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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:

Clarification regarding Geometrics standard magnetometers SX versions and the US Govt. export regulations In this brief review magnetometer specifications are given in terms of both nT/sq-rt-Hz RMS and in Peak-to-Peak (P-P) noise values as both forms are often used to describe instrument performance. The US Government specifies that an export license is required for magnetometers that have a sensitivity of better than (noise level less than) 0.02nT/sq-rt-Hz RMS. Obtaining an export license is not difficult but it does require approximately 6-8 weeks. Not all geophysical applications require export license sensitivity and so we offer SX models that have a noise floor of 0.02nT/sq-rt-Hz RMS. Compare this with our G-858 Magnetometer at 0.008nT/sq-rt-Hz RMS and our G-882 Marine Magnetometer at 0.004nT/sq-rt-Hz RMS. What does SX performance mean in the survey results? When the sensor is deployed at some distance from the “source” such as in above the shoulder mounting for geological surveys (G-859SX) or at some distance (several meters) from the seafloor for G-882SX surveys, the distance from the source provides some Natural filtering of the near surface response. This means that surveys not focused on small target detection (20mm ordnance rounds) where the sensor is deployed very close to the ground (<1m), SX performance is more than adequate. Let us consider the G-858 man-portable model. Under low noise laboratory conditions at a sample rate of 10 samples per second, the G-858SX will show approximately 0.125 nT of noise (peak-to-peak) compared to a standard G-858 of about 0.05nT P-P. To understand the significance of this, the Natural earth background noise due to geomagnetic micro-pulsations is about 0.02nT/sq-rt-Hz (about 0.125 nT peak-to-peak) at the quietest of times. Micro-pulsation amplitudes of 1 or 2 nT are common and, during active periods, they may be larger than 10 nT. Any magnetometer will produce a record of the combination of the background noise (micro-pulsations, diurnal drifts, etc) and its own internal noise. If the various noise components are not correlated with each other they will add as the square root of the sum of their squared amplitudes. In the case of the G-858SX, the combination of instrument noise and background micro-pulsations will be: √(0.125nT^2 + 0.125^2) = 0.18nT. For the standard G-858, this combination will be: √(0.05nT^2 + 0.125^2) = 0.13nT. That is, the SX model will exhibit about 30% more noise amplitude compared to the standard model if the atmospheric noise is typical. Unless the survey measurements are referenced to a high performance base station magnetometer equipped with a very accurate clock, the user will not be able to detect any difference between SX and standard performance. If such base station data were available, the greatest difference that would be seen should be no greater than about 0.05nT P-P in the average peak-to-peak amplitude. Such small differences cannot be seen or even detected in the total field contour maps made for exploration surveys which are typically contoured at 1nT or more. It should be remembered that the amplitude of the geomagnetic micro-pulsations in the frequency range from 5hz to 10hz is not constant; i.e., at most times they will be greater than 0.125nT and occasionally less than this value. Their intensity is governed by the average intensity of the instantaneous global thunderstorm activity and sunspot activity.

The hammer switch (trigger switch) that Geometrics seismographs use is a contact closure device that utilizes inertia force to create a momentary closure between the center rod and cylinder located in the enclosed device. The delay from strike to closure can vary based on orientation. The black dot on the switch and instructions were added to assist having a repeatable strike to closure time from usage to usage. However, this is only true for one hammer switch. From one hammer switch to the next there could be a variance however very slight indeed. In SCS there is a setting which lets the user change the sensitivity of the hammer switch to accommodate the Natural variance in the trigger switches. To build the switch, we use a tool to center the spring beam in the tube contact, which minimizes the variance in the triggering of the hammer switch between different orientations and Natural variations in different hammer switches. Any strike to closure time variance is more affected by the energy of the strike and the momentum produced on the hammer switch. The usage of the black dot to orient the hammer switch for most cases is probably extraneous. We center the beam accurately and suggest the usage of the black spot for mounting repeatability. We left it just to ensure any slight difference due to orientation could be eliminated. Additionally, both the tube and spring beam are gold plated to reduce and maintain contact resistance. After all this, it depends upon the repeatability of the user’s apparatus.

Your need of a magnetometer base station like the G-862RBS depends on the objective of the survey. If one is performing a geologic survey to investigate deep structure (exploration for mineral deposits, oil/gas, geology) then the wavelengths of the "target" body are typically “long” (long in meters, therefore long in data acquisition time). The rate at which the Earth's Natural magnetic field responds to interaction with the solar wind is also typically many seconds to minutes (diurnal variations). Since the geologic and diurnal variations are of similar wavelengths, a geologic mag survey usually requires a base station. Please read the introductory sections of the Applications Manual for Portable Magnetometers offered on our website for more details. If you are moving fast (fast in the sense of a brisk walk, ~1m/s) and looking for small targets (UXO, archaeological artifacts, environmental targets like drums, pipes, etc.) then you are “up and over” them in a matter of seconds and typically the earth’s field does not change in this time frame. So there is less need for a base station for these type surveys. Of course, it never hurts to have a base station running and if you are surveying over multiple days, having a reference station will allow easier “block leveling” of multiple day surveys.

