Gravity Gradient Survey Research Articles

3D unconstrained and geologically constrained stochastic inversion of airborne vertical gravity gradient data

We present an inversion tool for airborne gravity gradient data that yields a 3D density model using stochastic methods i.e. cokriging and conditional simulation. This method uses geostatistical properties of the measured gravity gradient to estimate a 3D density model whose gravity response fits the measured gravity gradient anomaly. Linearity between gravity gradient data and density allows estimation of the model (density) covariance using observed data, i.e. we adjust iteratively the density covariance matrix by fitting experimental and theoretical gravity gradient covariance matrices. Inversion can be constrained by including densities known at some locations. In addition we can explore various reasonable solutions that honour both the estimated density covariance model and the gravity gradient data using geostatistical simulation.The proposed method is first tested with two synthetic datasets generated from a sharp-boundary model and a smooth stochastic model respectively. The results show the method to be capable of retrieving models compatible with the true models; it also allows the integration of complex a priori information. The technique is then applied to gravity gradient survey data collected for the Geological Survey of Canada in the area of McFaulds Lake (Ontario, Canada) using the Falcon airborne gravity system. Unconstrained inversion returns a density model that is geologically plausible and the computed response exactly fits the observed gravity gradient anomaly.

Noise reduction procedures for gravity-gradiometer data

Noise suppression of airborne gravity-gradiometer data is a crucial part of the data processing stream. We considered two approaches to removing noise: kriging and directional filtering. Kriging is an estimation procedure for the interpolation of spatial data. The estimator is calculated from the data variogram, which characterizes the noise level and correlation length of the measurements. Directional filtering uses a user-defined operator that is oriented to preferentially smooth the data along the strike, but it leaves short-wavelength components in the cross-strike direction for definition of the trend edges. Both methods were applied to a recently collected offshore gravity gradient survey. The kriging and directional filtering results revealed a similar level of smoothness, but the main difference between them was the extra smoothing along the strike for the directionally filtered data. Because kriging is a data-driven procedure, it provides an objective estimate of the data noise level and degree of smoothness. The processing parameters required for directional filtering can then be chosen to give a similar level of smoothness and noise suppression to the kriging results, but with the added advantage of directional smoothing, which more effectively delineates geologic trends in the data.

Reconstructing the gravity gradient anomaly field from surveys with wide line spacing using equivalent source processing: an error analysis

ABSTRACTWhen anomalous gravity gradient signals provide a large signal‐to‐noise ratio, airborne and marine surveys can be considered with wide line spacing. In these cases, spatial resolution and sampling requirements become the limiting factors for specifying the line spacing, rather than anomaly detectability. This situation is analysed by generating known signals from a geological model and then sub‐sampling them using a simulated airborne gravity gradient survey with a line spacing much wider than the characteristic anomaly size. The data are processed using an equivalent source inversion, which is used subsequently to predict and grid the field in‐between the survey lines by means of forward calculations. Spatial and spectral error analysis is used to quantify the accuracy and resolution of the processed data and the advantages of acquiring multiple gravity gradient components are demonstrated. With measurements of the full tensor along survey lines spaced at 4 × 4 km, it is shown that the vertical gravity gradient can be reconstructed accurately over a bandwidth of 2 km with spatial root‐mean square errors less than 30%. A real airborne full‐tensor gravity gradient survey is presented to confirm the synthetic analysis in a practical situation.

Consequences of flight height and line spacing on airborne (helicopter) gravity gradient resolution in the Great Sand Dunes National Park and Preserve, Colorado

Line spacing and flight height are critical parameters in airborne gravity gradient surveys; the optimal trade-off between survey costs and desired resolution, however, is different for every situation. This article investigates the additional benefit of reducing the flight height and line spacing though a study of a survey conducted over the Great Sand Dunes National Park and Preserve, which is the highest-resolution public-domain airborne gravity gradient data set available, with overlapping high- and lower-resolution surveys. By using Fourier analysis and matched filtering, it is shown that while the lower-resolution survey delineates the target body, reducing the flight height from 80 m to 40 m and the line spacing from 100 m to 50 m improves the recoverable resolution even at basement depths.

