Recovering Magnetization of Rock Formations by Jointly Inverting Airborne Gravity Gradiometry and Total Magnetic Intensity Data

Conventional 3D magnetic inversion methods are based on the assumption that there is no remanent magnetization, and the inversion is run for magnetic susceptibility only. This approach is well-suited to targeting mineralization; however, it ignores the situation where the direction of magnetization of the rock formations is different from the direction of the induced magnetic field. We present a novel method of recovering a spatial distribution of magnetization vector within the rock formation based on joint inversion of airborne gravity gradiometry (AGG) and total magnetic intensity (TMI) data for a shared earth model. Increasing the number of inversion parameters (the scalar components of magnetization vector) results in a higher degree of non-uniqueness of the inverse problem. This increase of non-uniqueness rate can be remedied by joint inversion based on (1) Gramian constraints or (2) joint focusing stabilizers. The Gramian constraints enforce shared earth structure through a correlation of the model gradients. The joint focusing stabilizers also enforce the structural similarity and are implemented using minimum support or minimum gradient support approaches. Both novel approaches are applied to the interpretation of the airborne data collected over the Thunderbird V-Ti-Fe deposit in Ontario, Canada. By combining the complementary AGG and TMI data, we generate jointly inverted shared earth models that provide a congruent image of the rock formations hosting the mineral deposit.

On the Feasibility of Seafloor Topography Estimation from Airborne Gravity Gradients: Performance Analysis Using Real Data

Compared with airborne gravimetry, a technique frequently used to infer the seafloor topography at places inaccessible to ship soundings due to the presence of ice shelf or ice mélange, airborne gravity gradiometry inherently could achieve higher spatial resolution, thus it is promising for improved inference of seafloor topography. However, its estimation capability has not been demonstrated by real projects. Theoretical analysis through admittance shows that compared with gravity disturbance, gravity gradient is more sensitive to the short-wavelength seafloor topography but diminishes faster with the increase of the distance between the seafloor and airplane, indicating its superiority is recovering short-wavelength topographic features over shallow waters. We present the first numerical experiment that estimates seafloor topography from a 0.4-km resolution, real airborne gravity gradients. It is shown that airborne gravity gradiometry can recover smaller topographic features than typical airborne gravimetry, but the estimation accuracy is only ±17 m due to the presence of subsurface density variations. The long-wavelength effect of the subsurface density variations can be removed with the aid of constraining bathymetry inside the study area, whereas the short wavelengths cannot. This study expands the applications of airborne gravity gradiometry, and helps glaciologists understand its performance in seafloor topography estimation.

A fast equivalent source method for airborne gravity gradient data

Airborne gravity gradiometry measures several gravity gradient components unlike conventional gravimetry taking only the vertical gravity component into account. However, processing of multicomponent airborne gravity gradient (AGG) data without corrupting their internal consistency is often challenging. Therefore, we have developed an equivalent source technique to tackle this challenge. With a combination of Gauss-fast Fourier transform and the Landweber iteration, we have developed an efficient way to compute equivalent sources for AGG data. This method can handle two components simultaneously. We first examined its viability by applying this approach to a synthetic example. Afterward, we applied our method to real AGG data collected in the area of Karasjok, Norway. Our result is almost the same as the results that meet the industry standard but with great efficiency.

3D inversion of airborne gravity gradiometry data for the Budgell Harbour Stock, Newfoundland: A case history using a probabilistic approach

We have studied the Mesozoic Budgell Harbour Stock, a gabbroic intrusion in north-central Newfoundland, Canada, using 3D inversion of airborne gravity gradiometry data based on a probabilistic inversion method. Significantly, differences were observed between the results when inverting the single [Formula: see text] component and when inverting the 5C combination. We also found that the [Formula: see text] model failed to reproduce the long-wavelength signals from other components, whereas the model recovered from five components accommodated all of the signals from all of the components. To estimate the influence of long-wavelength signals from targets other than the intrusion, such as deeper bodies or large-scale terrain variations, inversion tests are performed on a synthetic model. The inversion results for the synthetic example indicate that the joint inversion of five components is more sensitive to long-wavelength signals, which can generate spurious structures to fit all of the signals from the five components. In contrast, the [Formula: see text] model is less affected by the long-wavelength signals and thus tends to produce a stable solution, despite failing to incorporate all of the long-wavelength signals from the tensor data. We found that gravity gradiometry data could be used to delineate the intrusion within this study area, which is also consistent with the susceptibility model recovered from inversion of aeromagnetic data and with results from a previous geophysical study. Moreover, the differences between the [Formula: see text] model and the 5C model may reflect the long-wavelength signals in the gravity gradiometry data.

Applying laterally varying density corrections to ground gravity and airborne gravity gradiometry data: a case study from the Bathurst Mining Camp

The influence of topography on gravity and gravity gradiometry measurements is profound and should be minimized prior to geological interpretation. The standard way of minimizing these effects is through the computation of a terrain correction. Terrain corrections require two inputs: topography and density. Often, geology and topography are inextricably intertwined: topography is caused by a change in geology. In geologic environments where there is a structural and (or) stratigraphic control on the near-surface mass distribution, using a single density value in the corrections leads to removal of the topographic effect of rocks having the chosen density. Any remaining gravity signal that correlates with topography is providing geological information. If the objective is to produce a gravity map with minimal topographic signal, then a regionally variable density correction is a means of compensating for this effect. In this paper, we demonstrate how to apply a spatially variable density correction using ground gravity and airborne gravity gradiometry data for the geologically complex Bathurst Mining Camp, northern New Brunswick, Canada. Ground gravity and airborne full tensor gravity gradiometry measurements are subdivided into a series of domains on the basis of the underlying tectonostratigraphic group. Terrain and Bouguer corrections are calculated for each domain using representative density values obtained from drill core and surface sampling throughout the Bathurst Mining Camp. The output from the spatially variable density correction is then compared with previous maps. Overall, the differences are subtle, but the spatially variably density allows for isolated anomalies to be better resolved.

Self-Gradient Compensation of Full-Tensor Airborne Gravity Gradiometer

In the process of airborne gravity gradiometry for the full-tensor airborne gravity gradiometer (FTAGG), the attitude of the carrier and the fuel mass will seriously affect the accuracy of gravity gradiometry. A self-gradient is the gravity gradient produced by the surrounding masses, and the surrounding masses include distribution mass for the carrier mass and fuel mass. In this paper, in order to improve the accuracy of airborne gravity gradiometry, a self-gradient compensation model is proposed for FTAGG. The self-gradient compensation model is a fuction of attitude for carrier and time, and it includes parameters ralated to the distribution mass for the carrier. The influence of carrier attitude and fuel mass on the self-gradient are simulated and analyzed. Simulation shows that the self-gradient tensor element Γ x x , Γ x y , Γ x z , Γ y z and Γ z z are greatly affected by the middle part of the carrier, and the self-gradient tensor element Γ y z is affected by the carrier’s fuel mass in three attitudes. Further simulation experiments show that the presented self-gradient compensation method is valid, and the error of the self-gradient compensation is within 0.1 Eu. Furthermore, this method can provide an important reference for improving the accuracy of aviation gravity gradiometry.