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Magnetometer
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===Magnetic surveys=== [[File:Ground surveying in Surprise Valley, California.jpg|thumb|right|Ground surveying in Surprise Valley, Cedarville, California]] Systematic surveys can be used to in searching for mineral deposits or locating lost objects. Such surveys are divided into: *[[Aeromagnetic survey]] *Borehole *Ground *Marine Aeromag datasets for Australia can be downloaded from the [https://web.archive.org/web/20110601172707/http://www.geoscience.gov.au/bin/mapserv36?map=%2Fpublic%2Fhttp%2Fwww%2Fgeoportal%2Fgadds%2Fgadds.map&mode=browse GADDS database]. Data can be divided in point located and image data, the latter of which is in ERMapper format. ====Magnetovision==== On the base of space measured distribution of magnetic field parameters (e.g. amplitude or direction), the [[magnetovision]] images may be generated. Such presentation of magnetic data is very useful for further analyse and [[data fusion]]. ====Gradiometer==== Magnetic [[gradiometer]]s are pairs of magnetometers with their sensors separated, usually horizontally, by a fixed distance. The readings are subtracted to measure the difference between the sensed magnetic fields, which gives the field gradients caused by magnetic anomalies. This is one way of compensating both for the variability in time of the Earth's magnetic field and for other sources of electromagnetic interference, thus allowing for more sensitive detection of anomalies. Because nearly equal values are being subtracted, the noise performance requirements for the magnetometers is more extreme. Gradiometers enhance shallow magnetic anomalies and are thus good for archaeological and site investigation work. They are also good for real-time work such as [[unexploded ordnance]] (UXO) location. It is twice as efficient to run a base station and use two (or more) mobile sensors to read parallel lines simultaneously (assuming data is stored and post-processed). In this manner, both along-line and cross-line gradients can be calculated. ====Position control of magnetic surveys==== In traditional mineral exploration and archaeological work, grid pegs placed by theodolite and tape measure were used to define the survey area. Some UXO surveys used ropes to define the lanes. Airborne surveys used radio triangulation beacons, such as Siledus. Non-magnetic electronic hipchain triggers were developed to trigger magnetometers. They used rotary shaft encoders to measure distance along disposable cotton reels. Modern explorers use a range of low-magnetic signature GPS units, including real-time kinematic GPS. ====Heading errors in magnetic surveys==== Magnetic surveys can suffer from noise coming from a range of sources. Different magnetometer technologies suffer different kinds of noise problems. Heading errors are one group of noise. They can come from three sources: *Sensor *Console *Operator Some total field sensors give different readings depending on their orientation. Magnetic materials in the sensor itself are the primary cause of this error. In some magnetometers, such as the vapor magnetometers (caesium, potassium, etc.), there are sources of heading error in the physics that contribute small amounts to the total heading error. Console noise comes from magnetic components on or within the console. These include ferrite in cores in inductors and transformers, steel frames around LCDs, legs on IC chips and steel cases in disposable batteries. Some popular MIL spec connectors also have steel springs. Operators must take care to be magnetically clean and should check the 'magnetic hygiene' of all apparel and items carried during a survey. [[Akubra]] hats are very popular in Australia, but their steel rims must be removed before use on magnetic surveys. Steel rings on notepads, steel capped boots and steel springs in overall eyelets can all cause unnecessary noise in surveys. Pens, mobile phones and stainless steel implants can also be problematic. The magnetic response (noise) from ferrous object on the operator and console can change with heading direction because of induction and remanence. Aeromagnetic survey aircraft and quad bike systems can use special compensators to correct for heading error noise. Heading errors look like [[herringbone pattern]]s in survey images. Alternate lines can also be corrugated. ====Image processing of magnetic data==== Recording data and image processing is superior to real-time work because subtle anomalies often missed by the operator (especially in magnetically noisy areas) can be correlated between lines, shapes and clusters better defined. A range of sophisticated enhancement techniques can also be used. There is also a hard copy and need for systematic coverage.
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