Airborne Gravity Gradiometry Research Articles

Efficient terrain correction in airborne and seaborne gravity gradiometry surveys

SUMMARY Proper terrain correction in airborne and seaborne gravity gradiometry surveys requires detailed digital elevation models (DEMs). However, due to standard low-pass filter techniques applied during acquisition, the required resolution of the DEMs is altered in commercial surveys. We quantify the ideal resolution for an example dataset with a new and practical method for performing terrain corrections based on Parker's 1972 Fourier domain calculation. In the same vein, we quantify the required spatial extents of DEMs for nominal cases. With these parameters quantified, we can optimize the terrain correction to improve efficiency in both gravity gradiometry forward modeling as well as terrain corrections. In an ideal case, we find that the required spatial extent of the DEM is roughly nine times the greatest relief in the terrain outside the survey area. For the specific case, we find that 20 meter resolution is suitable.

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.

Historical development of the gravity method in exploration

The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of global positioning systems (GPS) have led to a marked improvement in the quality of marine gravity and have transformed airborne gravity from a regional technique to a prospect-level exploration tool that is particularly applicable in remote areas or transition zones that are otherwise inaccessible. Recently, moving-platform gravity gradiometers have become available and promise to play an important role in future exploration. Data reduction, filtering, and visualization, together with low-cost, powerful personal computers and color graphics, have transformed the interpretation of gravity data. The state of the art is illustrated with three case histories: 3D modeling of gravity data to map aquifers in the Albuquerque Basin, the use of marine gravity gradiometry combined with 3D seismic data to map salt keels in the Gulf of Mexico, and the use of airborne gravity gradiometry in exploration for kimberlites in Canada.

Constrained Gravity 3D Litho-Inversion Applied to Broken Hill

We present an example of a constrained inversion method that uses a categorical property, lithology, as the primary model parameter. A 3D geological model is supplied as the starting model. The topology of this model is used as prior information, together with parameters that define the probability density function of a secondary property, density, and observations of the gravity field. The 3D geological model is a prediction of the geometry of geological interfaces given points where these interfaces have been observed, structural observations of the orientation of these interfaces and a table describing the relationships between the various geological units. The interfaces are modelled as equipotential surfaces, and cokriging is used to interpolate between the supplied points. The inversion method returns a number of models which are consistent with the supplied prior information. This ensemble of acceptable models can be analysed statistically to derive conclusions. A 3D geological model of Broken Hill was constructed from a range of geological inputs. Constrained inversion of ground gravity was then carried out. Prompted by the results of initial inversions, a number of adjustments were made to both the geological model and estimates of the density for each of the units. The final inversion was used to demonstrate a high degree of internal consistency amongst these amended forms of the prior information. The use of airborne gravity gradiometry (AGG) data was deferred pending modification of the forward modelling algorithm to incorporate the band-limited character of these observations.

ARKeX on the way to hatching an EGG

A new UK company is planning a breakthrough in airborne gravity gradiometry. Andrew McBarnet has been talking to its key executives.

Mapping basement relief with airborne gravity gradiometry

Airborne measurement of gravity tensor components provides a new means of high-resolution mapping of the gravity field. The tensor components attenuate more rapidly with increase in source depth than does gravity, so these methods are best suited to the investigation of shallow sources. A primary use of gravity studies in both petroleum and mineral exploration is the mapping of basement relief.In this study gravity tensor variations have been computed over a complex model representing a horizontally and vertically faulted basement surface. Simulated measurement noise was added to the model output to create synthetic data sets. This synthetic data was then inverted to investigate the limitations of recovering basement structure from the data.The model results presented in this study show that gravity gradients over wide and extensive vertical steps in a 0.4 gm/cc interface with amplitude +/- 10% of depth can be reasonably inverted up to noise levels of approximately 10 E.u rms. Multi-profile inversions substantially outperformed single profile inversions, indicating their value in suppressing random profile to profile noise.

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.

Constructing high resolution DEMs from airborne laser scanner data

In processing and interpreting the data collected during FALCON airborne gravity gradiometry surveys it is necessary to carefully compensate for topographic features. So that surveys may be performed even in areas where accurate DEMs (Digital Elevation Maps) are unavailable, the FALCON aircraft have been fitted with laser scanners, providing ground return data across a sufficiently wide swathe so that very adequate DEMs over the whole survey area can be produced. Additionally, in one aircraft, a laser profilometer has been fitted adjacent to the scanner, providing independent data to monitor the scanner integrity throughout a survey. This paper briefly describes the scanner features and details the post processing of the scanner ground returns through to gridded DEM format. The intrinsic accuracy of the scanner at low scan angles is demonstrated to be very good, accounting for a ground height error of less than 0.1m standard deviation. Taking into account that DGPS height errors are about 0.15m, the resulting ground height error is estimated as 0.2m standard deviation, which is confirmed by the analysis of height differences in the overlapping areas between adjacent lines. This estimate is characteristic of a lightly vegetated terrain. This level of topographic error will have a negligible impact on our ability to identify target anomalies arising from geological variations.

