures within the zone of interaction between the Eurasian and African lithospheric plates. Comprehensive geological and geophysical studies included magnetic prospecting, gravity prospecting and electric sounding (Dipole-axial sounding induced polarization DAS-IP) with geological routes along all rock outcrops on the Zmiyinyy island. Detailed integrated geophysical studies were: gravimetric observations over a network of 20×10 m, magnetometric — 10×5 m and electrometric (DAS-IP) — 40×5 m. For the first time on the island, gravimetric stations were set up, which will later become a support for gravimetric surveys. According to the results of magnetic field survey and DAZ-OP electrical exploration observations, an anomalous area was identified, which is perspective for archaeological research. Based on a comprehensive interpretation of geological and geophysical materials, a well was drilled to a depth of 120 m, which confirmed the constructed predictive stratigraphic section of about. Zmiyinyy. Based on the results of the search for sources of drinking water, three wells were drilled, with a fresh water flow rate of up to 10 m3 per day (each). Subsequently, fresh water with minor purification is used as drinking water.

Nonlinear vibration feature extraction based on power spectrum envelope adaptive empirical Fourier decomposition

Analyzing the vibration features of a shipboard stabilized platform (SSP) is significant to the design or vibration compensation of a marine gravimetric survey vibration isolation system. Empirical Fourier decomposition (EFD) is a recently developed method for nonlinear and non-stationary signal decomposition. However, the spectral segmentation boundary needs to be set in advance according to sophisticated experience, and it is easy to be disturbed by noise, and the decomposition result is inaccurate. In order to accurately extract the nonlinear vibration characteristics of SSP, this paper proposes a new method called power spectrum envelope adaptive empirical Fourier decomposition (PSEEFD). Firstly, the number of selected modal decomposition is determined based on the mutual information to realize adaptive segmentation. Then, the improved power spectrum envelope segmentation method is adopted to effectively diminish the interference of noise since the segmentation boundary is formed by the minimum of the adjacent extreme points enveloped by the maximum value of the power spectrum. The spectrum segments obtained from segmentation contain less interference. Finally, the component signal in each frequency band is reconstructed by inverse fast Fourier transform, and the instantaneous frequency signal component with physical significance is obtained. Through the analysis of vibration simulation signals and measured data of SSP, the proposed method is compared with EMD, AFVMD, EWT and EFD. The results show that PSEEFD has a well suppression of noise interference and can effectively extract the characteristics of nonlinear vibration signals.

Global geopotential models evaluation based on terrestrial gravity data over Ethiopia

Abstract The availability of high-degree and recent global geopotential models is a crucial resource for different geodetic and geophysical applications such as modelling of geoid and quasi-geoid and establishing global reference and height systems, estimating Moho depth, gravity anomaly and tectonics, other geopotential functional, and oceanography, which emphasizes the importance for model evaluation. We have therefore estimated complete Bouguer anomalies and Moho whose results are evaluated with the terrestrial values. We validate the recently released satellite-only and high-degree combined GGMs over Ethiopia using terrestrial gravity data based on a statistical comparison of the Bouguer anomaly, its residual and Moho depth. The terrestrial-derived gravity anomaly is more accurately estimated by EIGEN6C4. The assessment against a recently conducted high resolution (∼3 km) terrestrial and airborne gravimetric survey over Ethiopia shows that EIGEN6C4/SGG_UGM_1 and 2 have the highest accuracy (∼3.28/3.27 mGal). However, the comparison with such data hardly discriminates the qualities of other GGMs that have or are truncated to the same degree and order. Whereas, the validation results of GGMs against terrestrial and airborne data are identical. EIGEN6C4, SGG_UGM_2, XGM2016, XGM2019e_2159/SGG_UGM_1 have the best quality, and the accuracy of associated Moho is 4.89/4.90 km, and this value changes to 4.98/4.91/5.51 km when the EGM08/ITSG_Grace2018s/GOCO06S are assessed.

