Detailed Gravity Research Articles

Influence of thick lithomargic soil cover on Bouguer gravity low: Imprints from passive continental margin of southwestern India

The Periyar Plateau in central Kerala is characterized by a strong negative Bouguer gravity field around three major islands of a strongly negative field of the value of -150 mGal. Detailed gravity studies reveal that these islands of strong negative field are of a shallow nature and are not traceable in the 1/4° average gravity grids. Spectral analysis of Bouguer anomaly map and 2D gravity modelling of Periyar Plateau region reveal the average depth of the shallow source layer is as shallow as 700-1000 m, which is correlatable with the layer of lateritic/lithomargic soil cover and the underlying weathered country rock having a density of 2.2 g/cc. Such a strong influence on the regional gravity field is due to the wide contrast in the densities of the lateritic cover and the primary rocks of the basement. These results underline the need to recognize the strong influence of such alteration zones on the regional gravity fields, especially the tropics, a factor not always considered.

A detailed gravity picture of Nallamalai Fold Belt–Nellore Schist Belt (NSB) of Eastern Dharwar Craton with marginal Eastern Ghats Mobile Belt in Krishna Region, South India with special reference to Late Archaean NSB evolution

A detailed gravity picture of Nallamalai Fold Belt–Nellore Schist Belt (NSB) of Eastern Dharwar Craton with marginal Eastern Ghats Mobile Belt in Krishna Region, South India with special reference to Late Archaean NSB evolution

Stable downward continuation of the gravity potential field implemented using deep learning

Downward continuation (DC) of the gravity potential field is an important approach used to understand and interpret the density structure and boundary of anomalous bodies. It is widely used to delineate and highlight local and shallow anomalous sources. However, it is well known that direct DC transformation in the frequency domain is unstable and easily affected by high-frequency noise. Recent deep learning applications have led to the development of image recognition and resolution enhancement using the convolutional neural network technique. A similar deep learning architecture is also suitable for training a model for the DC problem. In this study, to solve the problems in existing DC methods, we constructed a dedicated model called DC-Net for the DC problem. We fully trained the DC-Net model on 38,400 pairs of gravity anomaly data at different altitudes using a convolutional neural network. We conducted several experiments and implemented a real-world example. The results demonstrate the following. First, several validation data subset and test data prediction results indicate that the DC-Net model was sufficiently trained. Moreover, it performed better than the traditional strategy in refining the upscaling of low-resolution images. Second, we performed tests on test datasets with changing levels of noise and demonstrated that the DC-Net model is noise-resistant and robust. Finally, we used the proposed model in a real-world example, which demonstrates that the DC-Net model is suitable for solving the DC problem and delineating the detailed gravity anomaly feature near the field source. For real data processing, noise in the gravity anomaly should be reduced in advance. Additionally, we recommend noise quantification of the gravity anomaly before network training.

Crustal configuration of the Naama and El Bayadh region of northwest Algeria: Inferences from gravity and magnetic analysis

Northwestern Algeria is located where the Saharan Atlas borders the Saharan Platform along the South Atlas Front. The origin of the South Atlas Front is controversial, thus a detailed gravity and magnetic analysis constrained by seismic reflection profiles and well data were conducted to determine the structural configuration of this region. A residual gravity anomaly map created using upward continuation and a reduced to the pole magnetic anomaly map indicated a series of northwest-trending maxima anomalies parallel to the Atlassic orogeny folds and faults and east-trending maxima within the Benoud foreland basin. These maxima mostly coincide with Paleozoic basement uplifts based on seismic reflection profiles. Depth analyses based on upward continuation, and two-dimensional forward modeling of the gravity and magnetic data indicated that the source of the maxima are mainly 5 km in depth with the magnetic sources being approximately 0.5 km deeper than the gravity sources. The gravity and magnetic models indicate that the source bodies are steep-sided and coincide with interpreted faults from the seismic reflection profiles. The maxima anomalies are mainly caused by basement uplifts instead of variations in density and/or magnetic susceptibility in the Paleozoic or older basement lithologies. The South Atlas Front and the Saharan Atlas on the central and eastern portions of Algeria is governed by a thin-skinned tectonics style involving the Mesozoic-Cenozoic sedimentary cover. While the western part was probably controlled by a mixed thin- and thick-skinned tectonic style with the reactivation of deeply rooted Triassic and Jurassic faults during Tertiary compressive events in the interpreted Paleozoic basement.Our gravity and magnetic models illustrate a crustal architecture model of the Paleozoic basement which is consistent with a north-dipping basement normal fault between less deformed Saharan Platform than the more deformed Atlassic domain. Such a crustal model aids in determining the nature of the Atlassic orogeny as geological and geophysical studies have been determining the variations of structural styles along its entire length.

