Waterborne Magnetic Surveying

Basic Concept

The magnetic method is used to measure the intensity of the earth’s magnetic field. It is one of the oldest geophysical methods, which was initially used for mineral exploration. Advances in equipment design have improved data resolution, quality, and quantity leading to new applications collected from the ground surface, air, water, and downhole. Waterborne magnetic data can be used to map geologic structures, characterize basement rock, aid high resolution near surface engineering, geotechnical, unexploded ordinance (UXO), and environmental investigations, discover archaeological objects, map contaminated sediments, and find ferrous debris (Thompson et al., 1980; Nabighian, M.N. et al., 2005; Sharma, 2012).

Whether from land surface or waterborne, the magnetometer passively measures the intensity of the earth’s magnetic field at points along the surface of the earth. Subsurface materials alter the intensity of the earth’s magnetic field due to their magnetic susceptibility. Materials with high magnetic susceptibility increase the intensity of earth’s magnetic field, and a heightened magnetic field intensity is observed. The magnetic method identifies anomalies of high magnetic susceptibility occurring within subsurface systems and thereby characterizes a site based on the measured anomalous zones (Burger, 1992; Telford, 1990).

For environmental waterborne applications, surveys are typically performed from a vessel towing a marine magnetometer mounted known as a “fish”.. A global positioning system (GPS) is used for location where the GPS sensor is mounted on the vessel towing the magnetometer with a constant offset equal to the distance from the GPS sensor to the towed magnetometer. Waterborne magnetometers can be a cesium vapor magnetometer, a proton precession magnetometer, or a flux-gate magnetometer. Each type of magnetic instrument has a specific purpose. Typically the results of a survey is a (2D) map of the magnetic-field intensity, which reveals the locations of ferrous, or materials containing iron, objects that have high magnetic susceptibility and produce high or low (positive or negative) responses as they alter the earth’s magnetic field. Additionally, waterborne magnetic gradiometric measurements can be used, where two or more sensors are used to remove the background magnetic field and accentuate the environmentally significant magnetic anomalies.

Theory

The earth’s magnetic field is generated by the outer core and is necessary to protect the earth from solar energy. This field behaves in a way that is analogous to what would occur if an earth-sized bar magnet ran along earth’s axis and connected the north and south poles. Similar to the earth, a magnet is dipole because it contains both a positive and a negative pole. Like poles are repelled from each other while opposite poles are attracted. This behavior results in the generation of magnetic fields observed at multiple scales.

Compared to the magnetic field of the earth, magnetic fields generated by objects in the earth's crust and in the near subsurface are relatively weaker, irregular, and/or anomalous (Burger, 1992). Depending on the orientation of the source object, its magnetic anomaly may have only a single pole (monopole) or both a high and low magnetic intensity (dipole). The polarity and magnitude of the total magnetic field anomaly allows for the interpretation of the source object's depth, location, and orientation. For example, if:

The anomaly is positive (+) if its orientation is in the direction of the external field
The anomaly is negative (-) if its orientation is in the opposite direction of the external field
There is no anomaly if its orientation is perpendicular to the external field

During formation, substances with high magnetic susceptibilities, also known as ferromagnetic substances, contain groups of iron atoms that align with the ambient direction of the earth’s magnetic field. However, this is dependent on the environment and will only occur if the formation is below the Curie temperature, above which, materials lose their permanent magnetic properties. Materials are classified based on the specific types of alignments: ferromagnetic (atoms aligned together), antiferromagnetic (antiparallel alignment), paramagnetic (positive alignment), diamagnetic (negative alignment), or ferrimagnetic (very strongly magnetic).

This alignment character of iron atoms determines the object’s magnetic susceptibility and, therefore, how intensely earth’s magnetic field is altered. Of course, objects containing no iron will have a very low magnetic susceptibility, and, hence, different materials will have different magnetic properties (Moskowitz, 2015). The goal of magnetic surveying is to reveal the subsurface locations and variations of such formations or objects.

Applications

Field applications of magnetic surveys first require an understanding that the earth’s magnetic field changes daily as solar storms may disrupt the earth’s field. As a result, a base station is required to monitor diurnal magnetic field fluctuations and can be used to correct the field survey data. The instrument used to meet the objectives of environmental investigations is usually a cesium vapor magnetometer measuring the magnetic field intensity in nanoTeslsa (nT; 1 tesla =1 newton/amp-meter).

Some environmental applications include identifying and/or mapping anomalous magnetic field intensities caused by discrete objects (e.g. UXO,tanks, utilities, or other subsurface objects that may be related to subsurface contamination). Additional applications of marine magnetic surveys include mapping shipwrecks or flooded archaeological sites, characterizing the sub-bottom for dredging, and mapping geologic structure, for development of conceptual site models (Bowens 2009; Boyce et. al., 2001; Boyce et. al., 2003). Complementary waterborne applications include bathymetric data for water-depth mapping and typically include sidescan sonar, sub-bottom profilers, or even remotely operated vehicles for imaging and/or sampling.

Because the magnetic method only responds to variation in the earth’s magnetic field, merging multiple methods to achieve a common interpretation is always good practice. For instance, the magnetic survey only identifies ferrous objects, so a complementary waterborne electromagnetic induction survey could aid in the location and interpretation of any non-ferrous objects in the subsurface. Within the environmental industry, applications of waterborne magnetic surveying include (Thompson et. al., 1980):

Locating subsurface or sub-aqueous ferrous utilities or objects (e.g. shipwrecks)
Locating underground storage tanks
Mapping sediment characteristics
General geologic mapping aiding development of conceptual site models(CSMs)
Locating unexploded ordinance (UXO)
Identify ferrous objects for “cleaning” EM surveys

Examples/Case Studies

Gamey, T.J., 2008, Magnetic Response of Clustered UXO Targets, Journal of Environment and Engineering Geophysics, v. 13, no. 3, p. 211-221,doi:10.2113/JEEG13.3.211.

