Airborne Gravity Gradiometry Survey Research Articles

Measuring gravity gradient in rugged terrain: HeliFalcon survey in Aso-Oguni, Japan

Since the first Falcon AGG survey in 1999 (van Leuwen et al., 2000), fixed-wing airborne gravity gradiometry (AGG) has proved to be a valuable tool for mining and oil and gas explora¬tion (Dransfield, 2007). The more compact digital Falcon AGG allowed the system to be installed in a helicopter and helicopter AGG surveys began in 2006 (Boggs et al., 2007). Compared with fixed-wing aircraft, the helicopter platform can fly surveys low and slow, offering superior spatial resolution as well as an improved signal-to-noise ratio (Dransfield, 2007). The superior spatial resolution and improved signal-to-noise ratio provided by helicopter AGG enhances its capability to detect smaller targets and better delineate subtler features. Dransfield and Christensen (2013) reported a HeliFalcon performance of 6 Eo RMS at 45 m resolution in vertical gravity gradient, by far the finest spatial resolution of any airborne AGG system. Another advantage afforded by helicopter AGG is its capa¬bility to follow terrains more closely especially in areas of high relief. Christensen and Hodges (2013) compared the vertical grav¬ity gradient from HeliFalcon with the simulated fixed-wing result over the Iron Range survey in the Canadian Rocky Mountains where the terrain variation reaches 1900 m. They showed that in this terrain, a fixed-wing survey would have to fly at ground clearances of more than 1000 m for much of the survey area. This high ground clearance would have greatly suppressed the signal at short wavelengths and degraded the spatial resolution. In this paper, we look at the HeliFalcon AGG survey conducted in the Aso-Oguni area exploring for geothermal fields, examin the technical challenges presented by the rugged terrains, describe some aspects of the data acquisition and processing, and present the survey results.

Airborne gravity gradiometer surveying of petroleum systems under Lake Tanganyika, Tanzania

The Lake Tanganyika South petroleum exploration block covers the southern portion of the Tanzanian side of Lake Tanganyika and is located within the East African Rift System. The rifting process has formed rotated fault blocks which provide numerous play types in the resulting basins. Interpretation of 2D seismic data from 1984 indicated that sufficient sediment thickness is present for hydrocarbon generation. The prospectivity of the lake sediment sequence is enhanced by large oil discoveries further north along the rift system at Lake Albert in Uganda. Airborne gravity gradiometry (AGG) has been used in the Lake Albert region to delineate the structural framework of sedimentary basins.Based on this analogy, in 2010 Beach Energy commissioned CGG to fly a FALCON AGG and high-resolution airborne magnetic survey over the Lake Tanganyika South block to provide data for mapping the basin architecture and estimating the depth to magnetic basement. A total of nearly 28000 line kilometres of data were acquired.The subsequent interpretation incorporated the AGG and magnetic data with available 2D seismic data, elevation model data, bathymetry, Landsat and regional geology information.The integrated data interpretation revealed that the Lake Tanganyika rifting structures occur as half-grabens that were formed through reactivation of Precambrian fault structures. Two major depocentres were identified in the magnetic depth-to-basement map in the north and in the west-central part of the survey area with sediment thicknesses in excess of 4 km and 3 km, respectively. Smaller, shallower depocentres (with less than 3 km of sediment) occur in the south-western region. This information was used to plan a 2100 km 2D marine seismic survey that was recorded in 2012. An interpretation of the results from the seismic survey confirmed a rifting structure similar to that encountered further north at Lake Albert in Uganda. Several targets were identified from the seismic sections for follow-up.

空中重力偏差法探査データから推定される霧島火山西部の密度構造

空中重力探査は海外を中心に近年測定精度が向上してきており，中でも航空機搭載型の重力偏差計測技術はめざましく進歩している。本研究では，鹿児島県霧島火山西部地域を対象としてJOGMEC（独立行政法人石油天然ガス・金属鉱物資源機構）によって行われた「地熱資源ポテンシャル調査のための空中物理探査」の空中重力偏差法探査データを用いて地下の3次元密度構造を推定し，得られた密度構造とこれまでに本地域で行われた地熱資源調査結果との比較を行った。この結果，本地域の深部（標高−1500～−1000m）では，大霧地熱発電所の主貯留層となっている銀湯地区や高温の噴気・温泉・変質帯などの地熱徴候が分布する調査地域北部および南東部高岡山周辺において高密度域が見られた。特に北部の高密度域には，本地域に見られる西南西−東北東方向の断層と平行する傾向が見られた。また，南東部の高密度域を取り囲むように低密度域が分布している。地熱貯留層が分布する標高−500～0mの範囲では，深部で見られた北部の高密度域はいくつかの小さな高密度域に分岐している。この内，銀湯地区と白水越地区の間に存在する高密度域は，標高−300mでの圧力分布や温度分布との比較から，両者の地熱貯留層を分けている可能性が高い。また，この高密度域の周囲には銀湯地区や白水越地区に卓越する西南西−東北東方向の断層と同じ方向性を持つ低密度域が存在する。この低密度域は高密度域との密度差が小さいことや坑井の温度分布との比較などから地熱貯留層が含まれる破砕帯や断裂帯を捉えている可能性が高い。空中重力偏差法探査は本地域のような地上重力測定点密度の稠密化が困難な火山地域において地下の詳細な密度構造推定に有効であることが明らかになった。

