A magnetic survey is a type of method which is being used for archeological geophysics. The survey is going to record the spatial variation of magnetic field of the Earth. In the field of archeology, the survey will be utilized for detecting and mapping the archeological features and artifacts. This is also being used for both the marine and terrestrial archeology.
A magnetometer is one main instrument being utilized for conducting this survey. This contains two different types, one having one sensor and another one having two sensors. The type with only one sensor gets the measurement of total strength in an Earth field. And the other type having two sensors gets the measurement for a fields gradient and these sensors are spatially separated. The aim of these ground magnetic surveys is to conduct an investigation of subsurface geology basing on the magnetic fields anomalies.
These anomalies will become the results for magnetic properties coming form the underlying rocks. Rocks are generally have magnetic contents and these are considered as extreme variables, depending on what type of rock and environment they are into. There are 3 common causes for anomalies including lava flows, dykes, and faults. In most geothermal environments, susceptibility is decreased due top the high temperatures.
For this survey method, the intensity will be measured and also the total, horizontal, and vertical gradients and components. Magnetism is considered to have similarities with gravity such as being a potential field. Another cause of these anomalies are the induced magnetism residues. And thus, this anomaly which is being induced will become the result for the induction of secondary magnetization in ferrous body.
The induced anomalies have the dimensions, amplitude, and shape, and these can be considered as the functions of geometry, depth, susceptibility, orientation, and size of body. And also functions of inclination and intensity of the field in a survey area. Typically, there are four important uses of this method and these are the following.
First, locate the metallic debris, pipes, buried tanks, and abandoned casings of a steel well. Second, map landfill boundaries and old waste sites. Third, map basic igneous intrusives and also basement faults. And fourth, investigate the archeological sites.
When exploring a geothermal energy, the ground data and the aero magnets data are both used in investigating the presence of a geothermal resource and this is done with combining gravity. In the aero magnets data, the anomalies usually are in correlation with volcanism expression surfaces. These include craters, localized basaltic lavas and plugs, or cones and domes.
Measuring the ground magnetism can be done through the use of some portable instruments that have regular intervals along with lines that are parallel and straight which will cover the area for survey. Intervals usually are in between the locations and these are lesser than those spaces between lines. Intense fields from the electromagnetic sources are common problems to these surveys.
To maintain the accuracy of mapping the anomalies, during the survey period, temporal changes must be considered. The normal changes are often called as diurnal drift. The corrections of diurnal drift are made through the repetition of instruments of the base station having frequent intervals. These measurements of stations are then made with correction for the temporal variations. Another thing is to avoid on conducting these surveys during severe storm that may occur infrequently.
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Author: Frank FoxThis author has published 1 articles so far.