Gravity Gradient Anomalies Research Articles

Three-dimensional estimate of the lithospheric effective elastic thickness of the Line ridge

Using a new bathymetry grid formed with vertical gravity gradient anomalies and ship soundings (BAT_VGG), a 1°×1° lithospheric effective elastic thickness (Te) grid of the Line ridge was calculated with the moving window admittance technique. As a comparison, both the GEBCO_08 and SIO V15.1 bathymetry datasets were used to calculate Te as well. The results show that BAT_VGG is suitable for the calculation of lithospheric effective elastic thickness. The lithospheric effective elastic thickness of the Line ridge is shown to be low, in the range of 5.5–13km, with an average of 8km and a standard deviation of 1.3km. Using the plate cooling model as a reference, most of the effective elastic thicknesses are controlled by the 150–300°C isotherm. Seamounts are primarily present in two zones, with lithospheric ages of 20–35Ma and 40–60Ma, at the time of loading. Unlike the Hawaiian-Emperor chain, the lithospheric effective elastic thickness of the Line ridge does not change monotonously. The tectonic setting of the Line ridge is discussed in detail based on our Te results and the seamount ages collected from the literature. The results show that thermal and fracture activities must have played an important role in the origin and evolution of the ridge.

Predicting Global Seafloor Topography Using Multi-Source Data

A new one-minute global seafloor topography model was derived from vertical gravity gradient anomalies (VGG), altimetric gravity anomalies, and ship soundings. Ship soundings are used to constrain seafloor topography at wavelengths longer than 200 km and to calibrate the topography to VGG (or gravity) ratios at short wavelengths area by area. VGG ratios are used to predict seafloor topography for wavelength bands of 100–200 km and to suppress the effect of crust isostasy. Gravity anomalies are used to recover seafloor topography at wavelengths shorter than 100 km. The data processing procedure is described in detail in this paper. The accuracy of the model is evaluated using ship soundings and existing models, including General Bathymetric Charts of the Oceans (GEBCO), DTU10, ETOPO1, and SIO V15.1. The results show that, in the discussed regions, the accuracy of the model is better than ETOPO1, GEBCO, and DTU10. Additionally, the model is comparable with V15.1, which is generally believed to have the highest accuracy. In the north-central Pacific Ocean, the accuracy of the model increased by approximately 29.5% compared with the V15.1 model. This indicates that a more accurate seafloor topography model can be formed by combining gravity anomalies, VGG, and ship soundings.

A program for bathymetry prediction from vertical gravity gradient anomalies and ship soundings

We design a program to calculate bathymetry model by combining vertical gravity gradients anomalies (VGG) and ship soundings. A bathymetry model in the experimental area (0 ∼ 30° N, 150 ∼ 210° E) from the North Pacific is calculated with the program. The bathymetry prediction procedure is validated by comparing the result to in situ ship depth soundings and existing models. The accuracy of the result is about 132.9 m, better than most of the existing models, such as GEBCO, DTU10, ETOPO1, and SIO V15.1. If VGG and gravity anomaly data are combined, the accuracy of recovered bathymetry model will increase by about 29.5 % to about 111.7 m, comparing to SIO V15.1. The bathymetry model constructed using VGG will be independent of gravity anomalies, and can be used to analyze the isostatic mechanism of ocean lithosphere with 3D admittance analysis technology. The accuracy of bathymetry prediction can be improved further by combining VGG and gravity anomaly data.

Bathymetry predicted from vertical gravity gradient anomalies and ship soundings

Abstract: In this paper, the admittance function between seafloor undulations and vertical gravity gradient anomalies was derived. Based on this admittance function, the bathymetry model of 1 minute resolution was predicted from vertical gravity gradient anomalies and ship soundings in the experimental area from the northwest Pacific. The accuracy of the model is evaluated using ship soundings and existing models, including ETOPOl, GEBCO, DTU10 and V15. 1 from SIO. The model's STD is 69.481m, comparable with V15. 1 which is generally believed to have the highest accuracy.

Joint Gravity and Gravity Gradient Inversion for Subsurface Object Detection

A novel approach of joint gravity and gravity gradient inversion is presented for passive subsurface object detection in this letter. Gravity and gravity gradient anomalies induced by an object can be measured and inversed to estimate the mass, orientation, and distance of the object. The new equations of gravity inversion are explored, and the weighted least squares estimation is constructed. Therefore, the two kinds of data are complementary to each other in the joint inversion via the self-adaptive weights. Simulation results show that the proposed approach is more efficient and robust than the previous gravity gradient tensor inversion.

