GeoWebinars’21. New-generation airborne geophysical technologies in hydrocarbon exploration

Airborne geophysical technologies in hydrocarbon exploration.

Airborne geophysical technologies in hydrocarbon exploration.

Abstract:

A TECHNOLOGICAL BREAKTHROUGH IN AIRBORNE GEOPHYSICS FOR
HYDROCARBON EXPLORATION
Alexey Leonidovich Meisner
JSC GNPP Aerogeofizika

Airborne geophysical survey technology has developed rapidly over the past 10 years. This technological breakthrough has been driven by the evolution of three principal components: airborne platforms, instrumentation and software. Alongside technological development, demand for airborne geophysical surveys has grown among oil and gas producers conducting geological exploration in increasingly challenging areas, in terms of geology, climate and infrastructure alike. The high mobility and relatively low cost of such surveys can substantially reduce the time and financial resources required for geological exploration while minimising environmental impact.

Methodology. When planning an airborne geophysical survey, the first requirement is to select the type of aircraft that will carry the survey equipment. Both operational and economic efficiency may depend strongly on this choice. Whereas only 5 years ago airborne gravity and magnetic surveys in Russia were conducted predominantly using AN-30 aircraft and Mi-8T helicopters, the range of available aircraft has now expanded substantially. Lighter yet productive Cessna aircraft and Robison helicopters have become available for airborne geophysical surveys in Russia. At the same time, reductions in survey equipment size and weight have made it possible to use ultralight airborne platforms such as AN-2 aircraft, Eurocopter helicopters and uncrewed aerial vehicles (UAVs). UAV use is now emerging as a separate field, and it can be said that the technology for using UAVs in airborne geophysical surveys is currently taking shape. Of the three principal airborne geophysical methods most effective in hydrocarbon exploration—gravity prospecting, magnetic prospecting and electromagnetic prospecting—only magnetic prospecting has so far been implemented in the field using UAVs as airborne platforms. Nevertheless, in 2020 a new strapdown gravimeter was tested whose weight and dimensions permit UAVs to be used as platforms for airborne gravity surveys. UAVs certainly have a future and enormous potential in airborne geophysics, just as production robotisation is logically to be expected globally over the coming decades. The advent of a new-generation strapdown gravimeter insensitive to vertical accelerations has made it possible to combine airborne gravity, magnetic and electromagnetic prospecting in a single system, whereas the first two methods had previously been methodologically incompatible with airborne electromagnetic prospecting. Airborne electromagnetic surveys require flight at a constant height above the ground while following the terrain, whereas vertical accelerations affected data obtained with previous-generation platform gravimeters. The new-generation strapdown gravimeter was tested in Russia’s Arctic regions. At a weight of 15 kg, it yields data of a quality comparable to that of the widely used GT gravimeter, which weighs 160 kg. Thus, given the wider choice of airborne platforms and modernised instrumentation, airborne geophysical services for hydrocarbon exploration can be grouped into three principal packages. The first, historically traditional package comprises gravimetry and magnetometry and addresses regional and exploration objectives relevant primarily when a producing company gains access to a licence area, assesses its geological exploration strategy and begins searching for hydrocarbon deposits. The second package includes airborne electromagnetic surveying, photography and laser scanning in addition to airborne gravity and magnetic prospecting, addressing regional and exploration objectives as well as preparation for ground-based geological exploration. A helicopter must be used as the airborne platform for this package. The third package supports the preparation and monitoring of geological exploration, and its suite of methods permits the use of ultralight aircraft and UAVs as platforms. Alongside the rapid development of the fieldwork and instrumentation components of airborne geophysical surveys, office-based processing and interpretation of the data remain a critical stage that profoundly affects the final product. Today, previously used approaches to processing and interpretation can no longer meet producing companies’ requirements for the final results of airborne geophysical surveys as part of oil and gas exploration. With this in mind, algorithms for processing and interpreting airborne geophysical data have been improved. In interpretation, particular attention has been given to integrating and jointly analysing potential-field data with ground-based electrical prospecting, seismic and drilling data. Processing requires an individual approach to each region studied rather than a uniform one. For example, removing the influence of the upper layer from gravity data is a complex task, not merely the application of a constant density to the upper part of the section. A terrain correction using a constant mean density of 2.3 or 2.67 g/cm3 was conventionally applied during processing. In reality, this assumption is far from always valid. To study areas with complex terrain and lateral density variations in near-surface layers, a method has been developed for constructing multilayer models that describe density variations in the upper part of the section in greater detail. Selecting the densities of upper-section layers may be an iterative process, and the final result is determined by the processing geophysicist and interpreter using a priori data and general understanding of the geological structure of the area studied. During interpretation, integration of airborne geophysical, seismic and drilling data yields a consistent geophysical model that can substantially reduce uncertainties in geological structure associated with lateral variations in the density, acoustic and geometric properties of sedimentary-cover and basement formations. Interpretation consists of a set of algorithms, each addressing a specific task. To solve interpretation problems that are difficult to formalise at the geophysical data integration stage, the company’s engineers use advanced machine-learning algorithms available as open-source libraries for the high-level programming language Python. This approach provides an individually optimised solution to a complex problem that accounts for the specific project and has proved highly effective. Integrated interpretation results are used to construct and refine geological models, which in turn support quantitative estimation of hydrocarbon volumes and the planning of further exploration. Considerable scope nevertheless remains for developing both airborne geophysical data processing and integrated interpretation, primarily in conjunction with seismic and drilling data. Conclusion. Technological development of the three principal components of airborne geophysics—airborne platforms, instrumentation and software—has substantially improved the operational and economic efficiency of these methods in hydrocarbon exploration. Airborne geophysical survey, processing and interpretation methods continue to evolve. New algorithms for exploration tasks draw on experience accumulated in airborne geophysical surveys in Russia and on the experience of foreign oil and gas producers and airborne geophysical companies. Airborne geophysical surveying makes it possible to obtain vast datasets quickly and without environmental impact, helping to refine understanding of the geological structure and hydrocarbon potential of the area studied. This subsequently allows more expensive seismic and drilling work to be focused on the most prospective sites.

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