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GEOPHYSICS, VOL. 70, NO. 6 (NOVEMBER-DECEMBER 2005); P. 33ND61ND, 6 FIGS.10.1190/1.2133784
The historical development of the magnetic method in exploration
M. N. Nabighian1, V. J. S. Grauch2, R. O. Hansen3, T. R. LaFehr4, Y. Li1, J. W. Peirce5,J. D. Phillips2, and M. E. Ruder6
The magnetic method, perhaps the oldest of geophysi-cal exploration techniques, blossomed after the advent ofairborne surveys in World War II. With improvements ininstrumentation, navigation, and platform compensation,it is now possible to map the entire crustal section at avariety of scales, from strongly magnetic basement at re-gional scale to weakly magnetic sedimentary contacts at lo-cal scale. Methods of data filtering, display, and interpreta-tion have also advanced, especially with the availability oflow-cost, high-performance personal computers and colorraster graphics. The magnetic method is the primary explo-
ration tool in the search for minerals. In other arenas, themagnetic method has evolved from its sole use for map-ping basement structure to include a wide range of newapplications, such as locating intrasedimentary faults,defining subtle lithologic contacts, mapping salt domesin weakly magnetic sediments, and better defining tar-gets through 3D inversion. These new applications haveincreased the methods utility in all realms of explo-ration in the search for minerals, oil and gas, geother-mal resources, and groundwater, and for a variety ofother purposes such as natural hazards assessment, map-ping impact structures, and engineering and environmentalstudies.
HISTORY OF MAGNETIC EXPLORATION
The earliest observations on magnets are supposedly tracedback to the Greek philosopher Thales in the sixth centuryB.C.E. (Appendix A). The Chinese were using the magneticcompass around A.D. 1100, western Europeans by 1187, Arabsby 1220, and Scandinavians by 1300. Some speculate that theChinese had discovered the orientating effect of magnetite, or
Manuscript received by the Editor May 19, 2005; revised manuscript received July 11, 2005; published online November 3, 2005.1Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401-1887. E-mail: email@example.com; firstname.lastname@example.orgU.S. Geological Survey, Box 25046, Federal Center MS 964, Denver, Colorado 80225. E-mail: email@example.com; firstname.lastname@example.orgPRJ Inc., 12640 West Cedar Drive, Suite 100, Lakewood, Colorado 80228. E-mail: email@example.comColorado School of Mines (retired), 1500 Illinois Street, Golden, Colorado 80401-1887. E-mail: firstname.lastname@example.orgGEDCO, 1200, 8158th Avenue SW, Calgary, Alberta, T2P 3E2, Canada. E-mail: email@example.comWintermoon Geotechnologies, Inc., 280 Columbine Street, Suite 301, Denver, Colorado 80206. E-mail: firstname.lastname@example.org.
c 2005 Society of Exploration Geophysicists. All rights reserved.
lodestone as early as the fourth century B.C.E. and that Chi-nese ships had reached the east coast of India for the first timein 101 BCE using a navigational compass.
Sir William Gilbert (15401603) made the first investigationof terrestrial magnetism. In De Magnete (abbreviated title) heshowed that the earths magnetic field can be approximated bythe field of a permanent magnet lying in a general north-southdirection near the earths rotational axis (Telford et al., 1990).
34ND Nabighian et al.
The attraction of compass needles to natural iron formationseventually led to their use as a prospecting tool by the 19thcentury. 1
As the association between magnetite and base metal de-posits became better understood, demand for more sensitiveinstruments grew. Until World War II, these instruments weremostly specialized adaptations of the vertical compass (dip-ping needle), although instruments based on rotating coil in-ductors were also developed and used both for ground andairborne surveys.
Victor Vacquier and his associates at Gulf Research andDevelopment Company were key players in developing thefirst fluxgate magnetometer for use in airborne submarinedetection during World War II (Reford and Sumner, 1964;Hanna, 1990). This instrument offered an order-of-magnitudeimprovement in sensitivity over previous designs. After thewar, this improvement initiated a new era in the use of air-borne magnetic surveys, both for the exploration industry andfor government efforts to map regional geology at nationalscales (Hanna, 1990; Hood, 1990).