A study of regional gravity field recovery from GOCE vertical gravity gradient data in the Auvergne test area using collocation

Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the first satellite mission that observes gravity gradients from the space, to be primarily used for the determination of high precision global gravity field models. However, the GOCE gradients, having a dense data distribution, may potentially provide better predictions of the regional gravity field than those obtained using a spherical harmonic Earth Geopotential Model (EGM). This is investigated in Auvergne test area using Least Squares Collocation (LSC) with GOCE vertical gravity gradient anomalies (Tzz), removing the long wavelength part from EGM2008 and the short wavelength part by residual terrain modelling (RTM). The results show that terrain effects on the vertical gravity gradient are significant at satellite altitude, reaching a level of 0.11 Eotvos unit (E.U.) in the mountainous areas. Removing the RTM effects from GOCE Tzz leads to significant improvements on the LSC predictions of surface gravity anomalies and quasigeoid heights. Comparison with ground truth data shows that using LSC surface free air gravity anomalies and quasi-geoid heights are recovered from GOCE Tzz with standard deviations of 11 mGal and 18 cm, which is better than those obtained by using GOCE EGMs, demonstrating that information beyond the maximal degree of the GOCE EGMs is present. Investigation of using covariance functions created separately from GOCE Tzz and terrestrial free air gravity anomalies, suggests that both covariance functions give almost identical predictions. However, using covariance function obtained from GOCE Tzz has the effect that the predicted formal average error estimates are considerably larger than the standard deviations of predicted minus observed gravity anomalies. Therefore, GOCE Tzz should be used with caution to determine the covariance functions in areas where surface gravity anomalies are not available, if error estimates are needed.

Automated gravity gradient tensor inversion for underwater object detection

Underwater abnormal object detection is a current need for the navigation security of autonomous underwater vehicles (AUVs). In this paper, an automated gravity gradient tensor inversion algorithm is proposed for the purpose of passive underwater object detection. Full-tensor gravity gradient anomalies induced by an object in the partial area can be measured with the technique of gravity gradiometry on an AUV. Then the automated algorithm utilizes the anomalies, using the inverse method to estimate the mass and barycentre location of the arbitrary-shaped object. A few tests on simple synthetic models will be illustrated, in order to evaluate the feasibility and accuracy of the new algorithm. Moreover, the method is applied to a complicated model of an abnormal object with gradiometer and AUV noise, and interference from a neighbouring illusive smaller object. In all cases tested, the estimated mass and barycentre location parameters are found to be in good agreement with the actual values.

Detection of High-Potential Oil and Gas Fields Using Normalized Full Gradient of Gravity Anomalies: A Case Study in the Tabas Basin, Eastern Iran

The normalized full gradient (NFG) represents the full gradient of the gravity anomaly at a point divided by the average of the full gradient at the same point. The NFG minimum between two maxima in an NFG section or a closed minimum surrounded by closed maxima on an NFG map may indicate density-deficient anomalies closely related to possible oil–gas reservoirs. On a cross-section, closed minima can be used to estimate the depth to centers of possible hydrocarbon reservoirs. The NFG map can also be used to locate oil–gas exploratory wells for estimation of the depth of possible reservoirs. The objective of this paper is to use two and three-dimensional (2D and 3D) NFG on gravity data of the Tabas basin in Yazd province, eastern Iran. A hypothetical model is first considered to explore the NFG characteristics and their relationship with the geometry of the model. The physical properties of the model are then studied to simplify the interpretation of real data. Finally 2D and 3D NFG models are developed for real gravity data to predict the location of any possible high potential oil–gas reservoirs. The results obtained indicate two zones in the northern and central parts of the Tabas basin suitable for hydrocarbon prospecting. However, the favorable zone located in the middle of the basin in which anticline E is detected at a depth of 5–7 km is more important for the purpose of hydrocarbon exploration.

Underwater Object Detection Based on Gravity Gradient

A novel method of underwater object detection based on gravity gradient is presented, which can be used on autonomous underwater vehicles (AUVs) to detect abnormal objects underwater. Gravity gradient anomalies of partial area, which are caused by the object, can be measured by a gravity gradiometer on an AUV. Then, anomalies can be inversed with a gravity gradient inversion algorithm, so the mass and barycenter of an object can be estimated. Simulation results show that approximate information of an object can be provided by the proposed method.