Results from FALCON® Airborne Gravity Gradiometer surveys over the Kauring AGG Test site

The Kauring Test Site in Western Australia was established in 2009 to provide a public benchmarking and comparison venue for new and existing airborne gravity and airborne gravity gradiometry (AGG) technology. Fugro Airborne Surveys flew the fixed-wing FALCON® AGG system over the Kauring AGG Test Site over three periods in July 2011, November 2011 and February 2012. Comparison between the FALCON AGG survey data and the high resolution ground gravity data over the Kauring AGG Test site indicates that the FALCON vertical gravity gradient, GDD, has an error of +/- 5.6 Eo, and that the FALCON vertical gravity, gD, has an error of +/- 0.18 mGal,. Comparison between the digital elevation model (DEM) derived from the fixed wing FALCON survey laser scanner data, and a high-resolution third party DEM, indicates that the error of the vertical position of the FALCON differential GPS is less than 0.5m. At 60m terrain clearance this corresponds to a subsequent error in AGG terrain correction of less than 2 eotvos. This terrain correction error is well within the Kauring AGG Test Site FALCON survey noise envelope of 5.6 Eo.

Robust 3D gravity gradient inversion by planting anomalous densities

We have developed a new gravity gradient inversion method for estimating a 3D density-contrast distribution defined on a grid of rectangular prisms. Our method consists of an iterative algorithm that does not require the solution of an equation system. Instead, the solution grows systematically around user-specified prismatic elements, called “seeds,” with given density contrasts. Each seed can be assigned a different density-contrast value, allowing the interpretation of multiple sources with different density contrasts and that produce interfering signals. In real world scenarios, some sources might not be targeted for the interpretation. Thus, we developed a robust procedure that neither requires the isolation of the signal of the targeted sources prior to the inversion nor requires substantial prior information about the nontargeted sources. In our iterative algorithm, the estimated sources grow by the accretion of prisms in the periphery of the current estimate. In addition, only the columns of the sensitivity matrix corresponding to the prisms in the periphery of the current estimate are needed for the computations. Therefore, the individual columns of the sensitivity matrix can be calculated on demand and deleted after an accretion takes place, greatly reducing the demand for computer memory and processing time. Tests on synthetic data show the ability of our method to correctly recover the geometry of the targeted sources, even when interfering signals produced by nontargeted sources are present. Inverting the data from an airborne gravity gradiometry survey flown over the iron ore province of Quadrilátero Ferrífero, southeastern Brazil, we estimated a compact iron ore body that is in agreement with geologic information and previous interpretations.

Practical aspects of terrain correction in airborne gravity gradiometry surveys

Proper terrain correction in onshore airborne gravity gradiometry surveys requires detailed digital elevation models (DEMs). However, the horizontal resolution requirement of DEMs is not well explored in literature. As a result, there is no standardized horizontal resolution used in commercial surveys. Also, the effect of requisite low-pass filtering applied during and subsequent to acquisition has not been discussed within this context. We address these issues by quantifying the optimal horizontal resolution using an example dataset and a practical method for performing terrain corrections in barometric surveys, based on Parker’s 1972 Fourier-domain calculation. We also quantify the required spatial extents of DEMs (including grid padding) by deriving empirical rules based on numerical simulations. With these parameters quantified, we provide the basis for optimising the terrain correction to improve efficiency in not only gravity gradiometry terrain corrections and forward modelling, but also in DEM acquisition survey design.

The Benefits of Wide Line Spaced Airborne Gravity Gradiometry On Regional Surveys

To stimulate the development of natural resources most governments maintain a basic national geophysical database to outline regional structural geology and basin geometries. Magnetics and gravity are normally the tools of choice. There is an obvious trade-off between cost and detail. Airborne gravity gradiometry can be configured to optimise this trade off. It is known that gravity gradiometry detects shorter wavelengths then what is possible with conventional airborne gravity but longer wavelengths are also captured on regional surveys as is demonstrated with an example survey. An airborne gravity gradiometry survey was conducted in Arnhem Land, Northern territory, that was previously covered by conventional airborne gravity. Analysis of the data shows that the conventional airborne gravity dataset has limited content at spatial wavelengths shorter than 4 kilometre whereas the gravity gradiometry data resolves wavelengths shorter than 1 km, while also maintaining the long wavelength information. The analysis indicates that airborne gravity gradiometry offers better resolution at the same line spacing. An additional benefit is the option to infill areas of interest to capture detail not possible with conventional airborne gravity. This enables more effective use of the regional gravity gradiometry data in an exploration programme.

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.