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| # | Post Title | Result Info | Date | User | Forum |
| Choosing the Right Lithium Polymer Battery for your MagEx | 2 Relevance | 3 years ago | Gretchen Schmauder | Hardware | |
| The MagEx uses a 3 cell Lithium Polymer battery to power the MagEx during surveys. The two main requirements for the battery are that it must fit into the battery compartment, and it must be nonmagnetic. Non-Magnetic Batteries: Some types of Lithium Polymer batteries are extremely magnetic. This is because the cell-to-cell connections are made with nickel strips (nickel is extremely magnetic). This makes them unsuitable for use in the MagEx since they will interfere with the background magnetic field that is being measured. Whether or not the batteries are magnetic is not something that appears on the data sheet, so it is important to choose batteries of a particular construction form factor that in practice has been shown to have a very low magnetic signature. Examples of this battery type will be shown below. There are many brand names for this battery type, and the brand names seems to change frequently. Evaluating the Magnetic Properties of a Battery: Batteries should be measured for magnetic signature before using them. This is especially true when trying a new battery brand just to be sure the battery is not going to affect the survey data. To perform this test you will need to start a survey with a stationary MagEx pointing north-south on a nonmagnetic platform (wooden sawhorses, cardboard box, etc). Hold the battery to be tested immediately over the battery compartment and rotate it in all orientations. Download the data and look for variations in the magnetic field that correlate with the battery rotation. There shouldn't be any correlation above 1 nT peak to peak. Make sure the operator is nonmagnetic when doing this test (shoes, belts, watches, cell phones, keys, etc. can all corrupt the results). Battery Size and Shape: The correct batteries are rectangular in shape and measure roughly 105x34x24mm. They are made from 3 flat cells stacked up measuring 11.1 volts nominal. They should be between 1800 and 6000 mAh (milliamp-hour). Higher capacity batteries will not physically fit in the battery compartment. Lower capacity batteries will work, but with a reduced run time. One 1800 mAh battery will run the MagEx for about two hours. The MagEx power connector is XT-60 so the battery must match. There are other power connector types, but XT-60 is commonly used. The 4-pin balance port connector is a JST-XH4 connector (though this is standard on most batteries). Where to Find Batteries: If you are in an Area that doesn't have strict controls on shipping Lithium Polymer batteries, then Amazon.com is a good source. Another good source is hobby stores, or anyplace that sells radio-controlled toy cars, boats, or airplanes. This is typically where this style of battery is used the most. What do the Battery Specifications Mean? 3S: This means it is a stack of three Li-Po cells Voltage: A fully charged 3 cell Li-Po battery measures 12.6 volts. A depleted battery will measure 9.6 volts. Thus, the voltage for this battery is typically labeled as 11.1 volt (the average of 12.6 and 9.6 volts. 35C (or any other "C" value): This is a rating on how much current can be safely drawn from the battery. To get the value in amps, take the milliamp-hour rating and divide by 1000 (to get amp-hours), and then multiply by the "C" value. For a 2200 mAh battery with a 35C rating multiply the 2.2 amp-hour capacity (2200 mAh / 1000) times the C value of 35, which gives a maximum discharge current of 77 amps. The MagEx draws about 0.6 amps, so any C value is fine - even if is down to 0.5. Battery Chargers: Most battery chargers being sold now are universal chargers which support a variety of rechargeable battery chemistries and output connectors. They come in many sizes and shapes, but most of them operate identically because the internal circuitry is the same. Most chargers will charge at a much faster rate than the MagEx discharges them, so you technically only need two batteries in the field. A nice feature to look for is the ability to power the charger off 12V as well as with AC power. This will allow charging in the field off a car battery. Be sure to charge in batteries in "Balanced Charge" mode using the battery balance JST-XH connector. This allows more charge current into cells that are more deeply discharged than the others and ensures that the battery gets all three cells completely charged. Battery Safety: Lithium Polymer batteries are small and light but store a tremendous amount of energy inside. This is good for running equipment for long periods of time between charges, but it also means that if something goes wrong and it releases all its energy at once it can be a serious fire hazard. Never charge a lithium battery unattended, charge only in a fireproof location. Batteries that are swollen or damaged should not be used. Dispose of these per local regulations. Be sure to follow all regulations for shipping or hand carrying Li-Po batteries. This may include packaging and labeling requirements, limiting the number of batteries, and discharging the batteries to 30% capacity before shipping. Do not discharge the battery below 9.6 volts (3.2 volts per cell). This damages the battery and could result in destructive decomposition and fire. If a battery that is discharged below a safe level is placed on the battery charger it will refuse to charge it. Batteries that are discharged below 9.6V should be removed from service and disposed of according to local regulations. Some example batteries are shown below: | |||||
| What naturally occuring magnetotelluric frequencies exist? | 2 Relevance | 3 years ago | Gretchen Schmauder | General Electromagnetic Info | |