Vertical gravity anomaly gradient effect of innermost zone on geoid-quasigeoid separation and an optimal integration radius in planar approximation

This study mainly focuses on the solution of the vertical gravity anomaly gradient and determination of its effect on the total geoid-quasigeoid separation. Due to its small effect on geoid-quasigeoid separation, the planar approximation of vertical gravity anomaly in the innermost zone has been implemented. The computation of a strongly singular integral expressing the vertical gravity anomaly gradient was used for this purpose for even order (up to n = 6) of the Simpson and Newton–Cotes integration technique. The derivation of the relationships for different integration radii has been made to obtain these solutions using gridded data of free air anomaly. The comparison of relationships for the different integration radii was made in order to select an optimum radius of the integration in planar approximation for the vertical gravity anomaly gradient dependent geoid-quasigeoid separation term. The integration radii of 3.1, 6.2 and 9.3 km show an increasing behaviour towards saturation. The effect of vertical gravity anomaly gradient has similar pattern on the geoid-quasigeoid separation term towards saturation. The saturation trend for vertical gravity anomaly gradient is comparatively faster than its corresponding geoid-quasigeoid separation dependent term. The results also show that 2nd order Newton–Cotes integration is found to be comparable with the approximate linear solution for the vertical gravity anomaly gradient given by Heiskanen and Moritz. The vertical gravity anomaly dependant term has a rather small effect on geoid-quasigeoid separation in the mid elevation range and ranges from −6.18 to 2.7 mm for n = 6. The findings of the study leads to the inferences that the order of integration should be selected either n = 4 or n = 6 for better estimates of vertical gravity anomaly gradient solution. This criterion is also valid for their effect on geoid-quasigeoid separation with planar approximation in the innermost zone for the low- to mid-range elevation areas.

Kimberlite exploration using integrated airborne geophysics

Airborne magnetic and electromagnetic surveys together with kimberlite indicator mineral geochemistry have been standard practice in the search for kimberlites. The recent advent of the airborne gravity gradiometer (AGG) showed that airborne gravity gradiometry could also be a successful tool in kimberlite exploration. The installation of a digital AGG system on a helicopter led to the first airborne gravity gradient cum magnetic cum electromagnetic survey. The survey was flown over the central part of the Ekati tenement within the Lac de Gras kimberlite province. Each of these three geophysical methods relies on a different physical property contrast for its success. A selected sub-area shows that no one method would have identified all known pipes. But all known pipes would have been discovered by integrating all three data sets. The pipes in the selected area are associated with conductivity and/or gravity gradient anomalies; a few with magnetic anomalies. New data alone are not sufficient to guarantee success in a mature exploration environment. The geophysical data were individually inverted to create 3D density, magnetic susceptibility and electrical conductivity models. Integrating and applying classification techniques to the three 3D models was used in the generation of new targets. Drill testing of the targets has begun leading to the discovery of a new pipe.

Investigation into the regional wrench tectonics of inner East Anatolia (Turkey) using potential field data

The residual aeromagnetic and gravity anomalies of inner East Anatolia, surveyed by the Mineral Research and Exploration (MTA) of Turkey, display complexities. Some faults, which are known and new lineaments, are drawn from maxspot map derived from the location of the horizontal gradient of gravity anomalies. Tectonic lineaments of inner East Anatolia exhibit similarities to the direction of East Anatolian Fault Zone. Anticlockwise rotation, approximately −30°, defined from disorientations of aeromagnetic anomalies. The lineaments obtained from maxspots map produced from the gravity anomalies and disoriented aeromagnetic anomalies are in-line with the mobilistic system revealed by the palaeomagnetic data. These Alpine age continental rotations caused westward wrenching of the global lithosphere and led to significant tectonic reactivation and deformations. GPS measurements, current tectonic knowledge and the results of the evaluation of potential field data were combined in a base map to demonstrate similarities.

Estimates of the temperatures of hydrocarbon generation in the region of the Sea of Okhotsk

Particular features of the tectonic structure and anomalous distribution of the geothermal, geomagnetic, and gravity fields in the region of the Sea of Okhotsk are considered. On the basis of heat flow data, the ages of large-scale structures in the Sea of Okhotsk are estimated at 65 Ma for the Central Okhotsk Rise and 36 Ma for the South Okhotsk Basin. The age of the South Okhotsk Basin is confirmed by the data on the kinematics and corresponds to a 50-km thickness of the lithosphere. This is in accordance with the thickness value obtained by magnetotelluric soundings. A comparative analysis of the model geothermal background and the measured heat flow values on the Akademii Nauk Rise is performed. The analysis points to an abnormally high (by approximately 20%) measured heat flow, which agrees with the high negative gradient of gravity anomalies. The estimates of the deep heat flow and the basement age of the riftogenic basins in the Sea of Okhotsk were carried out in the following areas: the Deryugin Basin (18 Ma, Early Miocene), the TINRO Basin (12 Ma, Middle Miocene), and the West Kamchatka Basin (23 Ma, Late Oligocene). The temperatures at the boundaries of the main lithological complexes of the sedimentary cover are calculated and the zones of oil and gas generation are defined. On the basis of geothermal, magnetic, structural, and other geological-geophysical data, a kinematic model of the region of the Sea of Okhotsk for a period of 36 Ma was calculated and constructed.

