The Earth's magnetic field has an origin in the nucleus and is modified by changing external components in the solid mass of the planet; in addition, local anomalies can be detected due to changes in the subsurface. Magnetic surveys are based on estimating the unequal distribution of magnetic forces within the earth's crust, and these anomalies are expected due to the variations in the physical properties of the rocks, such as magnetic susceptibility or the remaining magnetization of the rocks.
History of magnetic exploration
The first investigation about terrestrial magnetism was made by William Gilbert (1540-1603), summarizing all the knowledge about magnetism and recognized that the Earth behaves like a large magnet presenting the first unequivocal recognition of a geophysical property (Telford et al., 1990). LOWRY.
Since compass needles are attracted to natural iron formations prospecting tools based on this property were used by the 19th Century, and as the relationship between magnetite and base metal deposits became understood, new aims to develop more sensitive instruments grew.
The first fluxgate magnetometer was used in airborne submarine detection during World War II (Reford and Sumner, 1964; Hanna, 1990) and the precision in the instrument began a new period of airborne land and marine magnetic surveys for the exploration industry, government regional mapping and oceanography (Hanna, 1990; Hood, 1990).
In 1952, Scripps Institution of Oceanography began towing a magnetometer to map the ocean floor and this survey showed a pattern of magnetic stripes due to seafloor spreading during different periods of geomagnetic reversals (Vine and Mathews, 1963; Morley and Larochelle, 1964).
New instruments were developed from 1950 to 1970, increasing sensitivities for the proton precession and alkalivapor magnetometers, but after these developments, accuracy was the main factor to improve on magnetics surveys. After the availability of the global position system in the 1980s, exploration geologists began to find anomalies such as those caused by intrasedimentary sources (see papers in Peirce et al., 1998).
Usually, rock minerals have very low magnetic susceptibility, and magnetism is a result of a small proportion of minerals. Magnetic minerals are usually related to two main groups, the-iron- titanium-oxygen group creates minerals such as magnetite (Fe3O4) or hematite (Fe2O3) and the second group is the iron-sulphur group providing minerals such as pyrrhotite (FeS1+x, 0 < x < 0.15).
Igneous rocks, such as basic igneous rocks, are highly magnetic in contrast to acid rocks, but usually, this behavior is related to magnetite content. Metamorphic rocks are variable in their magnetic behavior, and sometimes magnetite is formed as an accessory mineral due to high oxygen and pressure. It is not easy to identify a rock based on his magnetic behavior alone, but magnetic anomalies over sedimentary areas are often related to igneous and metamorphic basement or by intrusions into the sedimentary layers.
Applications of magnetic measurements
Magnetics surveys have potential use in air, ground, space, water, boreholes covering different scales and applications such as minerals, hydrocarbons, groundwater, and geothermal resources. Some applications are presented here to show the versatility of the method and its importance.
The main use of magnetic applications is related to mineral exploration. Ground and airborne magnetic surveys can be used to detect economic mineralization such as iron oxide-copper-gold deposits, skarns, sulfides, heavy mineral sands or environments such as carbonatites, kimberlites, porphyritic intrusions, faulting, and hydrothermal alteration; and for general geologic mapping of prospective areas. Magnetic surveys and geology understanding led to detect the Far West Rand Goldfields gold system. (Roux, 1970).
Magnetic surveys are used to find continental and regional terrain boundaries, which usually have contrast on the magnetic fabric across the contact (e.g., Ross et al., 1994; examples in Finn, 2002). Crystalline basements have a high magnetic susceptibility and basin fill sediments have lower susceptibility, so it is possible to estimate the depth to a basement and sometimes map basement faults or structures (e.g., Prieto and Morton, 2003).
Buried igneous bodies have higher susceptibilities than the rocks they intrude and usually destroy hydrocarbon concentrations, however they also serve as structural traps or reservoirs, such brecciated igneous rocks [e.g., Eagle Springs Field, Nevada; Fabero Field, Mexico; Badejo and Linguado Fields, Brazil; Jatibarang Field, Indonesia; and reported potential in the Taranaki Basin, New Zealand, all cited in Batchelor and Gutmanis (2002)
High-Resolution Aeromagnetic surveys help exploration geologists to infer intrasedimentary faults if a marker bed with a specific amount of magnetite is present. The marker bed should present a very small anomaly when a fault is present helping to trace the corresponding fault system. The Albuquerque Basin presents a magnetic anomaly that shows relationships of bed thickness, offset, fault dip, etc. (Grauch et al., 2001).
Borehole magnetic surveys
Since the early 1950s Meter scale borehole measurements could be used to create correlations between wells and to determine the location and orientation of magnetic bodies in boreholes using fluxgate magnetometers Levanto (1959).
As noticed before, magnetics had an important role in mapping the seafloor. A new oceanic floor is being created at mid-ocean ridges, and the oceanic plates are moving away. The oceanic ridges had a positive or negative magnetic signature depending on the polarity epoch of the Earth’s magnetic field. Additional work is being done to calibrate absolute ages to the geomagnetic timescale (Geological Society of America’s 1999 geologic timescale).
Global magnetic measurements
Measurements on the global magnetic field are being conducted since 1964, and recent satellites have more sensitive scalar and vectormagnetometers to create a better approximation of the core field, crustal and external fields. The International Geomagnetic Reference Field (IGRF) is the result of the magnetic measurements and models compiled and serves as a tool to process exploration data.