The objective of many recent UXO surveys has been described as “wide-area assessment” with the purpose of obtaining better definition of a known problem area. The targets of interest are clusters of ordnance, fragments and debris which are all indicators of greater contamination, higher risk of UXO hazard and higher remediation or construction costs. This is a different problem from the detection and discrimination of individual anomalies.

This paper provides a definition of a “cluster” based on the amount of overlap between individual dipole signatures. In total field surveys, magnetic anomalies overlap significantly and show an increased amplitude response once the individual sources are spaced closer than 0.5 times the sensor height. When this condition is extended over a large area, such as the center of a target site, the result can be comparable to a horizontal sheet of dipoles.

The equations to simulate a horizontal sheet are derived, and from these the relative density of targets may be calculated from the measured data by assuming a nominal target moment. Two field tests support both the qualitative and quantitative predictions.

Extending this concept to field practice, we examine some of the implications for standard operational procedures. For example, if we accept that QA/QC metrics should represent the targets of interest, then we should require wide-area assessment surveys to create impractically large grids of surface frag. Likewise, for detection of clusters the concepts of detection probability and search radius based on single items are irrelevant. Discrimination techniques that rely on dipole fitting will be extremely inaccurate. Instead, QA parameters and models suitable for horizontal sheets will have to be derived.

Sambuelli, L., Comina, C., Bava, S., and Piatti, C., 2011, Magnetic, electrical, and GPR waterborne surveys of moraine deposits beneath a lake: A case history from Turin, Italy, GEOPHYSICS. VOL. 76, NO. 6 (NOVEMBER-DECEMBER 2011); P. B213–B224, 17 FIGS. DOI: 10.1190/GEO2011-0053.1.

Bathymetry and bottom sediment types of inland water basins provide meaningful information to estimate water reserves and possible connections between surface and groundwater. Waterborne geophysical surveys can be used to obtain several independent physical parameters to study the sediments. We explored the possibilities of retrieving information on both shallow and deep geological structures beneath a morainic lake by means of waterborne nonseismic methods. In this respect, we discuss simultaneous magnetic, electrical, and ground penetrating radar (GPR) waterborne surveys on the Candia morainic lake in northerly Turin (Italy).We used waterborne GPR to obtain information on the bottom sediment and the bathymetry needed to constrain the magnetic and electrical inversions. We obtained a map of the total magnetic field (TMF) over the lake from which we computed a 2D constrained compact magnetic inversion for selected profiles, along with a laterally constrained inversion for one electrical profile. The magnetic survey detected some deep anomalous bodies within the subbottom moraine. The electrical profiles gave information on the more superficial layer of bottom sediments. We identify where the coarse morainic material outcrops from the bottom finer sediments from a correspondence between high GPR reflectivity, resistivity, and magnetic anomalies.

References

Bowens, A., 2009, Underwater Archaeology: the NAS Guide to Principles and Practices (2^nd ed.), Wiley-Blackwell, London, p.240.

Boyce, J.I., Pozza, M. and Morris, W.A., 2001. High-resolution magnetic mapping of contaminated sediments in urbanized environments. The Leading Edge, 20: 886-890. https://doi.org/10.1190/1.1487301

Boyce, J.I., Reinhardt,E.G., Raban,A., Pozza,M.R., 2003.,Magnetic Mapping of Buried Hydraulic Concrete Harbour Structures: Kink Herod’s Harbor, Caesarea Maritima, Israel, Maritime Archaelogy Paper 03-01.

Burger, H.R., Sheehan, A.F., and Jones, C.H., 2006, Exploration Using the Magnetic Method, in Introduction to Applied Geophysics: Exploring the Shallow Subsurface: New York, W.W. Norton & Co., p. 389.

Gamey, T.J., 2008, Magnetic Response of Clustered UXO Targets, Journal of Environment and Engineering Geophysics, v. 13, no. 3, p. 211-221,doi:10.2113/JEEG13.3.211.

Moskowitz, B.M., Jackson, M., and Chandler, V., 2015, Geophysical Properties of the Near Surface Earth: Magnetic Properties, in Schubert, G., ed., Treatise on Geophysics, Second Edition: Elsevier, v. 11, p. 139-174.

Nabighian, M.N., Grauch, V.J. S., Hansen, R.O., LaFehr, T.R., Li,Y., Peirce, J. W., Phillips, J.D., and Ruder, M.E., 2005, The historical development of the magnetic method in exploration, Geophysics, v. 70, no. 6, 111 p., doi:10.1190/1.2133784.

Sharma, P.V., 2012, Magnetic surveying, Environmental and Engineering Geophysics: Cambridge, Cambridge University Press, p. 65-111,doi:10.1017/CBO9781139171168.004.

Telford, W.M., Geldart, L.P., and Sherriff, R.E., 1990, Applied Geophysics: Second Edition: Cambridge, Cambridge University Press, 770 p.

Thompson, R.J., Stober, J.C., Turner, G., Oldfield, F., Bloemendal, J., Dearing,J.A., and Rummery, T.A., 1980, Environmental applications of magnetic measurements, Science, v. 207, no. 4430, p. 481-486,doi:10.1126/science.207.4430.481.