Asbjorn Norlund Christensen

High resolution gravity surveys from a fixed wing aircraft

BHP has in operation two FALCON airborne gravity gradiometer (AGG) systems providing gravity surveys with sufficient sensitivity and resolution for mapping the gravity anomalies associated with mineral deposits. Their development was motivated by the need for greater selectivity in regional reconnaissance surveying. This is achieved through the addition of density information to magnetic susceptibility and electrical conductivity which are available from other airborne survey technologies. The capability of the FALCON AGG systems is demonstrated with three examples. The King George anomaly was identified as an isolated magnetic anomaly in government release regional magnetic survey data. A FALCON survey of 2500 line km over the anomaly was completed in 5 days and demonstrates a clearly defined gravity anomaly of 7 mGal coincident with the magnetic anomaly. This gravity information is sufficient to upgrade the significance of the magnetic anomaly as a potential iron-oxide-copper-gold (IOCG) target. The Middleback Ranges, South Australia are an iron ore province supplying the OneSteel steelworks at Whyalla. The survey area includes relief of 300 m and some sharp escarpments, requiring accurate topographic correction of the gravity data. The FALCON survey has been compared with the ground gravity data in both gravity and gravity gradient form.

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. An airborne gravity gradiometry and high-resolution aeromagnetic survey conducted in Lake Tanganyika within the East African Rift System has been interpreted to demonstrate sedimentary sequences and structures prospective for petroleum. Numerous play types analogous to the Lake Albert oil discoveries further north in Uganda are revealed by further surveys.

Noise and Repeatability of Airborne Gravity Gradiometry

When evaluating the capability of any Airborne Gravity Gradiometer (AGG) system some of the most useful inputs are data collected over areas with good-quality ground truth. A gravity test range has been established at Kauring in Western Australia for such comparisons. CGG (then 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, gD, has an error of /- 0.18 mGal, and that the FALCON vertical gravity gradient GDD has an error of /- 5.6 eotvos at 300m full wavelength low-pass filtering. Analysis of repeat surveying over the Kauring AGG Test site suggest slightly lower errors of the order of /- 0.10 mGal for the FALCON vertical gravity, gD; and that the FALCON vertical gravity gradient GDD has an error of /- 4.7 eotvos after 300m full wavelength low-pass filtering. We strongly recommend the collection and publication of comparative analyses over Kauring and areas with similar quality ground gravity data to establish the capability of AGG systems.