Gravity Survey on the Oil-Bearing Dammam Dome (Eastern Saudi Arabia) and Its Implications

A detailed gravity survey with 235 measurements was carried out at the King Fahd University of Petroleum and Minerals campus, which is located at the crest of the oil-bearing Dammam Dome (Eastern Province of Saudi Arabia). This survey allows us to better understand the geometry of the underlying Dammam Dome and its tectonic regime. The acquired data were processed using conventional gravity data reduction techniques. The effectiveness of terrain correction was evaluated using several recently developed algorithms. Afterward, processed data were subject to geophysical filters for edge detection (terracing transformation and horizontal gradient) and depth estimation (tilt derivative and 3D inversion). 3D Bouguer maps were generated and compared to the proposed geological models for the Dammam Dome. The results show the existence of ENE-WSW striking tectonic lines, where two nearly vertical, km long tectonic lines were predominant. The orientation of these tectonic lines defines an NNW-SSE trend for the least principal stress axis (σ3) and an ENE-WSW trend for the σ1σ2 stress plane of the driving stress regime, fitting well with the transtension stress regime recently suggested for the area. More importantly, the results of this study demonstrate that the Dammam Dome was affected by the intraplate stresses transferred from the convergence between the Arabian and Eurasian plates along the Zagros orogeny.

Density heterogeneity of the Earth’s crust of Ukraine and adjacent territories from three-dimensional gravity modelling

The work was performed to study the density heterogeneity of the crust of Ukraine and adjacent regions analyzing detailed 3D gravity models of the following tectonic regions and separate structures: Dnieper-Donets Basin and Donbas, Ingulsky megablock, Golovanivska suture zone, latitudinal zones of fractures of the Ukrainian Shield, Black Sea megadepression and surrounding areas. The models are constructed on the basis of geological, petrophysical and seismic data along geotravers, DSS profiles, the modern WARR methodsand are calculated using the automated software complex for interpretation of potential fields GMT-Auto. As a result, new information was obtained for the density distribution in the whole Earth’s crust.The density distributions construct for the entire crust of Ukraine and adjacent regions at certain depths (surface of the basement, 10, 20, 30 km, Moho discontinuity). The thickness was determined for conditionally distinguished «granite», «diorite», «basalt» layers of the Earth’s crust and crust-mantle mixture within the whole region of the study. Its values are graphically represented by isolines on the respective schematical maps. The resulting density inhomogeneity became the basis for constructing, for the first time in a three-dimensional version, a scheme for the predictive composition of the Earth’s crust with typification according to the power ratios of its constituent layers. The heterogeneity of the present-day consolidated crust of the whole study area is characterized by granitic, granitic-dioritic, dioritic and basaltoid types. To clearly identify the dismemberment of the lower layer of the crust which is considered inseparable by the classification of the DSS method the basaltoid type is divided into 3 subtypes. The subtype 1 with a content of 40—70 % of basalt, subtype 2, where concent-ration of basalt increases to 70—90 % and subtype 3, which is composed only of basalt.It has been established that the basicity of the Earth’s crust of the USh changes from west to east, respectively, from basalt to diorite type. Decrease in crustal basicity is also observed in suture zones, namely from diorite and basaltoid typein Golovanivskii suture to basaltoid and diorite type in the Inguletsko-Kryvorizckii suture and diorite type in the Orikhivsko-Pavlogradskii suture. In the DDB the crustal basicity increases from northwest to southeast in accordance with the segmentation of this feature. The basaltoid type of the crust is predominant in the Black Sea megadepression and adjacent territories. For the first time, a type of crust was identified that does not coincide with the known ones and is characterized by the presence in the section of the «basalt» and «granite» layers (with a zero or very low «diorite» thickness). A similar type of bark is present in the Ingul megablock of the USh, in the northern side zone of the DDB, as well as in the Black Sea megadepression.

