Airborne geophysical methods in hydrocarbon exploration

In recent years, the share of airborne geophysical methods in integrated oil and gas exploration has increased worldwide. This is primarily attributable to a new stage in the development of airborne geophysical technologies. Airborne geophysicists now have modern high-accuracy measuring equipment and the latest systems for geophysical and geological interpretation of the acquired data. The suite of airborne methods has itself expanded substantially with the inclusion of airborne gravimetry. The results make it possible to optimize exploration techniques and minimize expenditure on costly seismic and drilling operations.

Over recent decades, airborne geophysical methods and technologies have become firmly established in international practice for the geological and geophysical preparation of prospective oil and gas areas for exploration. Airborne geophysical surveys, predominantly aeromagnetometry, have been conducted at various times across all the world’s major sedimentary basins, providing a basis for planning and organizing hydrocarbon exploration through seismic surveys and drilling. Leading oil and gas producers and gas companies have funded airborne geophysical work, reducing and optimizing the cost of finding new deposits and significantly shortening the implementation time of new exploration projects. This has been made possible by the exceptional speed of airborne geophysical technologies, their economy, standardized methodology and exceptionally high statistical representativeness of the acquired data, combined with environmental compatibility and virtually universal accessibility.

In recent years, despite the apparently adequate airborne survey coverage of the world’s main oil and gas basins—mostly magnetometry and, less frequently, gamma-ray spectrometry—there has been a kind of boom in the use of airborne methods for oil and gas exploration, more pronounced abroad and less so in the Russian Federation. This is reflected both in new integrated airborne geophysical surveys of traditional hydrocarbon-producing regions surveyed by airborne methods several decades ago and in substantial survey volumes over new areas where surveys were conducted only recently.

This is readily understandable. In addressing exploration objectives, oil and gas company managers seek to expand airborne geophysical surveying because airborne geophysical technologies are now entering a new, truly revolutionary stage of development, characterized by:

• a fundamentally different level of quality in modern measuring equipment and the introduction of high-accuracy satellite navigation and measurement positioning;
• expansion of the airborne geophysical method suite to include airborne gravimetry, thermal infrared imaging and airborne gas surveys;
• fundamentally new systems for geophysical and geological data interpretation based on modern high-performance computing and advanced physical and geological theories.

Modern airborne geophysical methods and equipment.

Aeromagnetometry is one of the basic methods used to prepare oil- and gas-prospective areas for ground exploration. By the late 1960s, State aeromagnetic surveys had covered virtually the entire country. Nevertheless, airborne surveys using this method have been systematically conducted over previously studied areas in recent years. The reasons for repeat surveys become clear when comparing the technical and metrological characteristics of previous- and current-generation aeromagnetometers and the qualitative and quantitative differences between the information extracted previously and that available today.

Modern magnetometers have extremely high sensitivity (down to 0.0005 nanotesla, nT) and acquisition rates (from 10 measurements per second). This allows confident identification and analysis of metrologically reliable anomalies with amplitudes of a few nT or fractions of a nT.

The introduction of satellite navigation systems has had a particularly strong influence on airborne geophysical survey quality, including aeromagnetometry. Modern navigation systems use receivers for the combined GPS (USA) and GLONASS (Russia) constellations. These provide real-time observation-point positioning accuracy of 10–20 m, improving to less than 1 m when navigation data are differentially corrected in post-processing. This minimizes distortions in measured anomaly shapes caused by positioning errors.

These changes in the quality characteristics of aeromagnetic field surveying, achieved in less than a decade, have made it possible to investigate very low-contrast effects in the anomalous magnetic field associated with vertical and lateral heterogeneities in the sedimentary cover, including at depths from hundreds of metres to a few kilometres.

Gravity surveying has always been a distinctive component of geophysical investigations in sedimentary basins. In terms of its physical principles, gravimetry may formally be considered an analogue of seismic exploration because there is a direct correlation between the elastic properties of a medium and the densities of its constituent rocks. Its application is limited only by the greater ambiguity in interpreting measurements, arising from the method’s physical and mathematical theory. Nevertheless, the role of gravimetry in investigating areas difficult to study seismically, including the shelf and, above all, the transition zone, can hardly be overstated.