Geodynamic evolution of the lithosphere of the Sea of Okhotsk region from geophysical data

The tectonic structure and anomalous distributions of geophysical fields of the Sea of Okhotsk region are considered; the lack of reliable data on the age of the lithosphere beneath basins of various origins in the Sea of Okhotsk is noted. Model calculations based on geological and geophysical data yielded an age of 65 Ma (the Cretaceous-Paleocene boundary) for the Central Okhotsk rise underlain by the continental lithosphere. This estimate agrees with the age (the end of the Cretaceous) derived from seismostratigraphic data. A comparative analysis of theoretical and measured heat fluxes in the Akademii Nauk Rise, underlain by a thinned continental crust, is performed. The analysis points to a higher (by 20%) value of the measured thermal background of the rise, which is consistent with a high negative gradient of gravity anomalies in this area. Calculations yielded an age of 36 Ma (the Early Oligocene) and a lithosphere thickness of 50 km for the South Okhotsk depression, whose seafloor was formed by processes of backarc spreading. The estimated age of the depression is supported by kinematic data on the region; the calculated thickness of the lithosphere coincides with the value estimated from data of magnetotelluric sounding here. This indicates that the formation time (36 Ma) of the South Okhotsk depression was estimated correctly. Numerical modeling performed for the determination of the basement age of rifting basins in the Sea of Okhotsk gave the following estimates: 18 Ma (the Early Miocene) for the Deryugin basin, 12 Ma (the Middle Miocene) for the TINRO basin, and 23 Ma (the Late Oligocene) for the West Kamchatka trough. These estimates agree with the formation time (Oligocene-Quaternary) of the sedimentary cover in rifting basins of the Sea of Okhotsk derived from geological and geophysical data. Model temperature estimates are obtained for lithologic and stratigraphic boundaries of the sedimentary cover in the Deryugin and TINRO basins and the West Kamchatka trough; the temperature analysis indicates that the latter two structures are promising for oil and hydrocarbon gas generation; the West Kamchatka trough possesses better reservoir properties compared to the TINRO and Deryugin basins. The latter is promising for the generation of hydrocarbon gas. Paleogeodynamic reconstructions of the Sea of Okhotsk region evolution are obtained for times of 90, 66, and 36 Ma on the basis of kinematic, geomagnetic, structural, tectonic, geothermal, and other geological and geophysical data.

Analysis and Tectonic Interpretation to the Horizontal‐Gradient Map Calculated from Bouguer Gravity Data in the China Mainland

AbstractWe make a careful analysis and comparison to the colored map of horizontal gradient calculated from Bouguer gravity data of the China mainland, and obtain some new insights into tectonics. Taking roughly 105°E as the boundary in the map, the eastern part of the China mainland is characterized by low gradients (blue color) background, appearing belt‐shape structure of medium and high gradient as well as linear structure of gradient difference borders. The west part has a framework of belt‐shape structure with medium and high gradient (red color), inlaid by low gradient areas which contain shade‐line structure. In general, structures exhibited by horizontal gradient of gravity anomalies can be classified into three major sets. The first set includes high‐gradient zones along the NNE trending Da hinggan ling, Taihangshan, and Wulingshan Mountains, the Tanlu great fault zone which extends obliquely through East China, and three NNW trending great fault zones through western China. The second set presents an intersection pattern of NWW and NEE direction gradient zones in the west, as well as three NWW discontinuous shade zones with roughly equal spacing, and a single tectonic line in the east, plus NE trending low gradient lines in South China. And the third set consists of several EW tectonic zones through the whole mainland and a wide complex NS striking zone in the middle of the mainland, of which on the both sides spread NS direction shade lines which are short, discontinuous and of large intervals. These three sets of tectonic zones delineated by gravity gradient are associated with variations of topography and crustal thickness on a large scale. The inner structures and formation times as well as evolution histories of each set are likely different. They have overlapped each other to shape the current tectonic patterns of the China mainland. Each tectonic zone has varied response to the recent tectonic stress fields of large scales, leading to regional change of directions of principle stresses. The ubiquitous segmentation caused by conjunction between tectonic zones of different directions is an important issue for theoretical and applied research in structural geology.