Tectonic controls on the salt diapir-related Mississippi Valley-type lead-zinc mineralization of Fej El Adoum ore deposit (Northern Tunisian Atlas): Constrains from detailed gravity and drill hole data

Relationship between salt domes and base metal mineralization is widely recognized across many salt-bearing basins (e.g., US Gulf Coast, Southern Europe and North Africa). In the northern Tunisian Atlas, the Mississippi Valley-type Fej El Adoum lead-zinc deposit represents one of these salt-related ore deposits. It consists of a combination of stratabound, veins, stockworks and cavity filling ore bodies, hosted by dolostone and limestone units, formed along the transition between Triassic evaporites and Cretaceous limestone.In the aim to better understand the concealed complex geological structures across the area, new-acquired detailed gravity data, constrained with field investigation and drill hole data were analyzed. Particularly, edge source detection techniques such as vertical gradient, total horizontal gradient and theta angle map were applied to outline structural contacts related to subsurface density variations. The main detected lineaments are in N–S, E-W and NW-SE directions. The N–S direction represents the deepest and most important lineament in direct contact with the evaporitic Triassic, while the transverse E-W direction is related to dextral strike-slip faults that subdivide the area into blocks. These major deeper lineaments have controlled the growth of the Triassic salt dome mainly during the Albo-Aptian as revealed by pinch outs of these series in direction of the crest of the structure. The detected lineaments seem to play a significant role in: (1) providing favorable pathways for the migration and ascent of mineralized fluids and (2) creating open space filling mainly at the interferences of N–S and E-W trending faults present at the contact between Triassic evaporitic unit and Cretaceous limestone. Our results suggest further reserves mainly in the E-W Ain Said occurrence and West of the Dar N'Hal closed mine.Our study also revealed the importance of structural study in the understanding of complex structures such as highly deformed salt dome cap rocks such as the Fej El Adoum diapiric structure.

Integrated geological and geophysical studies for delineation of laterite covered chromiferous ultramafic bodies around Bhuban, southwestern part of Sukinda ultramafic complex, Odisha

Chromite and Platinum Group of Elements (PGE) mineralization in the Eastern Indian Shield region is found in the ultramafic rocks distributed in the Baula-Nuasahi and Sukinda massifs intrusive into the Iron Ore Group (IOG) of rocks. In addition to this, recent geological studies have also reported the occurrences of chromiferous ultramafic bodies within the granulite tracts of Eastern Ghat Group around Asurabandha and Maulabhanja areas in the southwestern part of Sukinda Massif. In the present study, detailed gravity and magnetic surveys (1510 stations) were conducted over a laterite covered area of ~0.16 km2 at Bhuban-Asurabandha to locate the anomalous zones/sources, the geometry of the chromite bearing host rocks and its depth continuity in the subsurface. Four distinct coincident positive gravity anomalies (up to ~0.18 to 0.36 mGal) and residual magnetic anomalies (~−200 to 350 nT) with the NE-SW strike indicate the presence of high density mafic/ultramafic bodies under the laterite cover. The results of 3-D Euler deconvolution and two-dimensional compact gravity inversion models in this region further suggest the presence of high-density material (~3100–3600 kg/m3) at a depth of 20–40 m in the subsurface. Exploratory drill hole data in the area confirms the subsurface occurrence of thin chromiferous serpentinite as lensoidal and sandwiched bodies within the granulite rocks of Eastern Ghat Group. Petrological studies of the drill core samples reveal the occurrences of disseminated and thin banded chromite within the serpentinites which is identical to the Sukinda ultramafics in terms of its mineralogy and alteration. On the other hand, the surrounding mafic granulite rocks contain occasional magnetite and sulfides within the granulite rocks. Hence, the chromiferous ultramafic bodies within the granulite rocks can be considered as the tectonically detached portions of the Sukinda ultramafic rocks during the regional thrusting.