Attempts to develop airborne gravity systems began in the 1970s, both in Russia and abroad. Satisfactory results, however, were obtained only with the practical introduction of modern satellite navigation systems. Operational systems with approximately similar characteristics are now available to several foreign companies—the North American companies CARLSON and FUGRO—and one domestic company, Aerogeofizika.

In terms of their specifications, airborne surveys performed using these systems meet the requirements for a scale of 1:200 000, but unlike comparable ground surveys, they provide uniform area coverage. Figure 1 compares ground and airborne gravity survey results in the White Sea area.

Fig. 1. Maps of Faye-reduced gravity-field anomalies (August 2000; arbitrary datum).
А — airborne survey data; В — ground survey data at 1:50 000

In recent years, the world’s leading oil companies have increasingly adopted airborne gamma-ray spectrometry, alongside aeromagnetic and gravity surveying, at the early stages of investigating oil- and gas-prospective areas. According to J. G. Morse et al., in 1995 the US Bureau of Mines published an information circular on the use of gamma-ray spectrometry in hydrocarbon exploration. Of 237 cases analysed, 85% reported positive results with varying degrees of confirmation. The radiation-field structure reflects both hydrocarbon accumulations themselves and elements of fault tectonics that may control fluid distribution.
The very low amplitude of useful anomalies against interference from the natural radioactivity background of surface formations places particular demands on equipment performance. High-quality surveys in Russia and abroad now generally employ digital 512–1024-channel spectrometers with Polyscin NaI(Tl) detectors having a total volume of up to 52 L and energy resolution at the 0.662 MeV line no worse than 9%. Recording the full gamma-ray spectrum allows not only natural radionuclide concentrations to be determined but also areas of anthropogenic contamination to be identified and radon concentrations in the near-ground atmosphere to be measured, which in turn enables mapping of active faults.
The first experience of airborne electromagnetic surveying for hydrocarbon exploration dates to methodological studies using a helicopter-borne transient electromagnetic (TEM) configuration at several sites in the Volga–Urals province in 1985–93. Positive results led to pilot operational surveys, which unfortunately were halted by the transformations then beginning in the country. Nevertheless, results obtained in Udmurtia, Tatarstan and the eastern Caspian region provided the basis for recommendations to include airborne electromagnetic surveying in the mandatory suite of advance investigations.
To assess the feasibility of mapping alteration halos above hydrocarbon accumulations, Geoterrex, the Canadian division of CGG, tested the Canadian GEOTEM airborne transient electromagnetic system over known oilfields in Nevada. Results were presented as a map of the transient decay time constant. Areas of positive values of this parameter were found to correlate well with the spatial distribution of producing wells.
Companies differ in their assessment of the potential use of thermal infrared surveys for hydrocarbon exploration. Some, predominantly foreign companies, use a model of anomalous temperature inversion over hydrocarbon accumulations to try to apply the method in routine exploration; others consider it supplementary but optional, while a third group, predominantly domestic companies, disregard its data entirely.
Without discussing the substance of this debate, we note only that, in our view, the method’s most effective application is monitoring and diagnosing hydrocarbon transport systems (oil and gas pipelines), as well as environmental surveys of oil- and gas-producing areas, including oil-product spills on water bodies.
Airborne gas surveying can serve similar purposes. This form of remote observation uses specialized open-path gas analysers to determine concentrations of nitrogen dioxide, sulphur dioxide, methane, butane, various aromatic hydrocarbons and similar substances in the near-ground atmosphere. Positive experience in using such surveys for exploration has been reported by the Krasnoyarskgeologiya Production Geological Association.
The listed methods for investigating oil- and gas-prospective areas provide, in the shortest possible time, with minimal funding and without any environmental consequences, a database unique in volume, integration and completeness. Its subsequent geological and geophysical interpretation ultimately minimizes costs and time and optimizes exploration techniques in prospective regions.

