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Methods: Magnetics

    Magnetics methods are commonly used for site investigations and construction/development projects where the following information is needed:
    WHAT IT MEASURES

    The magnetics method is a surface method that uses the response of either magnetic materials or atomic particles to an external magnetic field in order to measure the lateral variation in the intensity of the earth’s magnetic field. The magnetics method began its history in the oil industry to delineate petroleum basins and in the mining industry for mineral exploration; however, it is now used for many environmental and engineering types of applications. Because of its sensitivity to ferromagnetic (steel) objects and its great depth of detection, by far the most common shallow use for this method is to detect buried steel objects, such as USTs, abandoned steel-cased wells and steel piping. However, this method can also be useful for the lateral delineation of certain types of igneous bedrock, and so may be used to identify lateral geologic contacts as well as structural features in these rocks under certain conditions. The magnetics method is particularly useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively.  


    The earth’s background magnetic field generally varies from 30,000 nT at the magnetic equator to 65,000 nT at the magnetic poles. Anomalies in the earth’s field are often caused by the induction of a secondary magnetic field in a ferromagnetic material by the earth’s magnetic field. Anomalies in the earth’s magnetic field may also be caused by remanent magnetism in naturally occurring rocks and minerals, where remanent magnetism refers to the existing residual magnetism of a rock that is associated with the magnetic history of the rock. There are several types of remanent magnetism; however, in the case of exposed igneous rocks, the most likely type of remanent magnetism is thermoremanent magnetism, which occurs when igneous magma is cooled below the Curie point in the presence of an external field (the earth’s magnetic field). The direction of the remanent field depends on the direction of the earth’s field at the time and place where the rock originally cooled. Magnetic anomalies may also be associated with complex variations in the magnetism of rocks that are associated with the tectonic and structural features present at the time the rock cooled.


    In the case of induced magnetization, the magnitude of the induced field (and hence the anomaly) is proportional to the intensity of the earth’s magnetic field and the magnetic susceptibility of the underlying material. The shape and amplitude of an induced magnetic anomaly over a ferromagnetic object is dependent upon the geometry, size, and magnetic susceptibility of the object; depth to the object; and the inclination of the earth’s magnetic field in the survey area. In simple cases, such as abandoned steel-cased wells, the depth to the buried object may be calculated based on well-known half-width or slope methods. In North America, magnetic anomalies over buried objects such as drums, pipes, and buried metallic debris are generally dipolar and exhibit an asymmetric, south up, north down signature (maximum on the south side and minimum on the north side); anomalies over abandoned wells are typically monopolar with a strong positive amplitude. The shape and amplitude of naturally occurring magnetic anomalies depend on many factors, such as the amount of magnetite or pyrrhotite in the rock, the parent magnetic field strength and direction, the depth of the rock beneath the ground surface, and structural lineaments such as fractures and faults. For most magnetics applications Spectrum uses a Geometrics G-858 cesium vapor walking magnetometer (G-858) or a G-858-G cesium vapor walking gradiometer to collect magnetics data. Further investigation of anomalies is often done using a Schonstedt model GA-72CD hand-held magnetic locator (Schonstedt).

    HOW IT WORKS

    The G-858 is a self-oscillating split-beam cesium vapor (non-radioactive Cs133) magnetometer, where a gas cell in the sensor contains the cesium vapor. This magnetometer operates on the principle of optical pumping. In the absence of a magnetic field, valence electrons of alkali-metal atoms such as cesium have two normal and two excited energy states. However, in the presence of the earth’s magnetic field, these energy states are split into pairs of normal low- and high-energy and excited low- and high-energy states. When atoms in the cesium vapor are illuminated with a beam of visible polarized light, electrons in the normal high-energy state absorb light energy and move into the excited high-energy state. The vacant states are then available for transition of electrons from other energy states; this process is called optical pumping. Once the normal high- energy state is fully occupied, the application of RF energy to the cesium vapor cell then causes some electrons to be “bumped” back down to the normal low-energy state, and the process is repeated. Light absorbed when these electrons are “repumped” results in light flickering at the Larmor frequency, which is then measured with a light sensor. The Larmor frequency is related to the strength of the earth’s magnetic field by the gyromagnetic ratio of the electron. The gyromagnetic ratio is known to a precision of about 1 part in 107, making measurements with the G-858 magnetometer precise to about 0.01 nT.


    Optically pumped magnetometers benefit from continuous measurement of the earth’s total magnetic field intensity, from high precision, and from sensor readings taken close to the ground, where the response to small near-surface objects is maximized.


    During a typical G-858 magnetics survey Spectrum establishes a base survey grid for navigation, and then the geophysics crew collects nearly-continuous (approximately 1 foot apart using a 0.3-second sample rate) total field magnetics data along equidistant parallel lines using a NavCom SF- 2050G StarFireTM GPS receiver for positional tracking. In areas having severe cultural interference in the vicinity of the target (e.g. abundant surface or near-surface metallic debris; or surface cultural features such as buildings, fences, above-ground tanks or railroad tracks) vertical magnetic gradient data are collected with G-858G. The vertical magnetic gradient is the change in the total magnetic field over a short vertical distance and can be thought of as the first order vertical derivative of the total magnetic field. A vertical magnetic gradient contour map eliminates the effects of the background magnetic field at a site, and tends to narrow and sharpen anomalies of interest. Regardless of whether total magnetic field or vertical magnetic gradient data are collected, the data are downloaded to a laptop, processed in the field and used to generate color-enhanced contour maps to assist in identifying anomalous areas of interest.


    As previously stated, the Schonstedt (a fluxgate magnetometer) is often used to further investigate identified anomalies. Fluxgate magnetometers consist of two magnetic cores of material that have very high magnetic permeability at low magnetic fields, and are wound with primary and secondary coils. In the Schonstedt these cores are contained in a hand-held wand that is swept back and forth over the ground surface. Cores that are as identical as possible are mounted so that their coils are wound in opposite directions. Connected in series, the two primary windings are energized by a low-frequency alternating current. Twice each cycle, the maximum current magnetizes the cores to saturation but in opposite polarity. Core saturation is symmetric and opposite in sign in the absence of a buried ferromagnetic object, causing the outputs for the secondary windings to cancel one another out; whereby the instrument emits a constant 40-Hz audio tone. When the instrument passes over a ferromagnetic object, core saturation takes place in one core earlier than the other, resulting in an imbalance in output voltages from the secondary windings. The resulting imbalance causes the audio tone to increase to a pitch and frequency that peak when the instrument is directly over the object. The Schonstedt provides qualitative verification of targets, and therefore is best used for reconnaissance.


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