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Snell’s Law describes quantitatively how wave fronts refract or "bend" at boundaries between contrasting velocities. You've seen it manifest in light waves by the apparent bend of the straw in your glass of Water; light travels slower in Water than it does in air. Refraction is well illustrated using Huygen's Principle. Consider a wave front (for our purposes, a seismic one) emanating from a point energy source, as shown in the animation above. For simplification, assume we are far enough from the energy source that the wave front is essentially planar, and is approaching an abrupt change in seismic velocity: Applying Huygen's Principle, we see that after time t, the plane wave has advanced a distance d equal to the radii of a series of spherical wave fronts emanating from the plane wave: The radii of the spheres, i.e., the distance the plane wave travels in time t, is equal to V1t. The tangent to the spherical wave fronts is the new position of the plane wave. The planar wavefront continues at velocity V1. Again applying Huygen, we see that "every point on the wave front" (see discussion of Huygen above) includes the points where the wave front intersects the velocity boundary: As the planar wave front advances, the velocity boundary becomes a new source of spherical wave fronts expanding at V2. Hence, part of the plane wave (the tangent to the spherical wave fronts emanating from the velocity boundary) is now traveling at V2. Note that within V2, its direction of advance has changed. This is because in V2, which is higher than V1, Huygen's spheres grow faster during time t. The refracted wave front continues in the new direction until another velocity boundary is encountered. Here is a simplified version at higher speed: Zooming out, we see the effect of this on a spherical wave: It should be obvious from the above that in order for a wave front to refract, it must strike the velocity boundary at an angle other than 90 degrees. It should also be obvious that in the case of V2 < V1, refraction will be in the opposite direction, and if V1 = V1, no refraction will occur. Snell's Law quantifies refraction in terms of angle of incidence and velocity contrast. Combining the above diagrams and adding rays, we can now describe Snell's Law: In the figure above, i is the incident angle, and r is the refracted angle, measured between the ray and a line perpendicular to the refracting interface. In the example above, the velocity contrast is positive; V2 > V1. There are numerous derivations of Snell's Law on the web if you wish to understand the math. From the equation, you can see that for any given positive velocity contrast, as i increases, r increases faster: This is important; it is the property of refraction that allows us to use refracted energy to measure subsurface velocities. Conversely, a negative velocity contrast results in refraction in the opposite direction:

List of approved fluids is as follows: n-Decane Shell Sol-71 (Shell oil product) Isopar-G (Exxon oil Co.) Odorless mineral sprits (Naptha) Mineral Spirits (Naptha) Charcoal Lighter Fluid Kerosene lamp oil Kerosene Mineral Oil (medicinal Grade) diesel Fuel (may degrade diaphragm on marine systems) Camp Stove fuel White Gas Unleaded gas The preferred fluids are items 1-7 above. Items 3 & 4 are not the same as paint thinner. Paint thinner will damage the sensor coils, as will Acetone, and should not be used. Items 11 - 13 have a very low flash point and should be used with extra caution, although any petroleum product must be handled carefully. There are several Alcohols which will produce an acceptable signal, but will tend to absorb Water. This will degrade the performance of the sensor over time. These should be replaced as soon as possible with an alternate fluid from the above list. Usable alcohols: Ethanol (Grain Alcohol) at least 190 proof Methanol (wood alcohol) Denatured Alcohol (ethanol made poisonous) All of these fluids must be as clean as possible to ensure that no Water or contaminates (rust) are in them. Use a filter to make sure the fluids do not contain rust from the storage container.

The Earth's magnetic field changes throughout the day due to Solar wind and other factors. A base station's data is used to correct for these variations (diurnal variations), which can significantly impact the results of a survey. In general, diurnal variations do not change rapidly, and it is not required to record the Earth's magnetic field at high sample rates. However, during geomagnetic storms, diurnal variations can be so rapid that 10Hz or a higher sample rate is necessary to track the field change. Kp index is commonly used to measure the strength of geomagnetic activity. The diurnal variation on October 11th 2024, when the Kp index reached above 8, was shown in the plot below. Field changes as fast as 5nT/second were recorded. In comparison, the Kp index on October 15th was around 4. The diurnal variation was much slower. Sometimes if a survey site is close to man-made structures, such as railways, very rapid local magnetic field variations can also occur due to electrical current activities. A high sample rate magnetometer base station is also required in these situations, as well as some expertise in setting up the base station.

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.

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:

What Affects Geode Trigger Cycle Times? If you're trying to optimize your Geode system for faster trigger cycles—especially in high-repeat environments—there are a few key factors to consider. The goal is to ensure that the system completes its entire cycle (trigger → recording → data transfer → re-arming) before the next expected trigger. Here’s what influences that cycle: 🧠 Core Factors That Affect Cycle Times 1. File Size (Sampling Parameters) Your sample interval and record length directly affect the size of each data file.You can view the resulting file size in the Acquisition Parameters menu.Larger files take longer to transfer, which delays the re-arm process. 2. Data Transfer Rate The Geode typically transfers data at around 450–465 kb/sec.Reducing file size is the best way to reduce transfer time and speed up the cycle. 3. Calibration Frequency By default, the system may attempt to calibrate every N shots, which takes additional time.Go to Options > Calibration and set "calibrate every N shots" to a large number (e.g., 100000) to prevent unnecessary delays. 4. Recording Delay and Record Length If you're operating in a region with a consistently deep seafloor, you can add a recording delay and reduce record length accordingly.Example: If the Water column is always >0.3s, you can apply a delay of 0.2s and reduce record length by the same amount.This trims your file size and speeds up the transfer/re-arm process. ⚙️ Best Practices Use the Auto-Trigger function or set trigger sensitivity to the maximum value for testing.Monitor the cycle timing and adjust acquisition parameters to stay within your trigger window.It's often an iterative process to find the ideal configuration for your environment.