Производственная стажировка в Северо-Восточном аэрогеодезическом предприятии

The content of the author's production internship in the North-Eastern AGP GUGK under the Council of Ministers of the USSR in 1989 is considered. The technology of performing ice aviation gravimetric survey in the water area of the Chukchi Sea, as well as the technology of surveying the sea shelf of the Sea of Okhotsk, in which the author took part, is outlined.

Defining Lithospheric Limits with Deep Sourced Geophysics: The Case of São Francisco Craton and Borborema Province

The East Brazilian Shield is a product of the Neoproterozoic convergence and collage of rigid Archean lithospheric blocks, which stabilized at the end of the Paleoproterozoic Trans-Amazonian event (2.0 Ga). The areal extent of the São Francisco Craton is well understood by regional gravimetric surveys, but its northern limit with the Borborema Province is not so clear. The integration of gravity, magnetics and seismic velocity anomaly maps allows mapping this northern limit to the Borborema Province. Applying the vertical integration low-pass filter to satellite magnetics yields a remarkable match to the probable lithospheric suture zone. This magnetic map identifies the deep response from thinner/thicker lithospheres of Neoproterozoic mobile belts or stable cratons. One potential microcraton is the NE extension of the Borborema Province, one site of the oldest rocks in South America, a residue of the Benino-Nigerian Craton, separated from the main occurrence by the opening of the Atlantic. The Sergipana, Riacho do Pontal and Rio Preto Neoproterozoic fold-and-thrust belts mark areas of the foreland tectonism, hundreds of kilometers inside the São Francisco Craton lithospheric block. The presence of younger rocks inside the deep geophysical limits may be evidence of a large detachment of younger continental crust over the old cratonic lithosphere.

Improving the geodetic infrastructure for bathymetry and 3D navigation in the German exclusive economic zone of the North and Baltic Sea

Surveying and navigation became much easier, more accurate and operational thanks to the Global Positioning System. The use of this technology in height determination, bathymetry and 3-D navigation is not only limited by the reduced accuracy compared to the horizontal component. It supposes additional information about the geodetic height reference surface in order to leverage the full potential of this technology. The corresponding models, measurements and activities which are necessary to improve this part of the geodetic infrastructure are usually behind the curtain. This article emphasizes the need of a common cross-border geodetic infrastructure and the relevance of precise models of the height reference surface for GNSS-aided height determinations. The need of gravimetric surveys for the determination and improvement of these models is explained. Finally, it gives an overview about the gravimetric surveys which were carried out in the German Exclusive Zone of the North and Baltic Sea over more than one decade and provides some insight into practical aspects and challenges of this kind of surveys.

Mineralogical, geochemical, and isotopic data of a new special agpaitic dyke, enriched in high field strength elements (Eastern Part of Baltic Shield, Russia)

We present the petrography, bulk major and trace elements, and isotopic compositions for the recently identified Mohnatye Roga agpaitic syenite dyke, located 30 km from the Kovdor carbonatite complex, Kola Peninsula, Russia. The dyke was identified below Quaternary deposits as the result of a gravimetric survey and subsequent drilling. A K–Ar age on lamprophyllite of 368 ± 9 Ma, coincides with the age of the Devonian Kola Alkaline Carbonatite Province (KACP). In the total alkalis vs. silica diagram, the Mohnatye Roga dyke exhibits a compositional trend from lamprophyre to peralkaline phonolite as well as mineralogical and geochemical affinities with the nearby Niva intrusion. The dyke is composed of orthoclase, Ti-rich aegirine-augite, aenigmatite, lamprophyllite-group minerals, K-amphibole, and astrophyllite with accessory noonkanbahite, calcite, ilmenite, rutile, barite, apatite, and sulfides. Late stage minerals include natrolite and ferripyrophyllite. A notable feature of this dуke is its extremely high agpaicity and enrichment in HFSEs (high-field strength elements). The concentrations of a number of HFSEs are higher in this dyke than in the Lovozero rare metal deposit. Mohnatye Roga dyke and Niva intrusion are examples of a gradual transition from melt to hydrothermal fluid. Isotopic data indicate a mantle source for the dyke, and seemingly an absence of contamination by continental crust during its formation. The geochemical and isotopic data suggest that the source of the KACP rocks, including the Mohnatye Roga dyke, is FOZO mantle which is believed to be related to plume magmatism. • Origin and age of the recently identified lamprophyllite-bearing agpaitic syenite Mohnatye Roga dyke (Kola peninsula) • Great similarity with the Niva intrusion and the link to Devonian Kola Alkaline Carbonatite province • Agpaitic SiO 2 -undersatured melts preserving the depleted FOZO plume signature • Enrichment in HFSEs of Mohnatye Roga dyke and comparison with gigant Lovozero and Khibiny deposits.