Gravity Gradiometry for Emerging Applications

Gravity surveys have become an accepted part of the geophysical exploration process for hydrocarbons and minerals. Both scalar gravity and gravity gradient surveys are commonly conducted on land, s...

Mapeamento litológico por correlação entre dados de aeromagnetometria e aerogradiometria gravimétrica 3D-FTG no quadrilátero ferrífero, Minas Gerais, Brasil

O presente trabalho de pesquisa apresenta uma aplicação da relação de Poisson que pode ser utilizada em dados de magnetometria e gradiometria gravimétrica utilizando o exemplo na mesma região. A razão entre o contraste de suscetibilidade e o contraste de densidade (mapa litogeofísico) pode ser calculada. Como um teste destas técnicas, aplicamos esta relação nos dados de magnetometria e gradiometria gravimétrica 3D-FTG coletados na área do Baú-Gandarela no Quadrilátero Ferrífero, Minas Gerais, Brasil. A idéia principal foi estabelecer prioridades para prospecção de hematita na região supracitada. O mapa litogeofísico produzido mostra as regiões onde já existe uma mina de hematita e estabelece áreas para prospecção do minério.

Interpolation of gravity and gravity gradient data by using the generalized sampling expansion: Theory

The generalized sampling expansion is a method that can be used to combine gravity and gravity gradient data so that the line spacing of a survey can be increased without introducing aliasing into the measured signal. This reduces acquisition costs because the costs of shipborne and airborne gravity and gravity gradient surveys are related to the line spacing of the survey. The generalized sampling expansion combines the Fourier spectra of several undersampled gravity and gravity gradient data sets to produce an unaliased result. To achieve that result, input data are assumed to be noise free, infinite in extent, regularly sampled, and band limited. Deviations from this ideal induce errors, but these can be alleviated to a degree. In theory, the generalized sampling expan-sion can reconstruct an unaliased signal sampled at three times conventional line spacing. Unfortunately, edge effects and the presence of noise limit the practical increase in line spacing to twice normal line spacing. Six methods, each using different gradient components, can be used to achieve this doubling of line spacing. Edge effects introduce short-wavelength errors into several possible applications of the generalized sampling expansion, a problem that can be reduced by filtering the output of an expansion and by trimming away residual errors at its boundary. When applied to a deliberately decimated data set derived from data collected over the Cannington ore deposit in Australia, the generalized sampling expansion method successfully reduced aliasing in the signal and produced a more accurate interpolation than was obtained with a gradient-enhanced Akima spline method.

Detecting kimberlite pipes at Ekati with airborne gravity gradiometry

From late April to the end of July 2000, a 39,000 line km airborne gravity gradient survey was completed over the EkatiO mine property in the NWT, Canada. This was the world?s first airborne gravity gradient survey for the purpose of detecting kimberlite pipes. Preliminary data processing was done on site at the EkatiO diamond mine. Subsequent drilling of gravity anomalies in the year 2000 has resulted in the discovery of two new kimberlite pipes. More anomalies will be drilled in 2001. The AGG data shows that more than half of the known kimberlite pipes have associated gravity anomalies. Some pipes with a diameter as small as 100 m or less can be detected in the AGG data. The AGG data has a 300 m resolution with an average RMS noise of 7.6 Eotvos in the derived vertical gradient. Laser profilometer data and differential GPS data were also acquired in the survey to construct a detailed digital elevation model for terrain correction. Besides detecting kimberlite pipes, the AGG data is also useful for mapping details of geological structures. This is complementary to the magnetic data acquired simultaneously with the AGG data.