High-precision local gravity survey along planned motorway tunnel in the Slovak Karst

Abstract Results from a detailed gravity survey realized along the planned highway tunnel in the karstic area of Slovak Karst in the eastern Slovakia are presented. Detailed gravity profiles crossed an area of rugged topography, therefore the terrain corrections played a crucial role in the gravity data processing. The airborne laser scanning technique (LiDAR) was used in order to compile a high-resolution digital terrain model (DTM) of the surrounding area and to calculate terrain corrections properly. The difference between the Bouguer anomalies calculated with an available nationwide DTM and those with new LiDAR-based model can be significant in some places as it is presented in the paper. A new method for Bouguer correction density analysis based on surface data is presented. Special underground gravity measurements in the existing nearby railway tunnel were also conducted in order to determine the mean density of the topographic rocks. The Bouguer anomalies were used to interpret lithological contacts and tectonic/karstic discontinuities.

Gravity contribution to the geological study of grabens in the northwest region of Tunisia including the Kef Basin region

The northern Atlas region of Tunisia is characterized by northeast-trending compressional structures that affected the extensionally formed Late Permian to Mid Cretaceous Tunisian Trough. More recently, Late Cenozoic extensional forces have formed a series of northwest-trending grabens whose origins are partially controlled by geologic structures formed since the Late Permian. In order to aid in determining how these grabens were formed, a detailed gravity analysis was conducted in the western region of the Tunisian Trough. The gravity analysis included the construction of a residual gravity field using wavelength filtering which was further analyzed using derivatives, Euler deconvolution and three-dimensional modeling. The resulting residual gravity anomaly field consists of a series of short wavelength maxima that are mainly related to anticlinal structures and minima related to sediment-filled grabens. To better define the structure of the basins, three-dimensional (3D) gravity models were constructed, where the grabens were found to be approximately 3–5 km in thickness and mainly bounded by normal faults and underlain by low density Triassic evaporites. This evaporitic layer was not underlying the entire study area but only in the vicinity of the grabens. In particular, the Kef Basin region was found to be two separate grabens, the Kef and the Ksour located southeast of the Kef graben. The Kef graben has a maximum thickness of 3–4 km while the wider Ksour graben is between 4 and 5 km thick. The gravity analysis cannot determine exact origin of the grabens but extension is the most plausible cause of the grabens with strike-slip motion may have played a role in the formation of the Ksour graben if not the other grabens. The location of the grabens was controlled by the lateral extent of the Triassic evaporates which provided the rheological weakness to initiate the grabens.

GRAIL gravity observations of the transition from complex crater to peak-ring basin on the Moon: Implications for crustal structure and impact basin formation

High-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission provide the opportunity to analyze the detailed gravity and crustal structure of impact features in the morphological transition from complex craters to peak-ring basins on the Moon. We calculate average radial profiles of free-air anomalies and Bouguer anomalies for peak-ring basins, protobasins, and the largest complex craters. Complex craters and protobasins have free-air anomalies that are positively correlated with surface topography, unlike the prominent lunar mascons (positive free-air anomalies in areas of low elevation) associated with large basins. The Bouguer gravity anomaly profiles of complex craters are highly irregular, with central positive anomalies that are generally absent or not clearly tied to interior morphology. In contrast, gravity profiles for peak-ring basins (∼200km to 580km) are much more regular and are highly correlated with surface morphology. A central positive Bouguer anomaly is confined within the peak ring and a negative Bouguer anomaly annulus extends from the edge of the positive anomaly outward to about the rim crest. A number of degraded basins lacking interior peak rings have diameters and gravity patterns similar to those of well-preserved peak-ring basins. If these structures represent degraded peak-ring basins, the number of peak-ring basins on the Moon would increase by more than a factor of two to 34. The gravity anomalies within basins are interpreted to be due to uplift of the mantle confined within the peak ring and an annulus of thickened crust between the peak ring and rim crest. We hypothesize that mantle uplift is influenced by interaction between the transient cavity and the mantle. Further, mascon formation is generally disconnected from the number of basin rings formed and occurs over a wide range of basin sizes. These observations have important implications for models of basin and mascon formation on the Moon and other planetary bodies.

