The Importance of Airborne Geophysical Surveys for O&G Exploration
Airborne Magnetotelluric (MT) geophysical surveys are currently used worldwide to explore the deep structure and composition of the earth’s crust. The MT method is based on well-established theories published in the US, France and Russia and supported by successful field operations worldwide. The MT technology is recognized as having been developed by French geophysicist Louis Cagniard and Russian geophysicist Andrey Nikolayevich Tikhonov in the early 1950s. With almost continuous advances in instrumentation, processing and modeling since then, MT has become one of the most important tools for deep Earth research by academia and industry.
Since the 1950s sensors, receivers and data processing techniques have increasingly relied on digital electronics, becoming less expensive and more capable with each generation. Major advances in MT instrumentation and technique include the advent of remote referencing, GPS time-based synchronization, and 3D data acquisition and processing. Natural Electromagnetic (EM) signals or Telluric currents change as they pass through the earth and are distorted by the type and structure of the rock in their path. By recording signals in the 0.01 Hz to 10,000 Hz range, it is possible to determine the structure and composition of the earth. Each frequency measured has a different wavelength and each wavelength corresponds to a specific depth. O&G reservoirs are usually a combination of brackish water and dissolved hydrocarbons. When Telluric currents flow through the boundaries between hydrocarbons and water, they develop strong interfacial double layer electric charge effects. The accumulation of these charges results in an anomaly detected by airborne EMT system.
Over 75% of the world's hydrocarbon basins have some form of surface seeps. These seeps form plumes as they rise to the earth’s surface, which are also detected as a result of the cumulative charge effect and provide additional valuable information on the location of the reservoir. Hydrocarbon seeps can be mapped using a combination of active and passive fluorescence detectors, satellite imagery and airborne EMT system both onshore and offshore. The data from MT airborne surveys is ideal for integration with 2D and 3D seismic, satellite imagery and surface geochemistry.
For O&G exploration, MT is mainly used as a complement to the primary technique of reflection seismology exploration. While seismic imaging is able to image subsurface structure, it cannot detect the changes in resistivity associated with hydrocarbons and hydrocarbon-bearing formations. MT does detect resistivity variations in subsurface structures, which can differentiate between structures bearing hydrocarbons and those that do not. At a basic level of interpretation, resistivity is correlated with different rock types. High-velocity layers are typically highly resistive, whereas sediments – porous and permeable – are typically much less resistive. While high-velocity layers are an acoustic barrier and make seismic ineffective, their electrical resistivity means the magnetic signal passes through almost unimpeded. This allows MT to see deep beneath these acoustic barrier layers, complimenting the seismic data and assisting interpretation.
MT has been important in support of traditional seismic surveys in offshore hydrocarbon exploration. In places such as Brazil and Africa, airborne geophysical surveys have been vital in detecting linkages between basins, such as is the case in Brazil’s Equatorial Margin, where data indicates there may be connections between all of the offshore basins in the region, extending from the border with French Guiana all the way to Rio Grande do Norte, something to the tune of 2,000km in a straight line, or over 50,000 square kilometers when considering the size of each known basin. Complementary studies are ongoing to determine the exact extension of this mega-basin.