Hi Anton, We are not aware of any special grounding schemes for a large number of modules. In dealing with EMI interference, our experience is that you need to individually ground each Geode - how you do it depends on the conditions you are in, but generally a Metal stake with a wire to the grounding post is the best you can do. Sometimes adding a little Water to the soil around the stake can help. We have seen a majority of this type of noise occur in highly resistive environments (dry sands or hard rock), and grounding can help. If you are able to orient the lines perpendicular to the transmission lines that can also help. In general, with high voltage lines, the emanating field (50 or 60Hz) is likely coupling into the spread cable. Each geophone pair in the spread cable is a big antenna, but the pairs are twisted so any signal induce in one will also be induced (in varying degrees) into its neighboring wire. This will lead to a common mode voltage at the input to each Geode channel. So it would depend upon the common mode rejection (CMR) of the Geode’s input amplifier to reject this common mode signal. But the CMR of an amplifier is finite, and if the common mode signal is large, the common mode range of a given channel can be exceeded leading to the 50 or 60 signal being amplified by the front end differentially resulting in a lot of noise. This can happen even with very good CMR. Also any unevenness in the spread cable pair twists will result in a differential signal which will be amplified by the system. We would also suggest trying the lower gain setting, in case the front end is being over driven. You may need a bigger source to overcome the loss of amplification, but if you can get data, even with substantial 50 60z noise, a post processing 50 or 60Hz filter may be able to clean up the record enough to be useable. We haven’t tried this, but if everything else failed then this might be worth trying: It is possible that the noise is being induced in to the coils of the geophones. If that’s the case, then putting a metal can over the affected geophones might help. Note that the cans might have to be grounded with a metal stake driven into the earth.

Sensitivity is given as a frequency bandwidth product or nT/rt Hz RMS. This value is valid for ALL sample frequencies or sample rates. Sensitivity for the G-858 and G-859 is 0.008nT/rt Hz RMS Sensitivity for the G-823 and G-882 is 0.004nT/rt Hz RMS Noise levels can also be given as Peak-to-Peak numbers at certain sample rates. For instance at 10 Hz. RMS value at 1 Hz for either instrument is approximately equal to the sensitivity P-P value at 1 Hz is roughly equal to 3x or 4x the RMS value. Remember that the Root Mean Square is not operating on a sine wave but on some random noise components as well and thus the actual Root Mean Square would be about 0.024nT for the 858 and 0.012 for the 882 at 1 Hz. For higher frequencies, we basically take the square root of the sample rate and multiply that times the P-P at 1 Hz. So for 10 Hz we have 0.075nT for the 858 and 0.036 for the 882. This is approximately what we see in the field and that can be verified (looking at the noise on an 882 in very quiet area, deep Water, we see less than 0.050nT P-P.) So for 20 Hz the multiplier is 4.5 and for 40 Hz the multiplier is 6.3. So 882 noise at 40 Hz would be 6.3 x 0.012 = .075 nT P-P.

All wave phenomena, the most familiar being visible light, are subject to refraction, or a change in propagation direction, at interfaces between materials of contrasting propagation velocities. The bigger the difference in velocity, the more the energy is refracted or "bent". As the name implies, seismic refraction uses the travel times of refracted seismic energy to determine the seismic velocity of the earth. A short practical discussion of seismic refraction can be found here. The most important thing to keep in mind when learning how the seismic refraction method works is this: When doing seismic refraction, we are only interested in first-arrival energy at each geophone. The rest of the wave train -- reflected energy, surface waves, etc. are discarded and ignored. Except in the exceedingly rare case where the near-surface is slower than the speed of sound in air, the first-arrival energy will always be either direct or critically-refracted energy. Common applications of seismic refraction include: Estimating rippability prior to excavation Mapping depth to bedrock/bedrock topography Mapping depth to ground Water Measuring the thickness of the weathering zone Calculation of elastic moduli/assessment of rock quality Mapping thickness of landslides Identification and mapping of faults Seismic refraction uses body waves, most commonly, p-waves. Shear-wave refraction can also be done, but it has become largely supplanted by MASW.

different types of seismic waves travel at different velocities through any given material. In addition, different materials have different seismic properties, meaning that any one wave type can have a wide range of velocities, depending on the material properties. For instance, the p-wave velocity of shale can range from 800-3,700 m/s. Granite can range from 4,800-6,700 m/s. Because of this, by themselves, seismic velocities alone are not particularly diagnostic with regard to rock type. Ultimately, seismic velocity depends on the density and elastic properties of the material, whatever its composition. Specifically, Compressional-wave velocity depends on the “incompressibility” of the material, as embodied in the bulk modulus. The higher the bulk modulus, the less compressible the material, and the higher the p-wave velocity. Sound travels through Water about four times faster than it does through air. Similarly, shear-wave velocity depends on the rigidity of the material, or the resistance to shear. The higher the shear modulus, the higher the s-wave velocity. Mathematically, where K = bulk modulus µ = shear modulus ρ = density Note that Vp depends on both the bulk and shear modulus, while Vs depends only on the shear modulus. This observation implies two things: Shear waves always travel slower than compressional waves through a given material. Materials with zero rigidity – i.e., fluids – do not carry shear waves at all. Therefore, the absence or presence of groundwater has no effect on the shear wave velocity. It is interesting to note that, in general, seismic velocity increases with density – denser rocks tend to be much harder and faster. Yet in the above equations, density is in the denominator. This is known as the “velocity-density paradox”, the answer to which can be found in the fact that the elastic moduli tend to increase with density as well, and at a faster rate.