FEATURES OF THE GEOLOGICAL STRUCTURE AND SEISMOTECTONICS AT THE STS AREA BY EARTH REMOTE SENSING DATA

The general methodological approach on using specialized remote sensing data of the Earth (ERS) on the territory of the Semipalatinsk Test Site area to identify active faults and seismogenic structures potentially dangerous with respect to the occurrence of earthquakes is presented. Specialized ERS data include data from aerogeophysical surveys, satellite multispectral, radar and gravimetric surveys. Examples of different ERS data application to identify active faults and areas with excess mass of the geological environment associated with the seismicity at the studied area are shown.

Depth structure of the Gorishne-Plavninsk structure and general principles of geological and geophysical study of the Krivoy Rog-Kremenchug iron-ore strip

The article presents the results of the newest research performed by the State Enterprise «Ukrainian Geological Company» and the Institute of Geophysics of the National Academy of Sciences of Ukraine in 2021 to find out the morphology and elements of occurrence of an redeposit within the limits of the Gorishne-Plavninsk-Lavrikovsk area of the Krivoy Rog-Kremenchug iron-ore strip. In the process of geological and geophysical studies, a large body of new geological and geophysical data was obtained regarding the deep structural and morphological features of the construction of the Gorishne-Plavninsk structure (GPS), associated with the Krivoy-Rog deep fracture zone.
 Materials of reinterpretation of previously per formed ground-based magnetometric and gravimetric surveys, detailed high-altitude aeromagnetic studies and the study of the density and magnetic properties of the structure rocks, including the anisotropy of magnetic susceptibility using new equipment and techniques, gravimagnetic modeling with the construction of three-dimensional volume model of the structure lead to the conclusion that this structure is a typical drag fold with a pronounced reversal of its southern part by the right-hands hear. Its primary formation occurred as a faulted band of sedimentary-volcanogenic and extrusive formations of subvertical bedding and spreading to a depth of at least 1.5—2 km.
 A complete morphological and kinematic analogue of the GPS is the previously studied Likhmanovsk structure, located in the southern most part of the Krivoy Rog-Kremenchug strip. Many iron-ore structures of the Middle Bug area are close to it in structure and kinematics, the primary subvertical faulting and doubtful sedimentary genesis were mentioned earlier. All these data require a new approach to the study of iron-ore and some other deposits of the Ukrainian Shield associated with fault tectonics, as well as to the genesis problems and stratigraphy of rocks of the Early Precambrian.

Geomechanical behavior of crown pillar between ground surface and mined-out void roof in mining blind ore body on Sheregeshevsky deposit

Sheregesh Mine extracts ore reserves at the Levels of +255 ÷ +115 m in four sites at the depths of 470–700 m and deeper and produces annually more than 4.5 Mt of ore. The upper ore edge in Podruslovy site is spaced at 290–320 m from ground surface. The major ore reserves occur within the depth interval from 300 to 700 m below ground surface. Enclosing rock mass is composed of limestone, porphyry, skarn and other rocks which belong to stability class II. Considering the physical and mechanical properties of ore and enclosing rocks, the effective stresses and the geodynamic phenomena observable beneath the Level of +255 m, Sheregesh Mine operates in the rockburst-hazardous conditions. The geomechanical behavior of a crown pillar between ground surface and mined-out void roof in mining the ore body in Podruslovy site under conditions of rockburst hazard is assessed. The influence of the room-and-pillar and subsoil caving variants on the roof of the mined-out void is shown. The mathematical modeling delineates the concentration zones of stresses and inelastic strains in adjacent enclosing rock mass. The sizes of the crown pillar are estimated experimentally using geophysical methods. The pillar thickness ranged 420 to 300 m, then, three boreholes drilled from ground surface showed the thickness variation in the range from 309 to 12.7 m. Using the gravimeter survey in combination with orthophotomap, the outlines of the underground void and the ground surface subsidence are determined. The configuration layouts of holes for dry rock backfill by-pass for filling the void in Podruslovy site are presented.