Oceanographers quickly adapted early airborne magne-tometers to marine use. In 1948, Lamont Geological Obser-vatory borrowed a gimbal-mounted fluxgate magnetometerfrom the U. S. Geological Survey and towed it across the At-lantic (Heezen et al., 1953). Scripps Institution of Oceanogra-phy began towing a similar instrument in late 1952 and in 1955conducted the first 2D marine magnetic survey off the coast ofsouthern California (Mason, 1958). This now-famous marinemagnetic survey showed a pattern of magnetic stripes offsetby a fracture zone: the stripes were later attributed to seafloorspreading during periods of geomagnetic reversals (Vine andMathews, 1963; Morley and Larochelle, 1964).
As new instruments continued to be developed from the1950s to 1970s, sensitivity was increased from around 1 nTfor the proton precession magnetometer to 0.01 nT for alkali-vapor magnetometers. With higher sensitivities, the error bud-get for aeromagnetic surveys became dominated by locationaccuracy, heading errors, temporal variations of the magneticfield, and other external factors (e.g., Jensen, 1965). The de-velopment of magnetic gradiometer systems in the 1980s high-lighted the problem of maneuver noise caused by both the am-bient magnetic field of the platform and by currents inducedin the platform while moving in the earths magnetic field(Hardwick, 1984).
The availability of the Global Positioning System (GPS)by the early 1990s tremendously improved the location ac-curacy and thus the error budget of airborne surveys. At thesame time, explorationists began to design airborne surveysso they could resolve subtle magnetic-field variations suchas those caused by intrasedimentary sources (see papers inPeirce et al., 1998). The higher resolution was achieved pri-marily by tightening line spacing and lowering the flight al-titude. Today, high-resolution aeromagnetic (HRAM) sur-veys are considered industry standard, although exactly whatflight specifications constitute a high-resolution survey isill defined. Typical exploration HRAM surveys have flight
1There is a story that a Cretan shepherd named Magnes, while tend-ing sheep on the slopes of Mount Ida, found that the nails of his bootswere attracted to the ground. To find the source of the attraction hedug up the ground and found stones that we now refer to as lode-stones.
heights of 80150 m and line spacings of 250500 m (Mil-legan, 1998). Exploration surveys are generally flown lowerin Australia, at 6080 m above ground (e.g., Robson andSpencer, 1997), and even lower if acquired by the Geologi-cal Survey of Finland (3040 m flight height with 200m linespacing; http://www.gsf.fi/aerogeo/eng0.htm). (Airspace regu-lations, urban development, or rugged terrain prevent suchlow-altitude flying in many places.) In contrast to these typicalexploration specifications, aeromagnetic studies that requirehigh resolution of anomalies in plan view, such as those gearedtoward mapping complicated geology, usually entail uniformline spacings and flight heights, following the guidelines es-tablished by Reid (1980). Unmanned aerial systems are alsobecoming available and should be a cost-effective tool for ac-quiring low-altitude magnetic data in relatively unpopulatedareas, although their eventual role in exploration is difficult topredict.
THE EARTHS MAGNETIC FIELD
The largest component (8090%) of the earths field is be-lieved to originate from convection of liquid iron in the earthsouter core (Campbell, 1997), which is monitored and studiedusing a global network of magnetic observatories and vari-ous satellite magnetic surveys (Langel and Hinze, 1998). Toa first approximation, this field is dipolar and has a strength ofapproximately 50 000 nT, but there are significant additionalspherical harmonic components up to about order 13. Further-more, this field changes slowly with time and is believed toundergo collapse, often followed by reversal, on a time scaleof 100 000 years or so. Understanding the history of reversals,both as a pattern over time and in terms of decay and rebuild-ing of the primary dipole field, is the focus of paleomagneticstudies [see Cox (1973) and McElhinny (1973) for excellenthistorical perspectives].
Although the crustal field is the focus of exploration, mag-netic fields external to the earth have a large effect on mag-netic measurements and must be removed during data pro-cessing. These effects are the product of interaction betweenthe global field and magnetic fields associated with solar wind(Campbell, 1997). First, the earths field is compressed on thesunward side, giving rise to a daily (diurnal) variation; at mid-latitudes, diurnal variations are roughly 60 nT. Second, the in-teraction generates electrically charged particles that maintaina persistent ring current along the equator, called the equato-rial electrojet. Instabilities in the ring current give rise to un-predictable magnetic-field fluctuations of tens of nT near theearths surface. Finally, near the poles, entrainment of chargedparticles along field lines creates strong magnetic-field fluctu-ations during magnetic storms on time scales of a few hoursand with amplitudes in excess of 200 nT.
The remaining component of the earths field originatesin iron-bearing rocks near the earths surface where temper-atures are sufficiently low, i.e., less than about 580C (theCurie temperature of magnetite). This region is confined tothe