Magnetic Field Surveys and Data Reduction

Magnetic Field Surveys and Data Reduction

Geomagnetic field can be described by the declination D, the inclination I, and total field vector F.

Magnetic surveyis one of a number of methods used inexploration geophysics. Magnetic surveys record spatial variation in the Earth's magnetic field. Inexploration, magnetic surveys are used to detect and map rock types, structure, and ore bodies. Magnetic surveys are used in both terrestrial and marine exploration.

Magnetic surveys are typically used to:

Locate abandoned steel well casings

Locate buried tanks and pipes

Locate pits and trenches containing buried metallic debris

Detect buried unexploded ordnance (UXO)

Map old waste sites and landfill boundaries

Clear drilling locations

Map basement faults and geology

Investigate archaeological sites

Three types of magnetometers are frequently used in magnetic surveying – Proton magnetometer, Cesium vapor magnetometer (optically pumped), and Fluxgate magnetometer

Secular, diurnal and variations due to magnetic storms get incorporated into data acquired during field surveys. During field surveys, care must be taken to establish a base station to incorporate drift. Likewise the station should be in flat terrain, away from electromagnetic field sources, and easy to reoccupy. The surveyor must return to base at least every hour or continually record data using separate magnetometer. Wherever possible, one must conduct surveys perpendicular to strike. It is also necessary to observe magnetic cleanliness – beware of belt buckles, glasses, spiral bound notebooks, power lines, wire fences, field vehicles, and buildings with metal beams. Keep the sensor at least 1 m from ground else soil variations might dominate signal

Magnetometersused in geophysical survey may use a single sensor to measure the total magnetic field strength, or may use two (sometimes more) spatially separated sensors to measure the gradient of the magnetic field (the difference between the sensors). In most archaeological applications the latter (gradiometer) configuration is preferred because it provides better resolution of small, near-surface phenomena. Magnetometers may also use a variety of different sensor types. Proton precession magnetometers have largely been superseded by faster and more sensitive fluxgate and cesium instruments.

Every kind of material has unique magnetic properties, even those that we do not think of as being "magnetic". Different materials below the ground can cause local disturbances in the Earth’s magnetic field that are detectable with sensitive magnetometers. The chief limitation of magnetometer survey is that subtle features of interest may be obscured by highly magnetic geologic or modern materials.

Terrestrial Magnetic Surveys

In terrestrial archaeology, magnetic surveys are typically used for detailed mapping of archaeological features on known archaeological sites. More exceptionally, magnetometers are used for low-resolution exploratory surveys.

Several types of magnetometer are used in terrestrial archaeology. Early surveys, beginning in the 1950s, were conducted withproton precession magnetometers. Data collection with proton precession instruments was slow, making high sample density surveys impracticable. Data were manually recorded and plotted. The subsequent introduction ofFluxgateandcesium vapormagnetomers improved sesitivity, and greatly increased sampling speed, making high resolution surveys of large areas practical. Equally important was the development of computers to handle, process, and display large datasets.[1]

Magnetometers react very strongly to iron and steel, brick, burned soil, and many types of rock, and archaeological features composed of these materials are very detectable. Where these highly magnetic materials do not occur, it is often possible to detect very subtle anomalies caused by disturbed soils or decayed organic materials. Many types of sites and features have been successfully mapped with magnetometers, ranging from very ephemeral prehistoric campsites to large urban centers.

Marine Magnetic Surveys

Magnetic surveys are extremely useful in the excavation and exploration of underwater archaeological sites. The apparatus used on the water slightly differs from that on land. Marinemagnetometerscome in two types: Surface towed and near-bottom. Both are towed a sufficient distance (about two ship lengths) away from the ship to allow them to collect data without it being polluted by the ship's magnetic properties. Surface towed magnetometers allow for a wider range of detection at the price of precision accuracy that is afforded by the near-bottom magnetometers.

The most common type of magnetometer used for marine survey is thefluxgate magnetometer. Fluxgate magnetometers utilize two ferromagnetic cores each wound with a primary coil and an outer secondary coil attached to amp meter. When an alternating current (AC) is passed through the primary coils, it creates two opposing magnetic fields that vary in intensity based on the outside magnetic fields.[3]By floating them parallel to the seafloor, they can measure the changes in magnetic fields over the seabed.

Another common type is the newerproton precession magnetometer. This utilizes a container full of hydrogen rich liquids (commonlykeroseneormethanol) that, when agitated by a direct current (DC) or Radio Frequency (RF), cause the electrons to become energized and transfer that energy to the protons due to theOverhauser Effectbasically turning them into dipole magnets. When the stimulus is removed, the protons precess at a rate that can be interpreted to determine the magnetic forces of the area.[3]

In maritime archaeology, these are often used to map the geology of wreck sites and determine the composition of magnetic materials found on the seafloor. An Overhauser magnetometer (PPM) was used in 2001 to map Sebastos (the harbor ofCaesarea Maritima) and helped to identify components of theRoman concrete.

Data Reduction

The overall processing of magnetic data involves a number of steps.

Data Reductioncan be separated into ‘corrections’ and ‘data enhancements’

Diurnal Correction

Normal Field Correction -- essentially a correction for variations in field with latitude and longitude –use International Geomagnetic Reference Field (IGRF) to correct on large scale surveys

Elevation and Terrain Corrections -- vertical gradient is a maximum at poles (0.03nT/m) minimum at equator (-0.015nT/m). Elevation correction not required for ground surveys. Terrain correction difficult since we need to know magnetic properties in surrounding terrain

Derivatives

Emphasizing shorter wavelength features.

First vertical derivative emphasizes near surface features. It can be measured with gradiometer, or derived from corrected data

Second vertical derivative emphasizes boundaries of target zones