ABSTRACT: New Developments in Aeromagnetic Techniques for Oil Exploration
PRIDMORE, D., E. ETHERIDGE, I. CAMPBELL, and K. McKENNA, no affiliation
Aeromagnetic surveying has traditionally played an important role in reconnaissance exploration for hydrocarbons, principally as a basin mapping tool. Lately micromagnetic effects, generated at shallow depths by migrating gases or fluids above hydrocarbon accumulations have been targeted with aeromagnetic surveys.
The recent utilization of GPS satellite navigation, or similarly precise radio positioning, together with very low noise acquisition systems and recently developed image and data have increased, significantly, the potential role played by aeromagnetics.
High resolution surveys flown at line spacings of 500-2,000 m, 80-200 m mean terrain clearance and with aircraft manoeurve noise levels of less than 0.3nT (or gammas) provide data sets that can map structures within the sedimentary section.
Or course, the improved survey parameters allow basin mapping and detection of micromagnetic effects to be carried out with greater precision.
Linear or curvilinear features within enhanced images of the magnetic data can be identified as structures on the basis of correlation with other data sets, in some cases seismic. The wavelengths involved in these linear or curvilinear features indicate, unambiguously, source within the sedimentary section. Image processing of magnetic data is an essential step for the identification and mapping of sedimentary structures, since the magnetic effects being sought are often of the order of 1nT (gamma) or less.
These features have been observed in all sedimentary basins surveyed with high resolution, low noise, low altitude surveys over the past several years including the North Slope of Alaska, the West Permian basin of Texas, the Papuan basin of Papua New Guinea and the northwest shelf of Australia.
Where detailed comparison with seismic has been carried out, structures mapped within the sedimentary section from the magnetics can be correlated with those mapped by seismic, although not every structure mapped on the seismic can be traced on the magnetics.
The magnetic expression of the structure varies both from basin to basin and within basins. In some cases it is consistent with juxtaposition by faulting of differently magnetized beds, in others the structure corresponds to a zone where magnetic minerals have been uniformly destroyed or created, presumably by circulating fluids. A third response type, a narrow linear zone of chaotic magnetic response has been observed to correspond with structure.
Depth slicing techniques where the magnetic response of geology from different depth intervals within the earth is isolated are showing great promise. Inherent limitations of the potential field technique severely restrict the precision and sharpness of the depth intervals that can be windowed, relative to the seismic technique. However, useful insights into rock type and structures within the basement and sedimentary section are gained by generating depth slice maps and subsequent downward continuation of the fields to sharpen subtle responses.
Since the introduction of aeromagnetic surveying, sedimentary basin fill has been classified as nonmagnetic and to a large extent ignored. In fact, many sediments are weakly magnetic and over recent years it has been demonstrated that fluids (or gases) circulating within sedimentary basins effectively mobilize iron within the basin.
High resolution, low noise aeromagnetic surveys have demonstrated the capability to map structures within the sediments, enabling direct integration of seismic and magnetic interpretations. Modern aeromagnetic data complements both seismic survey planning, through knowledgeable placement of seismic lines, and seismic interpretation where the uniform and typically tight line spacing of aeromagnetic data allows structures to be interpolated between seismic lines.
A data base of magnetic properties of sediments, both susceptibility and remanance and better understanding of iron associated alteration effects resulting from fluid migration in sedimentary basins are urgently required before recent developments in airborne magnetic technology can be fully exploited.
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)