Lithological Mapping by Correlating Magnetic and Gravity Gradient Airborne Measurements

We report here two new applications of the Poisson relation useful when a joint magnetic and gravity gradient survey has been made. In the first, an apparent susceptibility contrast to density contrast ratio map (which we call the map) can be estimated. In the second, selected values of the above ratio are used in a filter to subtract from the magnetic field measurements the effect of a particular rock type. This would be especially useful as a filter of geological due perhaps to weathered magnetite products, particularly if the latter had an effect over a large area. As a test of these techniques we have applied them to a simulated joint airborne survey over a model which we designate Mini-Elura, in which the magnetic magnitude and vertical gravity gradient anomalies are not visible above the due to surficial variations. The computed pseudo-lithology map clearly distinguishes the regions of the map dominated by the alluvium source from those due to the Mini-Elura orebody, while the pseudo-magnetic filter does very effectively remove the noise due to the alluvium from the magnetic map.

Inversion of airborne gravity gradient data, southwestern Oklahoma

The diagonal elements of the gravity gradient tensor, as recorded by the Bell airborne Gravity Gradient Survey System (GGSS), are used to determine the basement topography in southwestern Oklahoma. This determination is accomplished through a nonlinear inverse procedure based on the conjugate gradient algorithm. The resulting model contains a ridge of shallow basement material (⩽3.0 km deep) trending east‐southeast. This ridge is bounded on the north and the south by basement troughs; the northern trough extends as deep as 10.0 km. The gradient field which results from this model fits most of the GGSS observations within their estimated errors of 12.0 E [Formula: see text]. The model is in general agreement with a set of available oil well depths to the basement and with inferred faults in these igneous rocks. To assess the derived solution, the problem was linearized about the final model and the linear parameter resolution and parameter covariances were computed. Generally, the basement depths are well resolved and the resolution matrix is diagonally dominant. Furthermore, the parameter standard errors are small: 72 parameters out of 98 have errors less than 1.0 km.

Resolution and variance operators of gravity and gravity gradiometry

Gravity gradiometry represents a new potential field data set which may better constrain the density structure of the earth. Using singular value (spectral) decomposition of the gravity and gravity gradient kernels, the model parameter resolution and model parameter variance of the two data types are compared using data from the Defense Mapping Agency and a recently acquired collection of airborne gradient measurements from Bell Aerospace Textron’s Gravity Gradient Survey System (GGSS). The GGSS was flown over a portion of southwestern Oklahoma, where the gravitational anomaly from the buried Wichita basement rocks is over 60 mGal. The corresponding maximum vertical gravity gradient was found to be 46.2 Eötvös. The determination of the subsurface density structure is cast as a linear inverse problem and, for comparison, a nonlinear inverse problem. For both the linear and nonlinear inversions, the gravity gradients improve the resolution and result in smaller variances than the vertical component of gravity. The density resolution and variance were computed for a subset of tracks from an airborne gravity gradient survey made in the summer of 1987. For the linear inversion, the resolution of the density is not adequate below the second layer (20 km). Furthermore, the estimated error of the actual gradient observations for a resolution of 0.9 km is 10E, for which the maximum error of the density values is [Formula: see text]. The linearized resolution of the boundary perturbations is better, with most parameters being well resolved. The standard errors for the layer perturbations are less than 1 km for the shallower layer (5.0 km) when using the gradiometer data. For the deeper layer (25.0 km), the maximum error is larger, 4.3 km.

Vertical gravity gradient surveys: Field results and interpretations in British Columbia, Canada

Two vertical gravity gradient (VGG) surveys were completed by the authors during 1977 in British Columbia, Canada. The VGG method utilizes a LaCoste and Romberg model D gravity meter in conjunction with a small gradient tripod. The VGG method is practical under most field conditions using a 2 person crew and yields results precise to ±20 E or better. The VGG work indicates that the “free‐air” effect ranges between 2600–2800 E for southwestern British Columbia, which is somewhat lower than the theoretical value of 3086 E. The usefulness of the method in mining exploration is doubtful, especially in hilly or mountainous terrain where VGG values are shown to be very terrain sensitive. However, the importance of knowing the regional VGG variations is emphasized by the work over the Hat Creek coal deposit, British Columbia.