Advanced helicopter aerogravity surveying system

Editor's note: Several new systems have been recently developed for acquiring airborne gravity data and airborne gravity gradiometry data. Therefore, the Meter Reader is currently running a series of articles that feature all commercial systems now available to the exploration industry. This is the second article in this series. In recent years, the uses of gravity data have advanced from basic delineation of sedimentary basins and identification/modeling of large structures in frontier basins to integration with seismic data in the construction of detailed velocity-depth models in salt dome basins. The demand for more precise gravity data often in some of the most remote places on earth has driven the 35-year evolution of the Carson Aerogravity System. This article discusses the most recent developments in that system. Before considering these in detail, it is worthwhile to look briefly at how this system has developed and what it has accomplished over the past 35 years. Carson's first commerical aerogravity system was developed in the 1960s, a “spinoff” of U.S. Department of Defense programs. Carson flew its first commercial aerogravity survey in 1978 and more than 100 000 line km were flown prior to a “blind” test of the system in 1982 by a consortium of petroleum companies. General industry acceptance of the system increased rapidly after those carefully supervised trials. International patents on the system followed—the first issued in 1984 and the latest in 1997. The first field surveys with GPS began in 1991. Continuous improvements in accelerometers, platform, and gravimeter throughout the 1990s culminated in the system discussed in …

Singularity and Branching: A Path-Following Formalism For Geophysical Inverse Problems

dSUMMARY A path-following technique is presented as an alternative to the direct minimization of a misfit functional. The algorithm is based upon the iterative adjustment of an Earth model to fit a set of generated ‘data’. These values vary from a purely artificial set, produced from some prior model, to the observed data. The changes in the model that are required to satisfy the conditions for minimum misfit are calculated at each iteration. Multiple solutions appear when the path-following equations become singular and linear approximations fail. Using techniques from singularity theory in Hilbert space, the number, type and stability of solutions appearing at a singular point can be calculated. It is possible to project beyond a singular point and follow the additional solutions that emerge. Thus the algorithm allows one to track multiple solutions to a given inverse problem. The technique is used to infer velocity variations between two boreholes at the Conoco Borehole Test Facility. From a set of 471 arrival times, three velocity models are deduced. All of the models satisfy the conditions for minimum misfit. One model is very similar to the solution produced by a conjugate gradient algorithm. The three models all contain high velocities overlying low velocities. The path-following algorithm is also applied to a set of airborne gravity gradiometry data gathered in SW Oklahoma. An upthrown block of high-density basement material is found to coincide with a positive vertical gravity gradient anomaly. This conclusion agrees with the depths to basement observed in numerous oil wells in the region.

The use of FFT techniques in physical geodesy

The fast Fourier transform (FFT) technique is a very powerful tool for the efficient evaluation of gravity field convolution integrals. It can handle heterogeneous and noisy data, and thus presents a very attractive alternative to the classical, time consuming approaches, provided gridded data are available. This paper reviews the mathematics of the FFT methods as well as their practical problems, and presents examples from physical geodesy where the application of these methods is especially advantageous. The spectral evaluation of Stokes', Vening Meinesz' and Molodensky's integrals, least-squares collocation in the frequency domain, integrals for terrain reductions and for airborne gravity gradiometry, and the computation of covariance and power spectral density functions are treated in detail. Numerical examples illustrate the efficiency and accuracy of the FFT methods.

Establishing Requirements for Gravity Surveys for Very Accurate Inertial Navigation

Many types of gravity surveys have been conducted or proposed to support very accurate inertial navigation. For example, gravimetry and satellite altimetry data now exist over many areas of the world and new techniques are being developed such as satellite-to-satellite tracking and airborne gravity gradiometry. This paper presents a universal approach for analyzing the navigation system improvements afforded by these surveys. Comparisons are shown of the navigation RMS errors for five survey methods. These comparisons reveal the tradeoff between gravity survey resolution and inertial navigation system accuracy.

On optimal estimation of gravity from gravity gradients at aircraft altitude

Least squares collocation in physical geodesy is now a standard technique for estimating gravity‐related quantities from discrete observations in a limited region. It is an optimal method because estimation errors due to measurement noise and the discreteness and finite extent of observations are minimized. However, for large observation sets, the computational problem of collocation (i.e., the problem of solving a large, possibly ill‐conditioned, linear system of equations) is also well known and is very clearly illustrated with airborne gravity gradiometry. Natural alternatives to rigorous least squares collocation are therefore examined and are categorized as being either nonoptimal, suboptimal, or virtually optimal. All choices of estimation method are directed at reducing the computational requirements with an awareness of the desired estimation accuracy. An example of a nonoptimal procedure is the use of integral formulas of the Stokes or Vening‐Meinesz type; suboptimal estimation is exemplified by frequency domain collocation; and a case of virtual optimality is the approximate collocation estimator obtained with a few iterations of the conjugate gradient method used to invert the autocovariance matrix.