Channel Remapping Channel remapping allows you to change: the order of channels on each analog spread cable that connects to the Geode reorder the Geode boxes. You would use this option if your cables were wired opposite to the default order normally used in Geometrics wiring, if you wished to turn your line around to have the low channels at the opposite end, or if your cables had a wiring error. Channel remapping is also often necessary when using more that a single network cable. Default cable wiring of Geometrics seismographs Default order is defined as the Natural electrical order in which channels are oriented when the system first powers up before remapping. Refer to Section 3 under Connector Wiring that discusses standard wiring configurations. You may have requested a custom wiring configuration from Geometrics. If you are confused about your wiring, contact the factory and refer to the serial number and job number. Geode cables are typically wired in a ‘high-side configuration’, meaning that the Geode connects closest to the highest numbered channel on the analog cable. The 149 figure above shows this configuration for a single box system, with 24 channels. Multiple Geodes The following diagram shows a default single digital line (one network card) system with 3 Geodes. Note that Geode one is always closest to the controller in a default configuration. Multiple Network Lines The next diagram below shows a default configuration with two digital lines (two network cards) with the controller positioned in the middle. Line 1 is on the left and line 2 is on the right. One might use two lines to increase data throughput to reduce time between shots. Like the configuration above, the Geodes are numbered starting closest to the controller. The seismic controller software labels all of the channels contiguously even though they are on two separate digital lines. However, if the lines are collinear, the first line will have the channels ordered backwards. This can be easily rectified with the remapping feature. There are two ways of remapping channels: automatic mode and manual mode. Automatic mode settings are listed on the top of the remapping dialog box, and manual mode on the bottom. Automatic Channel Remapping Automatic channel remapping allows you to reverse either the order of the Geodes on the line, or reverse the order of the channels on the spread cable. The above diagram shows the result after both channels and Geodes have been reversed, renumbering the line so that low channels start on the left hand side and increase towards the right. In the dialog box, the automatic remapping boxes referencing line 2 remain unchecked, since the default orientation on line two was correct. Manual Channel Remapping Channels can be remapped on an individual basis using the Manual Map Mode. Select the appropriate check box, and enter the order in which you would like the channels that differs from the default order. You can specify individual channels separated by a comma (1, 3, 4, 6 etc) or a range of channels (1-13, 24-14 etc). For example, if you wanted the channels ordered backwards on a 24-channel system, you would enter 24-1. If you wished to reverse the order of channels 1- 12 in a 24 channel system, you would type 12-1, 13-24. Other examples are shown opposite, and are available by pressing the See Examples button on the remapping menu.

We get occasional calls asking how to use one of our seismographs as a vibration monitor. The method for this is described below, but it should be noted that while true amplitudes can be obtained, this method of measuring them would probably not stand up in court. True vibration monitors – seismographs designed specifically for this task – have a built-in geophone. The voltage output of the geophone per unit vibration is known to a very high degree of accuracy, and the system is calibrated by the manufacturer regularly (usually once a year). If you are measuring vibrations in a situation in which litigation might be involved, you should use a true vibration monitor. One of the more popular ones is the Blastmate by Instantel. Vibrations are generally quantified in units of particle velocity, the first derivative of displacement. Geophones are particle velocity sensors – output is directly proportional to particle velocity. If you know the response function (sensitivity) of your geophone – the voltage output per unit velocity input – you can convert voltage (as measured by the seismograph) to mechanical vibration in terms of particle velocity. The sensitivity of your geophone can be obtained by the geophone manufacturer, and will be expressed as a function of frequency. A typical graph of geophone sensitivity is shown below: It is best to used a geophone that has a Natural frequency at or lower than the lowest frequency of interest. Seismic data files are stored in a SEG format. The first step is to convert the SEG output of the seismograph to an ASCII columnar format. If you are using an ES-3000 or Geode, your controller PC should have this icon for Tape Reader on the desktop: If not, download Tape Reader. Run the program and click on File>>Open: Read in the file you wish to convert to ASCII. Now, click on File>>Save Displayed Data to Ascii File: After making your format choices (be sure to convert to mV), press Export. The record will be written in an ASCII format that can then be imported to Excel. From here you can calculate the frequency spectra and particle velocities using the response function of the geophone.