ПРИМЕНЕНИЕ ВОЗДУШНЫХ ЛИДАРОВ В ВЫСОКОТОЧНОЙ ГРАВИРАЗВЕДКЕ (НА ПРИМЕРЕ ВОСТОЧНОГО ЗАБАЙКАЛЬЯ)

In the production of high-precision gravimetric studies at ore deposits, an important factor is the methodical correctness of the work, which directly affects the subsequent costs and life cycle of a geological exploration enterprise. It is known, that gravimetric survey, in addition to the actual stage of measurements on the ground, includes the stage of eliminating errors introduced into the measurements by external fields and objects, in particular, the terrain. Methods existing and adopted at the state level involve the use of materials from cartographic funds (topographic maps at a scale of 1:100,000 - 1:25,000 and larger). It is also allowed to use open (free) terrain models. These materials are distinguished by a common characteristic feature - the lack of detail in thedisplay of the microrelief and steep inclined surfaces (slopes, walls, faults, incisions). These elements have a significant effect on the field values, measured by gravimeters, especially when measurements, are taken at points located at a small distance from the specified forms. In addition, the existing methods do not involve calculations using digital elevation models built from initial data with a high density of elevations (with a step of a few meters or more). All this creates the prerequisites for an insufficiently complete consideration of corrections in gravimetric measurements. At the same time, the technical capabilities of modern aviation remote sensing equipment make it possible to quickly obtain a high-precision digital model of the terrain over large areas. The most accurate, versatile and promising technology in this respect is the method of airborne laser (lidar) scanning. In this paper, the authors consider a methodology for performing work using both airborne laser scanning of the relief and other types of data (topographic maps 1:25,000, open data models). As part of the research, calculations were also made of the final corrections for the influence of the terrain and a comparison of the results obtained with each other. With the help of mathematical modeling and cartographic algebra, matrices (grids) of design parameters were constructed. The results of the study are clearly demonstrated in diagrams and diagrams, substantiating and illustrating the high information content of the study

Using High–Low Flight Gravity Data to Improve Geoid Model Precision: A Case Study in U.S. Virgin Islands (July 2021)

The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project collected airborne gravity data over Puerto Rico (PR) and the U.S. Virgin Islands (VI). There was one large N-S/E-W survey at about 11-km altitude running from N17 to N21 latitude and E291 to E297 longitude. Considering the very limited surface gravity coverage on VI, only about 500 points, additional flights were conducted at lower altitudes (1.5–1.8 km). By adding these additional low altitude aerial gravity data, the geoid model precision is improved from 6.55 cm with no airborne data to 2.55 cm with both the high and low altitude data. Using only the high altitude flights achieves only 4.99-cm geoid model precision. The ~500 surface gravity data points in the target area do not improve the model precision significantly, which justifies the rationale of carrying out these lower altitude flights. Noticing the nearly three times geoid model precision improvements from the low flight data, we suggest to fly in low altitudes in future airborne gravimetric survey for geoid modeling in the areas that have limited surface gravity coverage and/or small topographic features like islands that are not well-sampled in high-altitude, wider-spaced data.