Geophysical characterization of saltwater intrusion in a coastal aquifer: The case of Martil-Alila plain (North Morocco)

Several factors can affect the quantity and the quality of groundwater resources, but in coastal aquifers seawater intrusion is often the most significant issue regarding freshwater supply. Further, saltwater intrusion is a worldwide issue because about seventy percent of the world's population lives in coastal regions.Generally, fresh groundwater not affected by saltwater intrusion is characterized by low salinity and therefore low electrical conductivity (EC) values. Consequently, high values of EC in groundwater along the coastline are usually associated to seawater intrusion. This effect is amplified if the coastal aquifer is overexploited with a subsequent gradual displacement of the freshwater-saltwater interface towards the continent.Delineation of marine intrusion in coastal aquifers has traditionally relied upon observation wells and collection of water samples. This approach may miss important hydrologic features related to saltwater intrusion in areas where access is difficult and where wells are widely spaced. Consequently, the scarcity of sampling points and sometimes their total absence makes the number of data available limited and most of the time not representative for mapping the spatial and temporal variability of groundwater salinity.In this study, we use a series of geophysical methods for characterizing the aquifer geometry and the extension of saltwater intrusion in the Martil-Alila coastal region (Morocco) as a complement to geological and hydrogeochemical data. For this reason, we carried out three geophysical surveys: Gravity, Electrical Resistivity and Frequency Domain Electromagnetic. The geometry of the basin has been determined from the interpretation of a detailed gravity survey. Electrical resistivity models derived from vertical electrical soundings allowed to characterize the vertical and the lateral extensions of aquifer formations. Finally, frequency domain electromagnetic methods allowed delineating the extension of the saltwater intrusion.

Redefined crustal structure of the Buchan Rift, northeast Victoria: evidence from potential field modelling of newly acquired land-based gravity data

ABSTRACTThe Buchan Rift, in northeastern Victoria, is a north–south-trending basin, which formed in response to east–west crustal extension in the Early Devonian. The rift is filled mostly with Lower Devonian volcanic and volcaniclastic rock of the Snowy River Volcanics. Although the structure and geometry of the Buchan Rift and its major bounding faults are well mapped at the surface, a discrepancy exists between the surface distribution of the thickest rift fill and its expected potential field response. To investigate this variation, two new detailed land-based gravity surveys, which span the rift and surrounding basement rocks in an east–west orientation, have been acquired and integrated with pre-existing government data. Qualitative interpretation of the observed magnetic data suggests the highly magnetic rocks of the Snowy River Volcanics have a wider extent at depth than can be mapped at the surface. Forward modelling of both land-based gravity data and aeromagnetic data supports this interpretation. With the Snowy River Volcanics largely confined within the Buchan Rift, resolved geometries also allow for the interpretation of rift boundaries that are wider at depth. These geometries are unusual. Unlike typical basin inversions that involve reactivation of rift-dipping faults, the bounding faults of the Buchan Rift dip away from the rift axis and thus appear unrelated to the preceding rifting episode. Limited inversion of previous extensional rift faults to deform the rift-fill sequences (e.g. Buchan Synclinorium) appears to have been followed by the initiation of new reverse faults in outboard positions, possibly because the relatively strong igneous rift fill began to act as a rigid basement ramp during continued E–W crustal shortening in the Middle Devonian Tabberabberan Orogeny. Overthrusting of the rift margins by older sediments and granite intrusions of the adjacent Tabberabbera and Kuark zones narrowed the exposed rift width at surface. This scenario may help explain the steep-sided geometries and geophysical expressions of other rift basins in the Tasmanides and elsewhere, particularly where relatively mechanically strong basin fill is known or suspected.

