Field Mapping and Geologic Exploration

A first step in the mineral exploration of a region is a magnetic survey to reveal the Earth's crust structure below the exposed geology (see Plate 7). Such measurements are made with sensitive magnetic field sensors carried through a grid pattern by researchers while walking over the area or by flying over it in airplanes (aeromagnetics) in low-altitude grid patterns (Figure 2.14), or by using ships with cables attached to sensors that are towed just above the continental shelf. The local steady (main) magnetic field background and the recurring regular daily variation fields are subtracted from the observations to reveal the pattern of the magnetic materials within the crust. By measuring short-period magnetic field fluctuations and their companion electric fields

Magrveiometer

"Stinger'

Magrveiometer

"Stinger'

Aeromagnetic Method

Crus tal Magnetic Anomalie

FIGURE 2.14 ► Mineral exploration programs rely on aeromagnetic measurements of the crustal magnetic anomalies to reveal the subsurface geology.

Crus tal Magnetic Anomalie

FIGURE 2.14 ► Mineral exploration programs rely on aeromagnetic measurements of the crustal magnetic anomalies to reveal the subsurface geology.

(magnetotelluric method) over a survey area, an even greater resolution of the Earth's crust substructure is obtained (see Section 5.2.3, p. 128).

To the experienced eye of the exploration geophysicist, magnetic and magnetotelluric charts show the deep crustal geological features and potential economically important mineral sites. Seismologists set small explosions and read the delay time for the sound reflections from the subsurface layers. Then the search is sharpened with complex seismic modeling of the region's responses to the passage of the explosion's sound waves. All the information is assembled to reveal the location of major mineral emplacements. The valuable Alaskan north-slope oil fields were first delineated using aeromagnetic surveys. During naturally occurring magnetic disturbances, wise surveyors delay their work until the quieter periods return so that the natural magnetic noise (see Figure 1.13) does not dominate their magnetic recordings. National solar-terrestrial disturbance forecasting centers provide the geophysi-

FIGURE 2.15 ► Example of an aeromagnetic map of ocean-surface field changes that revealed an ancient sunken sailing ship near the ocean's continental coast. Shading indicates contours of similar field strength.

cists with predictions of quiet magnetic periods. We will examine the sources of magnetic disturbances later in our tour (Chapter 4).

The search for and discovery of other than mineral deposits also depend on the magnetic surveys. Buried archeological formations can be mapped (ar-chaeomagnetism) when the ancient building materials have magnetic properties differing from the local environment. The Roman walls in England have been outlined using surface magnetic survey mapping. Even sixteenth-century sunken galleons of the Caribbean have been located using aeromagnetic charts responding to the field effects of metal armament, iron nails, and construction braces that were used on the old ships (Figure 2.15). In 1991, just north of the Yucatan peninsula of Mexico, aeromagnetic surveys located the 120-mile-diameter crater carved by the asteroid that covered the Earth with a

vegetation-destroying dust and resulted in the demise of the dinosaurs about 65 million years ago.

In early North American land exploration and development, local boundaries often depended on directions given by compass readings alone. This was particularly true in the gold and silver mining regions of the United States during the late nineteenth century. Unfortunately, mineral-rich mountain areas typically abound in magnetic field anomalies that arise from buried magnetic materials. As a result, skewed compass lines of ownership demarcation appear on the property maps of those days. Today, the reestablishment of the old mining boundaries is often a difficult process, requiring exact knowledge of the early geomagnetic field alignment and reference field models that need to be extended back to the original land survey date by the geomagnetic specialist.

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