ОПРЕДЕЛЕНИЕ ПЕРЕМЕННОЙ ПЛОТНОСТИ ПРОМЕЖУТОЧНОГО СЛОЯ НА ГУБАХИНСКОМ УЧАСТКЕ (ПЕРМСКИЙ КРАЙ)

Because of the improvement of instrumentation and technology progress in the conducting of gravity survey, there is a continuous increase in the accuracy of calculating the gravity Bouguer anomalies. At the same time, due to inadequate assessment of the intermediate layer density taken as a constant for reduction, the calculated anomalies may contain significant relative error. The article describes the experience of using the three-point formula of V.M. Berezkin to build a map of the areal distribution of the density of the intermediate layer and calculating, on its basis, anomalies in the Bouguer reduction according to high-precision gravimetric surveys at the Gubakha area.

Integrated assessment and prospectivity mapping of geothermal resources for EGS in Brazil

Studies in Brazil related to geothermal resources have been carried out mainly by the National Observatory. Based on the results vast areas of Brazil have been delimited with a surface heat flow of over 60 mW/m2 and an excess temperature above 100 °C at depths between 3 and 6 km. The nature of the heat source and the mode of heat dissipation in these areas are largely unknown. Geothermal resources that might be associated with these areas can occur in hot sedimentary aquifers (HSA) and units of hot dry rocks (HDR) that are hosted, respectively, in intracratonic sedimentary basins and high heat producing granites (HHPG). The focus of our study is on finding resources of high enthalpy (above 150 °C) in HHPG, which regardless of their hydrological properties (porosity, permeability) could be considered as potential targets for enhanced geothermal systems (EGS). The structural provinces of Tocantins and Borborema were chosen as pilot regions based on both the indication of HDR and the wealth of open access georeferenced information. A comprehensive database was compiled and processed under GIS that includes geology, faults, soil typology, radiogenic heat, heat flow and geothermal gradient from wells, thermal spring locations, airborne gamma-ray spectrometry and magnetic surveys, terrestrial gravimetric surveys, and earthquake locations. In parallel, various critical factors were defined primarily to identify and characterize the most promising HHPG under insulating sedimentary rocks for EGS. These critical factors were grouped into three main categories labelled as: ‘thermal source’, ‘thermal insulation’ and ‘active pathways’. In the absence of exploratory data on existing EGS in Brazil these critical factors were used in this study along three separate work streams to develop a knowledge-driven model based on fuzzy logic for improving the knowledge on the prospectivity of geothermal resources. The developed model was further tested for its validation. The selected approach is expected to be applicable to mapping areas with EGS potential in other regions with similar geology and open access georeferenced databases.

An Assessment of Recently Released High-Degree Global Geopotential Models Based on Heterogeneous Geodetic and Ocean Data

The development of the global geopotential model (GGM) broadens its applications in ocean science, which emphasizes the importance for model assessment. We assess the recently released high-degree GGMs over the South China Sea through heterogeneous geodetic observations and synthetic/ocean reanalysis data. The comparisons with a high resolution (∼3 km) airborne gravimetric survey over the Paracel Islands show that XGM2019e_2159 has relatively high quality, where the standard deviation (SD) of the misfits against the airborne gravity data is ∼3.1 mGal. However, the comparisons with local airborne/shipborne gravity data hardly discriminate the qualities of other GGMs that have or truncated to the same expansion degree. Whereas, the comparisons with the synthetic/ocean reanalysis data demonstrate that the qualities of the values derived from different GGMs are not identical, and the ones derived from XGM2019e_2159 have better performances. The SD of the misfits between the mean dynamic topography (MDT) derived from XGM2019e_2159 and the ocean data is 2.5 cm; and this value changes to 7.1 cm/s (6.8 cm/s) when the associated zonal (meridian) geostrophic velocities are assessed. In contrast, the values derived from the other GGMs show deteriorated qualities compared to those derived from XGM2019e_2159. In particular, the contents computed from the widely used EGM2008 have relatively poor qualities, which is reduced by 3.9 cm when the MDT is assessed, and by 4.0 cm/s (5.5 cm/s) when the zonal (meridian) velocities are assessed, compared to the results derived from XGM2019e_2159. The results suggest that the choice of a GGM in oceanographic study is crucial, especially over coastal zones. Moreover, the synthetic/ocean data sets may be served as additional data sources for global/regional gravity field assessment, which are useful in regions that lack of high-quality geodetic data.