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A Brief History of Micromagnetic Surveying
Micromagnetics is the petroleum exploration form of magnetic survey for recording and interpreting all frequencies of the magnetic field from the shortest wavelength anomalies sourced in the shallowest sedimentary formations to the longest wavelength anomalies sourced at or beneath the basement surface. This is accomplished by either using hand- held magnetometers on the ground surface or with airborne magnetometers flown at a terrain clearance in the order of 500 feet. The objective is to map all magnetic indications of structure from the top of the sedimentary column to crystalline basement.
The earliest magnetometers allowed measurement accuracies of between 10 and 25 gammas (nT). In 1936, W.P. Jenny wrote that the method had been "revolutionized" by improvements in the instruments and in field techniques that allowed accuracies of plus or minus 2 gammas. Magnetometers involved in the early mappings of sedimentary responses were mechanical, tripod-mounted, magnetic balances that measured either the vertical or horizontal component of the Earth's field. These and later mechanical devices, eventually improved to allow accuracies of plus-or-minus one gamma, were the mainstays for surface work until the coming of the proton magnetometer in the 1960's with its one-gamma sensitivity that eventually improved to 0.1 gamma.
In 1946, the Gulf Airborne Fluxgate Magnetometer, developed during World War II for anti- submarine warfare, became available for commercial use. This tool that allowed the rapid inventory of mineral resources regardless of terrain and accessibility, caused a revolution in reconnaissance survey, worldwide. The Mark II Gulf instrument with its analog data and noise envelope in the order of 0.25 gamma was the primary tool until the digitally-recorded, optically-pumped, cesium vapor magnetometer arrived in 1962. Its sensitivity and sampling rate have been improved over the years to .001 gamma and .01 second. In airborne work, an important factor in improving data quality has been the improvement in flight path identification from the snapshot photographic technique patented in 1952 by Homer Jensen of Aero Service Corp. to the current digital recording of each sample points geographic coordinates using data from the Global Positioning System satellite network.
Perhaps the first printed mention of the term micromagnetics was in the 1936 W.P. Jenny article, "Micromagnetic Surveys, Gulf Coast Structures May Be Outlined By This New Method" in which he stated that" The basic assumptions for the interpretation of magnetic anomalies have been changed completely. In former years we thought that all magnetic anomalies were caused by the basement or shallow igneous rocks. Today we know that sufficient variations in magnetic susceptibility exist within the sedimentary column to produce noticeable magnetic anomalies above structures of commercial interest". In the years that followed, Jenny carried out many surface magnetic surveys and published many articles on the subject of micromagnetics.
When the airborne magnetometer became commercially available in 1946, Jenny became perhaps airborne micromagnetics' most ardent advocate and practitioner. In 1953, under contract with The Aluminum Company of America (Alcoa) he designed and managed an airborne micromagnetic survey of the Frio-Miocene Trend of the Texas and Louisiana Gulf Coast. The 45,000 square mile program produced a unique perspective of the region's leads to sedimentary structure that were then systematically investigated and evaluated using subsurface geology and site-specific seismic in a very successful development of drilling prospects.
The next major point in the history of micromagnetics came in 1979 with T.J. Donovan's observations of sedimentary magnetic responses coincident with geochemical alteration of surface formations and production at the Cement Field in Oklahoma. Also, within the production area boundary a shallow subsurface zone was found by drill cutting analysis to have a higher magnetic susceptibility than at the same depth outside the production area. His conclusion that the upward microseepage of hydrocarbons from a reservoir causes geochemical alteration of iron minerals in the overlying sediments, has subsequently been well documented. We now know that depending on whether the magnetic minerals thus created are magnetic or non-magnetic, positive or negative magnetic responses will be the end product.
At this point in the history of micromagnetics, we know that when near-surface sediments are structurally displaced, a measurable magnetic anomaly is created. Such an anomaly can signify a local change in rock properties, commonly the geochemical alteration that accompanies the microseepage of hydrocarbons. Geophysics alone will not tell us whether hydrocarbons are trapped in the underlying sediments, however, micromagnetics is now a reconnaissance tool that can resolve local anomalies having the potential of being hydrocarbon-related.