| If you are looking for natural magnetotelluric (MT) frequencies that are nearly always observable then you can count on the Schumann resonances. You can assume very low signal strength in the low micro Volt range. The foundational Schumann Resonance is the strongest at 7.83 Hz (around 8Hz) and it generally has a strength measured around 1 microV/root Hz in the San Francisco Bay Area. The Schumann and other lightening generated frequencies are propagated into the atmosphere, the atmosphere acting as a wave guide due to the electromagnetic signal reflecting off the ionosphere. As electromagnetic waves interact with the Earth’s surface they act as displacement currents going vertically into the Earth. These displacement currents then create secondary currents that flow horizontally in the Earth. MT signals are assumed to be plane waves since the source is far enough away to be several skin depths distant. The assumption of plane wave and multiple polarizations of the signal allows magnetotelluric calculations to be made without consideration of the source parameters. The general MT signals will come and go depending on atmospheric conditions, time of day, time of year, location, and general distant lightning activity. Signals below about 0.1 Hz are typically from the ionosphere, generated by variations in the solar winds and how they press on the ionosphere, and not by lightning strikes. Traditional deep MT measurements will use natural magnetotelluric signals from 0.001 Hz (1,000 second period) and even lower frequencies with instruments that are capable. The dominant Schumann Resonant frequencies are 7.83Hz, 14.3, 20.8, 27.3, and 30.8Hz. There exist a magnetotelluric “dead-zone” in the 800 Hz to 4 kHz range, and this dead-zone is the result of certain frequencies not being contained in the atmospheric wave guide, instead simply dissipating into space. If you are working with AMT measurements generally the limit is somewhere between 0.1 Hz to around 2 Hz but that is because of instrumentation not the existence of the fields. The Stratagem EH4 went to 10 Hz and the Geode EM3D goes to 0.1 Hz. All the power line harmonics of 60 Hz in North America and 50 Hz in other parts of the world will give strong signal but are considered noise as far as MT measurements go and need to be avoided and filtered out. Another noise problem are the world-wide very low frequency (VLF) signals that also need to be filter and avoided. VLF signals are military signals from stations around the world and they swamp out the much lower natural magnetotelluric fields. There is a geophysical method that actually uses the man-made VLF signal to detect linear conductive geologic structures but VLF are a problem for MT measurements. For more information on the Schumann Resonances, which are the predominant natural magnetotelluric currents that exist, watch the video by Geophysicist Stefan Burns below: | |||||
| Magnetometer Survey Planning Considerations | 2 Relevance | 3 years ago | Gretchen Schmauder | General Magnetometer Info | |
| A common question many have with magnetic surveys is "How wide of a survey swath does a single magnetometer sensor cover on a single pass?" The answer is it depends on what is being searched for. Magnetometers are passive instruments, meaning they don’t actively send out signals or have a limited swath or depth of exploration. When planning a magnetic survey the grid (line spacing and waypoint spacing) should be designed using the best possible model of the target. There are some general rules of thumb that can be used to determine typical detection ranges for common iron objects. For example, a 10lb sledgehammer has been lost and needs to be found, and assuming this is 10lbs of pure iron, it would be expected to see a 1nT anomaly when the magnetometer sensor passes 6 meters over the top of the tool. Knowing this, survey line spacings should not be any narrower than 6 meters. With a line spacing of 3 meters, the chances of getting a clear anomaly goes up 8 fold as the 10lb iron sledgehammer would be at a minimum a 8nT anomaly vs a 1nT anomaly. In a geological sense, let's say we have a mafic dike intrusion that we believe is running E-W and it extends at least 25 meters in the near-surface in a somewhat linear fashion. It's difficult to model the amount of iron in a geological structure like this, so the survey should be designed to cross the dike perpendicularly every 5 meters or so, making sure to cross over the dike several times. Each pass over the dike may exhibit an anomaly of similar amplitude, and the feature will show up as a clear linear feature in the final processed map. For general mapping of geology, you have a lot of options. Most commonly mineral exploration surveys are done over very large Areas, so the line spacing is wider to save time as the lower resolution model that results still accomplishes the task of finding large mineral deposits. If more detail is required, then a more fine-grained survey can be done later. 20m-50m line spacing is typical for mineral exploration surveys. Design a survey grid to completely encompass the Area of interest (i.e. make sure you get some data outside of the Areas of interest, in case an interesting anomaly lies right along the edge). The founder of Geometrics the late Sheldon Breiner called this the Law of Search, as he often found his targets of interest along the edges of his archaeological magnetic surveys. It is important to make sure the operator of the magnetometer is magnetically clean before surveying with the magnetometer. This means no steel toe boots, glasses or hats with metal fittings, cellphone, belt buckle, etc. Magnetometer data acquisition is fairly simple, but data interpretation can be complex. You may need a base-station too. Please refer to the Base-Station information. | |||||
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