New technologies for geological and geophysical interpretation and prediction of hydrocarbon deposits using airborne survey results.

The geological and geophysical interpretation of integrated airborne survey data is based on assumptions about subsurface hydrocarbon-deposit localization patterns (exploration criteria), both empirically established and repeatedly confirmed and still hypothetical, and about how hydrocarbon accumulations themselves, or phenomena associated with their formation, alteration and destruction, are expressed in geological and geophysical fields (exploration indicators).
Interpretation technologies have been developed to identify and map predictive exploration criteria from airborne geophysical survey data, relying primarily on potential-field methods: magnetometry and gravimetry. These generally aim to divide the vertical rock section of the study area into separate subhorizontal components—markers—differing in depth and in magnetic and density properties. In ancient sedimentary basins, the principal marker is usually the platform’s folded basement, or more precisely its surface; less pronounced markers are weakly magnetic terrigenous interbeds within carbonate or clay-rich formations that are practically nonmagnetic. Accordingly, the main objective of geophysical interpretation of the magnetic field is to identify marker surfaces or layers and map their relief. Gravity-field interpretation is approached from the same standpoint.
Relief structures on marker-horizon surfaces may then indicate various structural hydrocarbon traps, while analysis of thicknesses between individual horizons can identify some nonstructural traps.
Analysis of the interpretation technologies investigated and used has shown that the geological structure of the folded basement itself in ancient sedimentary basins is a very important component of hydrocarbon prediction. It too can be mapped through geological interpretation of magnetic and gravity data.
An analysis of interpretation technologies known from published sources identified at least three groups of solutions within this approach. In the first group, predominantly foreign publications, modern quantitative interpretation methods are used only minimally. Interpretation is usually limited to simple transformations followed by recognition of the resulting patterns and attempts at tectonic regionalization. Only occasionally are simple singular-point analysis techniques, such as Euler deconvolution, used to estimate source depths.
The second and third groups of potential-field interpretation solutions became feasible through, on the one hand, exceptionally high field-measurement accuracy and, on the other, modern quantitative interpretation methodology. Their differences essentially reduce to the predominance of two-dimensional interpretation models in the second group and interpretation based on three-dimensional solutions in the third. These differences reflect both the capabilities of the computing equipment used and the availability of software implementing modern advances in physics and mathematics for interpretation in large-scale routine operations.

Fig. 2. Results of magnetic and gravity data interpretation for mapping the relief and geological structure of folded basement. 1, 2 — maps of rock magnetization and density, respectively; 3 — rock classification by magnetic properties; 4 — map of petromagnetic sections; 5 — depth map; 6 — geological-tectonic map of the upper basement section; 7 — map of relief structures. Fig. 3. Results of magnetic data interpretation for mapping relief structures of marker horizons in the sedimentary cover. 1, 2 — structural maps of magnetically active horizons М-1 and М-2 in the sedimentary cover; 3, 4 — maps of thickness variations between horizon М-1 and basement, and between horizons М-1 and М2; 5 — effective magnetization of rocks in horizon М-1; 6, 7 — map of fold and fault structures of the reference magnetically active horizons

