New technologies signal a renaissance in airborne geophysics

By Stewart Walter, Fugro Gravity and Magnetic Services

As oil companies continue to push the boundaries of exploration into ever more frontier areas with increasing costs and risk of failure, it becomes necessary to review all possible data that can be used to reduce the risk and ensure that the exploration budget is well spent.

HIGH RESOLUTION AEROMAGNETICS FOR OIL AND GAS

Airborne geophysics has historically played a significant part in exploration for oil and gas. Aeromagnetics was the first airborne technology and was originally regarded as a reconnaissance technique to screen acreage and plan expensive seismic campaigns. The advent of faster sampling acquisition systems and GPS positioning in the 1990's saw the introduction from the mining sector to the oil and gas industry of high resolution aeromagnetics, with survey line spacings down to 250m. The consequent identification of low amplitude anomalies associated with intra-sedimentary faulting allowed such data to be used not only at the start of the exploration phase but also with seismic data to confirm structures and so help to de-risk prospects. The companies that now make up Fugro Airborne Surveys were at the forefront of this revolution and have now introduced further cutting-edge technology developed for mineral exploration to the oil industry: the Falcon airborne gravity gradiometry system.

Figure 1. High resolution magnetic data separated into frequency bands to show sources at different depths

To fully appreciate the significance of the Falcon system it is first necessary to review the history of airborne gravity data. Gravity data is one of the oldest geophysical methods and many hydrocarbon discoveries were made by drilling anticlines or other structural highs predicted by gravity data. Acquisition of land gravity is slow and expensive in many exploration areas, where access is difficult or dangerous, and it has long been the goal of geophysicists to find a methodology to acquire airborne gravity data of sufficient accuracy to be useful at more than a regional level.

AIRBORNE GRAVITY CHALLENGES

The measurement of gravity from an aircraft is technically very challenging since the accelerations due to the aircraft are several orders of magnitude larger than the gravity differences due to changes in rock type. Again the advent of GPS provided a breakthrough, allowing the external measurement of the aircraft motion, so that airborne gravity has for the last decade been another tool in the explorationist's toolbox.

There is some discussion as to the resolution that can be achieved by airborne gravity systems and this is dependent on a number of factors including the type of gravity meter, the aircraft platform, weather conditions, skill of the pilots, etc, but in all cases the data must be filtered to remove residual high frequency noise and this also removes the high frequency signal which is generated by shallow sources, generally in the top 2 to 3 km of the section. This means that airborne gravity is still something of a basin-wide tool, used to identify deep structural features, sedimentary thickness and major geological trends. Fugro have successfully flown many such surveys, including country-wide programs in Angola and Ghana, to stimulate onshore exploration and so airborne gravity certainly has a place. However, for more detailed work the interpretative value is limited. For instance salt domes coming through to surface cannot be accurately interpreted and a series of en-echelon basin bounding faults cannot be distinguished from a single continuous fault.

Figure 2. Typical survey aircraft with external stinger-mounted magnetometer at rear and gravity meter or gravity gradiometer with ancillary equipment onboard.

TECHNOLOGY BREAKTHROUGH WITH FALCON GRAVITY GRADIOMETER

It was with these limitations in mind that BHPBilliton undertook a 5-year collaboration with Lockheed Martin to produce an airborne gravity gradiometer sufficiently sensitive to map the low amplitude, near surface anomalies associated with mineral exploration. The result was the Falcon gravity gradiometer, the only system developed specifically for airborne use. Fugro provided operational support for a number of years, before purchasing the technology from BHPBilliton in 2008 and making it commercially available to the oil and gas industry. Falcon relies on multiple sensors which use differential measurement to remove the effects of the aircraft motion. With a resolution 20 times better than airborne gravity, the Falcon system allows acquisition of gravity data comparable in accuracy to land gravity. This enables the interpretation of even very shallow geological features and reduces the ambiguity in the interpretation of fault systems, diapirs etc.

The Falcon system measures gravity gradients and the vertical gravity field (the more familiar quantity as measured by land gravimeters) is derived from them. The gradients are themselves an interpretative tool and their availability has been coupled with improvements in the Fugro interpretation software, the latest versions of which offer simultaneous inversion of gravity and magnetic total field and gradient data. These simultaneous inversion techniques reduce the inherent ambiguity in potential field data and when combined with some additional control from a seismic line the interpreter can feel confident in the robustness of his or her interpretation.

UNDERSTANDING THE OBEJECTIVES IS VITAL

The interpretation software is also vital in the survey planning stage. Modelling the responses of the likely geological structures allows the geophysicist to select the best technology and determine a realistic budget for the survey. The increased resolution of the Falcon airborne gravity system along the survey lines can only be fully exploited by flying closely-spaced survey lines, as close as 330m in some cases, to allow production of detailed grids of data.  Compared to airborne gravity this may be a six-fold increase in the data volume and consequently the associated cost. It is therefore important to know that the increase in resolution is necessary to achieve the objectives. Equally, it is clear that choosing airborne gravity simply because of budget constraints without understanding the target responses is also a mistake.

The addition of the Falcon airborne gravity gradiometer to Fugro's airborne capability is an exciting development and uniquely allows Fugro to offer gradiometry alongside airborne gravity and high resolution magnetics. Explorationists can also feel confident that they will get impartial specialist advice from Fugro geophysicists on which of the airborne potential field techniques will provide the best solution to reducing their exploration risk.

About

Stewart Walter

International Business Development

Fugro Gravity and Magnetic Services

Fugro House

Wallingford OX10 9RB

United Kingdom

Tel: +44-1491-820600

Email: s.walter@fugromcs.co.uk