Integrated application of gravity and seismic methods for determining the dip angle of a fault plane: Case of Mahjouba fault (Central Tunisian Atlas Province, North Africa)

A procedure for a dip angle determination of a fault plane from gravity field data is presented to constrain a seismic profile interpretation. This procedure is applied on Mahjouba normal fault at the western border of Kalaa Khesba graben (Central Tunisian Atlas Province, North Africa). Seismic and detailed gravity data, in this region, were analyzed to provide more constraints on the geometry of the fault dip angle. The Mahjouba fault is mapped as three major parallel lineaments extended for 2 km with a NW-SE to N-S trend. The dip of the Mahjouba fault is estimated from the gravity modeling data to be 45°E. This study reveals that integrating gravity and seismic data provides accurate mapping of faults geometry and such result provides useful information and constraints on the exploration of natural resources.

Geophysics of the Tropicana gold deposit Minerals keynote paper

The Tropicana Gold Mine is located 330km east-northeast of Kalgoorlie, Western Australia. Discovered in August 2005 the deposit is the first world-class gold resource discovered in high-metamorphic grade gneissic rocks, in an Archean terrane not previously thought to be prospective for gold. The contrast in petrophysical properties of host rocks observed across the Tropicana gold mine enable geophysical methods to assist in mapping the deposit. Geophysical methods applied at Tropicana include; regional aeromagnetics and gravity, high-resolution airborne magnetics, gradient array Induced Polarisation (IP), pole-dipole IP, detailed gravity, MIMDAS IP, 2D seismic reflection, 3D seismic reflection, helicopter TEM, and SPECTREM. Initial gradient array IP combined with geochemical analysis of aircore drilling samples provided the most cost effective method to direct early diamond and RC drill testing of auger and soil anomalies. Integration of all available data within a 3D Common Earth Model (CEM) facilitates lithology constrained 3D potential field and 3D IP inversions. When combined with the lithological packages, structural architecture, alteration assemblages and zonation, and geochemical signatures the 3D CEM provides a powerful means of delineating ore positions and exploration targets.

Spatial and Spectral Representations of the Geoid-to-Quasigeoid Correction

In geodesy, the geoid and the quasigeoid are used as a reference surface for heights. Despite some similarities between these two concepts, the differences between the geoid and the quasigeoid (i.e. the geoid-to-quasigeoid correction) have to be taken into consideration in some specific applications which require a high accuracy. Over the world’s oceans and marginal seas, the quasigeoid and the geoid are identical. Over the continents, however, the geoid-to-quasigeoid correction could reach up to several metres especially in the mountainous, polar and geologically complex regions. Various methods have been developed and applied to compute this correction regionally in the spatial domain using detailed gravity, terrain and crustal density data. These methods utilize the gravimetric forward modelling of the topographic density structure and the direct/inverse solutions to the boundary-value problems in physical geodesy. In this article, we provide a brief summary of existing theoretical and numerical studies on the geoid-to-quasigeoid correction. We then compare these methods with the newly developed procedure and discuss some numerical and practical aspects of computing this correction. In global applications, the geoid-to-quasigeoid correction can conveniently be computed in the spectral domain. For this purpose, we derive and present also the spectral expressions for computing this correction based on applying methods for a spherical harmonic analysis and synthesis of global gravity, terrain and crustal structure models. We argue that the newly developed procedure for the regional gravity-to-potential conversion, applied for computing the geoid-to-quasigeoid correction in the spatial domain, is numerically more stable than the existing inverse models which utilize the gravity downward continuation. Moreover, compared to existing spectral expressions, our definition in the spectral domain takes not only the terrain geometry but also the mass density heterogeneities within the whole Earth into consideration. In this way, the geoid-to-quasigeoid correction and the respective geoid model could be determined more accurately.

Inner zone terrain correction calculation using interpolated heights

Abstract The discrepancy between real heights of gravity points and the elevation model has a significant impact on the terrain corrections calculation especially within the inner zone. The concept of interpolated heights of calculation points used instead of measured ones within the specified inner zone can considerably decrease the resulting errors. The choice of appropriate radius of the inner zone for use of interpolated heights is analysed on synthetic topography model as well as real data. The tests with synthetic models showed the appropriate radius of this zone is proportional to the deformation wavelength of the model. Simple statistical analysis of a particular elevation model can give an estimate of the appropriate radius for the calculation using interpolated heights. A concept with interpolated heights in the zone 0–250 m is used in actual practice in Slovakia. The analysis of regional gravity data from the Tatry Mountains test area indicates the searched radius should be about 100 m. Detailed gravity measurements from different areas showed the searched radius does not play so important role but the use of interpolated heights instead of measured ones is still relevant. The more reasonable method instead of using interpolated heights is also presented when calculating the topographic effect.

