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by Graham Chandler on March 27, 2014 applied
Inversion of Airborne Gravity Gradient data helped to enhance earth models and interpretation of Norway’s Karasjok Greenstone Belt shown above.
In the early 1980s, the Geological Survey of Norway (NGU) conducted an exploration program in the northern county of Finnmark, a highly prospective greenfield area which encompasses the Karasjok Greenstone Belt. It was the first aeromagnetic and gravity survey of the area and served to establish a preliminary description of its lithology, stratigraphy and mineralizations.
In the ensuing decades, however, it became necessary to revise the data as it was no longer providing adequate mineralization information. For example, many of the original gravity points were acquired on the ground using snowmobile transportation.
“The original gravity anomaly is dominated by a regional field and so it was difficult to link local anomalies to the local geology,” explains Dr. Jörg Ebbing of the NGU. He adds that those efforts were accompanied by petrophysical sampling even though large parts of the areas are covered by overburden. The sampling showed that the amphibolites and komatiites of the greenstone belt have a higher density and magnetization than the surrounding gneiss and migmatite complexes.
The Karasjok greenstone belt is a continuation/part of the Central Lapland greenstone belts. In Finland, high-resolution gravity data and seismic are available, and the results of their interpretation by the Geological Survey of Finland (GTK) showed the need to improve the geophysical data base in Norway on the way to increasing the geological understanding of the whole northern Fennoscandian Shield.
Improved datasets were thus in order. “In recent years, Store Norsk Gull [a Norwegian exploration company] has identified Finnmark as the most prospective greenfield area in Norway,” says Ebbing, adding that the company’s efforts were focused on the Karasjok Greenstone Belt. Further geochemical and geological mapping, combined with fixed-wing aeromagnetics and drilling, confirmed the existence of copper and gold mineralization. Apart from these limited efforts, no systematic remapping of the area was done; nor was anything done specifically to test the greenstone belt’s extension at depth. Says Ebbing: “Depth extents to mid-crustal or even lower-crustal level are suggested for other greenstone belts, [such as] the Barberton in South Africa, the Pilbara Craton in Australia, and the Kuhmo greenstone belt of Finland.”
Regional field sources. At left: Residual IGRF corrected RTP, upward continued 1km. At right: Gzz Bouguer, upward continued 1km with >0 RTP contours.
Unconstrained AGG density model (left) compared with the IRI focused AGG density model (right), sliced to northern portion of the survey area. Notice improved amplitude recovery and sharpening of thrust fault contact. Colour legend density contrast range for both models.
Comparison of magnetization source depth. Looking due East. Top is unconstrained: highly magnetized sources >0.1 normalized units, source depths reach <2km. Bottom is constrained by high density contrasts: highly magnetized sources >0.1 normalized units, source depths reach <4km.
So in 2011, NGU launched a program called Mapping of Mineral Resources in Northern Norway (MINN). The program collects and assembles geophysical, geological and geochemical background data so that mineral resources could be mapped and studied. (See the data on www.ngu.no.)
As part of this program, aeromagnetic and gravity gradient data were collected in the area of the Karasjok Greenstone Belt. These efforts, Ebbing notes, were co-funded by Store Norske Gull AS. Fugro Airborne Surveys (now CGG) flew the program, and the gradient data were measured using the FALCON Airborne Gravity Gradiometer (AGG). A combined total of 3,291 line-kilometres of data were flown with an average drape height of 160 metres with 200 metre in-line and 5 km tie-line spacing.
The particular combination of data types – magnetic and AGG -- was significant. “The combined acquisition of gravity gradient and magnetic data has become increasingly popular for industrial applications,” says Ebbing, adding that both data sets are highly sensitive to near-surface structures. “The greenstone belt of the Karasjok area was of interest for acquiring this novel type of data set as mineral deposits often are found in connection with them.”
Significant, too, was NGU’s choice of 3D inversion techniques in Geosoft VOXI Earth Modelling to interpret the data. “The VOXI inversion tools offer a straightforward inversion for gravity gradients, which is not yet a standard in other software packages,” Ebbing says. “Greenstone belts in general are an ideal target for gravity and/or magnetic studies as they often feature higher density and magnetization than the surrounding bedrock.” “From petrophysical studies in Finnmark we could expect that the komatiite components of the belt are to be characterized by positive density contrasts [greater than 50-250 kg/m3] and higher magnetization than the surrounding bedrock,” he says, adding that an advantage of gravity gradients is that their characteristics are comparable with magnetic field behaviour. “Gravity gradients, in contrast to conventional gravity anomalies, are more sensitive to the upper crustal structures.”
This application of VOXI Earth Modelling was done with enhancements: for the first time, improvements in the prismatic approximation of surfaces and Iterative Reweighting Inversion (IRI) methods for reverse-modelling of gravity gradient data were used. This was followed by a co-modelling exercise using the combined AGG and aeromagnetic data to interpret the greenstone belt in terms of its petrophysical properties.
Ebbing endorses the approach: “While it is challenging to estimate the depth from gravity and magnetic data alone, the inversion can give a minimum/maximum bound of depth extension within the parameter range provided. Furthermore, the sequential inversion of the density and magnetization allowed us to identify areas with different characteristics. As an example, we expected the dominant high magnetization regions to coincide with the dense mafic units to be identified as the greenstone belt. From the two high-density regions identified in the upper to mid-crust, one shows such a characteristic.” He adds that the western branch is identified by a slightly lower density contrast, “but with a significantly lower contrast in magnetization (lower susceptibility contrasts), which indicates a more granitic, felsic domain within the belt.”
The results were revealing. The tilt derivative of the gravity gradients shows a structure that extends from northwest to southeast and then swings into a northeast-southwest trend. “This anomaly,” Ebbing notes, “most likely reflects the greenstone belt, which underlies the geology observed at the surface and cannot as easily be identified from the gravity anomaly, not even from the magnetic anomaly map.”
Ebbing believes the project was a vast improvement over older methods of inversion. The main improvement is that the measured AGG data can be directly used for the inversion. Other software packages still require that the vertical gravity component be used, which requires calculation of the conformed gravity field.
The second improvement is that the AGG-derived density structure can be used as an input into the magnetic vector inversion and as a constraint during the IRI. This helps to force some of the magnetization to greater source depths than those created by running an inversion on the magnetics alone.
The depth aspect was significant, Ebbing says. “The unconstrained inversion of magnetic data would lead to near-surface distribution of the magnetic sources with a maximum source depth of less than two kilometres. The use of the AGG-derived density structure as input led to a distribution to four kilometres depth and within a reasonable range for magnetic susceptibility.”
The inversion has helped to outline the Karasjok Greenstone Belt and the NGU is prepared to evaluate different concepts as to its structure and evolution. The next step is to link the inversion results with a geological model in 3D.
Says Ebbing: “This will allow us to redefine the geological map and link the small-scale features like zones of mineralization to the semi-regional subsurface model. The Karasjok Greenstone Belt can serve as a case example for the use of geological and geophysical techniques both for mineral exploration and geology in 3D. For this next step, the integration of all available information through GIS and within the 3D model is vital.”
Efforts are also underway to make the MINN project data more accessible to explorers. All the geophysical data from NGU, including the MINN project, will be published and distributed through a Geosoft DAP Server from early 2014.
Our current database system is outdated and is limited to display of existing data but does not provide any download facilities,” says Ebbing. “The new web solution, powered by DAP, will be modern and more easily accessible, enabling all clients to download the data of interest for a specific area.”