Consider several examples of interpretation solutions based on airborne geophysical survey data for three-dimensional mapping of ancient sedimentary basin structure.
The first important component of such a workflow is usually determination of the depth to the folded, and sometimes metamorphosed, basement of platform regions, together with geological mapping of its upper section. The first part of this task can be addressed using, among other techniques, ROMGAS, developed at GNPP Aerogeofizika. It uses a spectral representation of potential fields to estimate the depth to a magnetically active (gravity-producing) surface, which in most cases may be associated with basement depth.
The second part is addressed using REIST, which models the basement magnetic field by fitting the effective magnetization of its constituent rocks. The same approach is used in gravity-field interpretation to obtain a model of the density-property distribution in basement rocks.
Figure 2 presents results of mapping the structures and surface of the folded basement of an ancient sedimentary basin in an oil- and gas-prospective region. Comparing estimated basement depths with available drilling data usually yields mean relative errors of approximately 5–10%.
To investigate the sedimentary cover, residual gravity and magnetic fields associated with vertical and lateral heterogeneities within it are isolated and then analysed to identify and map these heterogeneities.
The geological tasks here differ little from those encountered in geological investigations of the basement—subdivision, mapping and so forth. Methodologically and technologically, however, this is a much more difficult problem. This is primarily because structures and formations in the sedimentary cover contribute far less to the total magnetic field than comparable basement objects. In the example above, the basement magnetic model obtained using REIST accounts for the original field down to a residual standard deviation of 12 nT. Given that the original field had a standard deviation of approximately 300 nT, the contribution from sedimentary-cover formations can be estimated at approximately 0.16% of the total field energy.
This imposes particularly stringent requirements on the quality of the source data and makes surveys from earlier years unsuitable for addressing this task.
The most rigorous way to calculate residual fields is to reduce the original field by subtracting the basement-model field, which can be obtained using techniques such as REIST.
Under favourable conditions, the surface relief of reference horizons within the sedimentary cover can be estimated using ROMGAS. It is generally possible to map the depths of relatively thin rock units (tens of metres) with excess magnetic susceptibility of tens of 10-5 SI units.

Figure 3 presents the results of such estimates. Geological interpretation includes lithological and stratigraphic identification of the magnetically active horizons and construction of maps of fold and fault structures. Another important element is analysis of the thicknesses between identified lithological-stratigraphic units, or markers. This allows structures to be classified by age and formation conditions. In some cases, in carbonate sections, anomalous increases in thickness between terrigenous markers can be detected and are often associated with reef buildups.
The maps of basement geology, basement surface relief and sedimentary-cover structures resulting from geological and geophysical interpretation enable construction of a geological model of an ancient sedimentary basin and identification of important individual stages in its formation. All these data are important for identifying structural and nonstructural hydrocarbon traps—predictive exploration criteria—which subsequently form the basis for optimizing the design of ground geophysical and exploration drilling operations.
Exploration indicators of hydrocarbon accumulations remain insufficiently studied. To date, no direct indicators that unequivocally identify hydrocarbon accumulations and are unambiguously understood and interpreted have been established for fields measured by airborne methods. Indirect indicators are expressions in geological and geophysical fields of oil and gas accumulations and associated phenomena that may also reflect other geological formations unrelated to hydrocarbon accumulation. Many such indirect indicators are now recognized.
Identification of indirect deposit-exploration indicators is usually based on a model of specific epigenetic processes occurring above oil and gas accumulations in the overlying rocks owing to hydrocarbon migration. Some researchers consider the characteristic anomalous expressions of these processes to coincide approximately with accumulation boundaries, others with field margins, and others with areas of enhanced tectonic fracturing.
Many researchers point to possible changes in the physical properties of overlying strata under the influence of vertically percolating liquid fluids. They suggest that hydrocarbon fluids may substantially reduce the magnetic properties of rocks hosting accumulations and that aeromagnetometry may identify such anomalous zones. This enables hydrocarbon-deposit prediction from aeromagnetic data through joint analysis of structural maps of the sedimentary cover and magnetization distributions in reference horizons. Figure 3.5 shows an example of this type of anomaly derived from aeromagnetic survey data.
For radiogeochemical methods, it is assumed that deposit formation is accompanied by superimposed dispersion halos produced by long-distance migration of metals in mobile forms. Such anomalies have been investigated above known deposits in Western Siberia, Belarus and the Volga–Urals region.
Despite the debatable and hypothetical nature of the assumptions underlying the method, this pattern of geochemical transformations is clearly expressed in large-scale experiments investigating the consequences of environmental disasters involving oil releases and spills and wastewater spills. For remote methods, geochemical-field structure is reflected in the particular expression of natural radionuclide anomalies (U, Th, K).

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The data presented in this review and many years of experience in applying airborne geophysical technologies across a broad range of geological research and operational practice, including oil and gas exploration, suggest that their effectiveness in investigating the petroleum potential of ancient sedimentary basins will continue to increase.