Density-thermal dependence of sedimentary associations calls to reinterpreting detailed gravity surveys

<p>The modern gravimetric equipment allows to register very small effects of gravity field changes and can be applied solving different geological, geophysical and environmental problems. However, sometimes insufficient calculation of various kinds of geological noise complicates effective application of detailed gravity field analysis. One of such factors is the temperature regime over and outside different buried objects of investigation. In this paper temperature changes in a subvertical zone over a hydrocarbon deposit and outside its contour are analyzed. The integrated density change and corresponding gravity effects are calculated for the Muradkhanly oil deposit situated within the south-east part of the Middle Kura Depression (central Azerbaijan). Calculation of these effects on the basis of density- temperature data correlation analysis could significantly improve the microgravity field examination over the hydrocarbon deposits. The similar correction procedure may be applied also over underground water horizons and some environmental targets.</p>

New Gravity Bases in Jabera-Damoh Vindhyan Basin, Central India

Abstract To map the geological features of long debated Jabera-Damoh region of Vindhyan basin, a detailed gravity survey was carried out. In this survey, gravity observations were taken at 1500 stations and new gravity bases were established at 40 locations. This paper provides information about the acquired gravity data along with absolute gravity values calculated at gravity bases for the use of general scientific community. All the necessary gravity data correction steps are given with formula except terrain and geoid corrections as they were not applicable to the data used in the present study. This will be very helpful for the students as well as young researchers, who want to venture in this field. Elevation, Bouguer and free air anomaly maps are also prepared, which may be useful for future planning. The Bouguer anomaly map of this region has brought several interesting features like (1) possible extension of Jabera-Damoh basin further south beyond NSL and (2) possibility of occurrence of Kimberlite pipes below Mahakoshal group of rocks, situated Northwest of Damoh towards Panna diamond field. Researchers who are interested in raw gravity digital data for the future research work can approach Director, NGRI, Hyderabad.

The importance of the geophysical methods for the determination of the urban subsurface structures

In this work, it is intended to highlight the indispensable significance of the geophysical methods at the level of primary decision-making at urban planning studies and to help resolving a range of problems connected to risk assessment.To this purpose, a research approach that integrated different geophysical methods for local subsoil evaluation using for seismic microzonation studies was carried out in the city of Leonessa. A detailed gravity survey and microtremor observations were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits. The Leonessa basin, the test area of this study, is one of the major intra-mountain tectonic depressions of Central Apennines boarded on the southern side by the great normal fault, here referred to as the Leonessa fault.The study involved a test area of 50 km2 covered by 300 gravity stations and by 30 sites of 3-component single-site observations. The gravity measurements were performed using a LaCoste & Romberg gravimeter mod.D60. Instead, the acquisition of the ambient seismic noise was performed using the REF TEK (Third Generation Broadband Seismic Recorder) model 13 0S-01 equipped with three components 1 Hz sensors. Stations were accurately located with differential GPS (Ashtech Z-Xtreme dual-frequency GPS) that provided centimetric accuracy in elevation. The microtremor data were analyzed by means of the horizontal-to-vertical spectral ratio (H/V). A realistic density of the unconsolidated Quaternary deposits (1.75-2.00 g/cm3) determined on core samples and a density of 2.60 g/cm3 for the basement limestone were used to constrain the 2D gravimetric models. The gravity and microtremor data were compared with a collection of 69 boreholes, 5 well logs, 130 geotechnical field testing and 61 geophysical surveys (MASW and Down Hole) made available by the the municipality of the Leonessa. The 2D models match quite well with all these data.Finally, the data obtained with these two different geophysical techniques allowed us to evaluate the thickness and the fundamental resonant frequency of the Quaternary sedimentary infilling, and to reconstruct the 3D bedrock configuration.We think that, in the process of creating earthquake resistant cities, geological, geophysical and geotechnical investigations have an indispensable significance in the evaluation of suitability for settlement and land use decisions.