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	Potential Field Methods
	Magnetic Method The magnetic method involves the measurement of the earth's magnetic field intensity. Typically the total magnetic field and/or vertical magnetic gradient is measured. Measurements of the horizontal or vertical component or horizontal gradient of the magnetic field may also be made. Anomalies in the earth's magnetic field are caused by induced or remanent magnetism. Induced magnetic anomalies are the result of secondary magnetization induced in a ferrous body by the earth’s magnetic field. The shape, dimensions, and amplitude of an induced magnetic anomaly is a function of the orientation, geometry, size, depth, and magnetic susceptibility of the body as well as the intensity and inclination of the earth's magnetic field in the survey area. Buried ferrous metallic objects, such as pipes, drums, tanks, and debris generally give rise to dipolar anomalies with a positive response south and a negative response north of the object. The magnetic method is an effective way to search for small metallic objects because magnetic anomalies have spatial dimensions much larger than those of the objects. Magnetometers are not able to detect nonferrous metals such as aluminum and brass. Magnetometer surveys are used to: Detect buried UXO Locate subsurface utilities Investigate archaeological sites Map bedrock faults and geology Locate abandoned steel well casings Locate underground storage tanks (USTs) Locate pits and trenches containing drums and metal debris Gravity Method The gravity method involves measuring the gravitational attraction exerted by the earth at a measurement station on the surface. The strength of the gravitational field is directly proportional to the mass and therefore the density of subsurface materials. Anomalies in the earth’s gravitational field result from lateral variations in the density of subsurface materials. The gravity method is typically used during environmental and geotechnical investigations to: Locate underground caverns Locate abandoned mine shafts Map regional geologic structure Spontaneous Potential Method (SP) Also known as spontaneous potential or streaming potential SP is one of the oldest geophysical methods. SP has been widely employed as a prospecting tool for sulphide ores and graphite deposits around the world. Soviet scientists pioneered and had considerable success detecting seepage and drainage structures in embankment dams in the latter part of the last century. In the US, SP has gained acceptance as a method to detect seepage and karst structures. SP effects originate from multiple sources including electrochemical, electrokinetic and thermoelctric mechanisms. SP borehole logs can be used to detect changes in permeability, pore fluid and lithology. The practical considerations of an SP survey are relatively straight forward. Non-polarizing electrodes are used to measure potential differences between a base electrode and a roving electrode that is moved to discrete stations in a predetermined grid. As with many other types of geophysical methods, care must be taken to account for cultural interference that may have a profound effect on data quality. SP is typically used to: Detect flow patterns in karst Detect and map seepage paths in dams Detect lithologic and fluid variations in boreholes