Orthometric corrections from leveling, gravity, density and elevation data: a case study in Taiwan

A new orthometric correction (OC) formula is presented and tested with various mean gravity reduction methods using leveling, gravity, elevation, and density data. For mean gravity computations, the Helmert method, a modified Helmert method with variable density and gravity anomaly gradient, and a modified Mader method were used. An improved method of terrain correction computation based on Gaussian quadrature is used in the modified Mader method. These methods produce different results and yield OCs that are greater than 10 cm between adjacent benchmarks (separated by ∼2 km) at elevations over 3000 m. Applying OC reduces misclosures at closed leveling circuits and improves the results of leveling network adjustments. Variable density yields variation of OC at millimeter level everywhere, while gravity anomaly gradient introduces variation of OC of greater than 10 cm at higher elevations, suggesting that these quantities must be considered in OC. The modified Mader method is recommended for computing OC.

Detection of reservoirs from normalized full gradient of gravity anomalies and its application to Shengli oil field, east China

We present how to detect reservoirs by the 3‐D normalized full gradient (NFG) of gravity anomalies constrained by seismic and drilling data. The normalized full gradient represents the full gradient of the gravity anomaly at a point divided by the average of the full gradient at the datum. The closed minima on an NFG map indicate the occurrence and horizontal locations of the centers of mass of density anomalies related to oil reservoirs. This information can be used to select well locations in an oil field. On a cross‐section, the closed minima can be used to estimate the depth to centers of mass of possible hydrocarbon reservoirs. Some characteristics of the NFG are calculated for a hypothetical case of an anticline saturated with oil and gas. The relationship of these characteristics to the geometry and physical parameters of the anticline is studied. Modeling studies show that the NFG largely depends on the number of terms in the Fourier series used to calculate it, and it closely related to the length of the gravity profile or the size of the study area. Applying the NFG method to the Shengli oil field, i.e., using closed minima of NFG alongside drilling data, showed an effectiveness of up to 70%. These results were used to select well locations with the result that some high‐production gas‐bearing strata were discovered by drilling.

A comparison of gravimetric techniques for measuringsubsurface void signals

To demonstrate the potential of gravity gradiometry in thedetection of underground structures or voids, an experiment was carriedout at an underground missile launch facility at Vandenberg Air ForceBase, California. Gravity data were collected using a LaCoste-RombergModel G gravimeter and gravity gradient measurements were made using avertical spin-axis Lockheed Martin developed arms control verificationgravity gradiometer. After performing all necessary corrections, the datawere compared to modelled signatures of the launch facility. Peak gravityand gravity gradient anomalies of approximately -75 µGal (1 µGal = 10-8 m s-1) and 30 E (1 E = 10-9 s-2) respectivelywere measured over the centreline of the largest capsule. The presence ofthe buried launch facility was apparent in both the observed gravity andgravity gradient data. The gravity gradients were unambiguous in resolvingthe underground facility's signature and thus revealing its location. Incontrast, identifying the launch complex solely from the gravity anomalydata would have proven difficult due to a strong regional trend across thetarget area.}\\fnm{4}{National Imagery and Mapping Agency, Patrick AFB,FL 32925-3438, USA.}\\fnm{5}{MITRE Corporation, Bedford, Massachusetts 01730-1420, USA.

Predicting Bathymetry from the Earth's Gravity Gradient Anomalies

Historically, prediction of ocean floor depth, or bathymetry, has been based on the isostatic modeling and linearized relationships between gravity anomalies and bathymetry. The need for isostatic modeling limits the application of the resulting bathymetry predictions as constraints in geophysical models. An alternative technique making use of the Earth's vertical gravity gradient for predicting bathymetry is explored in this paper. This technique is based on the fact that the observed gravity gradient anomalies result primarily from local mass concentrations on the ocean floor, and that mass compensation by the oceanic crust has an insignificant effect on the gravity gradients, and can be neglected. The resulting bathymetry prediction therefore is independent of isostatic modeling assumptions, allowing it to be used as a constraint on models of lithospheric compensation and for other geodetic and geophysical applications.

Gravity Gradiometry as a Tool for Underground Facility Detection

This paper is an examination of the feasibility of using gravity gradiometry for underground facility detection. Predicted gravity gradient anomalies for these structures have been calculated and some sample results are presented. Some work is done with respect to the noise due to variation of parameters such as density and terrain which might be expected in a survey, and the comparative usefulness of the different elements of the gravity gradient tensor for underground facility detection. The design parameters of the Gravity Gradiometer Survey System (GGSS) are compared with the requirements for underground facility detection as applied to the Cloud Chamber and the Yucca Mountain Tunnel.