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Environmental Technology Listing
Title of Listing:
SMARTMAG Cesium Vapor Magnetometer
Category:
Characterization Technologies
Subcategory:
*Waste, Soil, All Listings
Media:
Soil, Solid, Sediment, Water
Contaminants:
Buried Ferrous Metals, Soil Types, Bedrock Stratigraphy
Web Site:
http://www.scintrexltd.com
Email:
scintrex@scintexltd.com
Technology Description:

The SMARTMAG consists of a staff-mounted cesium-vapour magnetometer and processor, an optional memory console, optional staff-mounted display, optional headphones, and belt-mounted battery pack for the processor.

          There are three versions of the SMARTMAG available.

          1. A sweep version, consisting of the magnetometer and processor with a belt-mounted battery pack, staff-mounted display and headphones.

          2. A mapping version, consisting of the magnetometer and processor with a belt-mounted battery pack and memory console.

          3. A survey version consisting of the magnetometer and processor with a belt-mounted battery pack, memory console, staff-mounted display and headphones.

          A cesium-vapour magnetometer sensor consists of a miniature atomic absorption unit from which a signal proportional to the intensity of the ambient magnetic field is derived. A signal processor converts the signal into the magnetic field strength in nanoteslas (nT) for display and recording.

          The three main elements of the sensor are a cesium lamp, an absorption cell containing cesium vapour and a photosensitive diode, all mounted in a common optical axis. Strictly speaking, these magnetometers should be called optically-pumped, optically-monitored cesium magnetometers, in which a proprietary feedback technique is used to create a quantum mechanical oscillator resonating at a frequency proportional to the intensity of the ambient magnetic field. This resonant frequency is called the Larmor Frequency.

          The energy radiated from the cesium lamp causes the cesium electrons in the absorption cell to align their spins with the magnetic field vector so they are in a singlet spin state. This is called "optical pumping." When the electron spins are aligned, the absorption of the cell is low and the photosensitive diode receives a maximum level of light, causing it to output a maximum current.

          The photosensitive diode output is then fed back into the coil wound around the absorption cell. The magnetic field generated by this coil causes the electron spins to distribute themselves evenly between all possible spin states, thus destroying the optically-pumped state. However, as the optically-pumped state is dissipated, the electrons again begin interacting with the radiation from the cesium lamp, increasing the absorption and decreasing the photosensitive diode current.

          The entire system is in oscillation and the photosensitive diode current varies sinusoidally at some critical frequency. This critical frequency is called the Larmor Frequency and, as mentioned above is proportional to the intensity of the ambient magnetic field. It therefore remains only to amplify the signal and count the frequency to derive the magnetic field.

          The constant of proportionality which relates the Larmor Frequency to the intensity of the magnetic field is called the gyromagnetic ratio of electrons. For the Cs133 atom it is known very accurately to be equal to 3.49857 Hz/nT.

          The inherent resolution of a Scintrex SMARTMAG Cesium-Vapour sensor is 0.005 nT.

          The Scintrex Smartmag System can provide measurements continuously in a sweep mode setting, or at individual grid points in a survey mode.
Performance Status/
Limitations:

As is the case with all magnetometer, the instrument readings can be affected by cultural features such as overhead power lines. The operator should also be devoid of any metallic object on his person, such as belt buckles, rock picks, steel-toe boots and compasses. The cultural features and metallic objects will interfere with the normal reading taken by the magnetometer and produce spurious data.

          Also the operator must be aware that the earth's magnetic field varies with time. As the earth rotates, the outer layers of the ionosphere interact with the solar wind to cause minor fluctuations in the background values of the magnetic field. Depending on the duration and intensity of these fluctuations, they are given different names.

          Fluctuations with a period lasting about one day are called diurnal variations. These variations are fairly predictable and are not a problem in magnetometer surveys. The diurnal drift can cause a variation of the order of 50 nT/Hour.

          Erratic, short-term blips or spikes in the magnetic field are called micro-pulsations. These can range in intensity from a few through tens or hundreds of nanoTeslas in intensity. These can present a problem when you are carrying out a magnetometer survey in that they may appear similar to anomalies caused by buried objects.

          When the timing and duration of micro-pulsations becomes severe it is then called a magnetic storm. Typical micro- pulsations last a few hours whereas magnetic storms can last for days. Needless to say, it is not recommended to carry out a total field magnetometer survey during a magnetic storm, as you may not be able to remove all of the rapidly changing variations in the magnetic field, giving rise to false anomalies. You can obtain magnetic activity forecasts, much like weather forecasts, from several agencies worldwide.

          Depending upon the requirements of your survey site, you may choose to remove, or not to remove, these variations in time of the magnetic field from your collected magnetic data.

          There are three ways in which you can remove these variations:

          1. Use a base station magnetometer to record all the changes in time and then use this data to remove the change from the readings in the field magnetometer. This is the most accurate way of doing it, but also it is more expensive, as two complete instruments are required.

          2. Use a tie-line method while doing the total field survey. This assumes that the field is changing slowly and evenly between the first time you measured the value at a station and the next time you check-in to that station again. This method is not as accurate as using a base station, but if the field is not changing rapidly, it is quite adequate to locate an anomaly. This technique may be cost effective, as it only requires one magnetometer.

          3. Carrying out a vertical gradient survey. Since you are measuring the rate of change between two sensors, any changes in the background will apply to both sensors and you will not see any of the noise effects. This technique is quite effective for near-surface anomalies.
Topics
Geophysics, Technologies, Characterization
Additional Topics/Tags/Keywords
Magnetometer, Monitor, Subsurface


Organization:
Scintrex Ltd.
Address:
222 Snidercroft Road
Phone:
DescriptionNumber
City:
Concord
1.
Primary (905) 669-2280
State/Province/Territory:
Ontario
2.
Zip/Postal Code:
L4K 1B5
3.
Country:
Canada
4.
Fax:(905) 669-6403
Branch Locations: