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| # | Post Title | Result Info | Date | User | Forum |
| Differences Between our Standard Cesium Magnetometers and the SX Model | 11 Relevance | 2 years ago | Gretchen Schmauder | General Magnetometer Info | |
| 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. | |||||
| Is MagArrow's "Altitude" with respect to ellipsoid or geoid? | 8 Relevance | 2 years ago | Rui Zhang | Hardware | |
| Altitude refers to Meters Above Mean Sea Level. For both MagArrow I and II, the altitude is ellipsoidal and the earth Model is WGS84. For additional information: Overview Reported elevations from MagArrow (and G-864 and MagEx) are the unedited values from the elevation field in the GNSS's GGA NMEA string. That value is the GNSS's calculated height above geoid; height above geoid is the standard meaning of the elevation field in the GGA NMEA string. But what does calculated height above geoid mean? Definitions GNSS - Global Navigation Satellite System GPS - The GNSS operated by the United States. Other systems include GLONASS(Russia), Galileo (Europe), BeiDou (China), QZSS (Japan), IRNSS (India). Ellipsoid - A comparatively simple or abstract geometric Model of Earth's surface. Geoid - A more complex Model of Earth's surface that takes the place of what was previously called Mean Sea Level. At any particular latitude or longitude, the geoid's surface may be above or below the ellipsoid's by as much as a few hundred meters, depending on regional and local geography and geology. Reference datum - A specific Model of Earth's shape (such as WGS84, EGM96...), including references to specific landmarks. Calculations A particular GNSS, for example the GPS system run by the United States, provides timing data to a receiver to calculate the receiver's position above or below a particular latitude and longitude on the surface of the ellipsoid. The GNSS receiver first uses that timing data to calculate its height over the ellipsoid, and then subtracts from it the local height of the geoid over the ellipsoid (or HAE), to arrive at the local height of the receiver over the geoid (or in old-fashioned terms, elevation over mean sea level): h - calculated height above geoid. This is the value reported in the GGA elevation field. H - height of the receiver over the ellipsoid (calculated from GNSS timing signals) N - local height of geoid over the ellipsoid, or HAE, per a lookup table or other local reference. h = H - N Because the local height of the geoid over the ellipsoid is not provided by the GNSS, it must be provided locally, i.e. by the GNSS receiver, which may contain an internal database from which the local geoid height over ellipsoid (or HAE) can be found, based on the receiver's latitude and longitude. Small GNSS receivers contain small HAE databases, so the HAE value will not be exact. Some small receivers contain no HAE table at all; in this case HAE is deemed to be zero, so that the reported elevation is the uncorrected height over ellipsoid. A user of elevation data from Geometrics' MagArrow, G-864, and MagEx magnetometers may evaluate or adjust the reported values of the GGA elevation field and the GGA HAE field, by comparing the GGA HAE values to another source of local HAE data; this may particularly be useful for GNSSes that report a HAE equal to zero. Geometrics magnetometers do not currently record the values of a VDOP calculation, which offers an additional statistical estimate of the accuracy of the GNSS elevation measurement. | |||||
| How to integrate elevation data in SeisImager? | 6 Relevance | 6 months ago | Kolby Pedrie | SeisImager Software | |
| 🧭 How to Import Geophone Elevation Data into Plotrefa If you have measured relative or absolute geophone elevations, you can incorporate these into your velocity Model in Plotrefa to produce more geologically realistic results. 📄 Step 1: Prepare the Elevation File Create an ASCII text file with two columns: The left column contains geophone horizontal positions (in meters or feet). The right column contains the corresponding elevation values. Example: 0.0 100.0 5.0 101.2 10.0 101.5 ... 100.0 89.0 ✅ Tip: While it’s recommended to have an elevation for each geophone, it’s not strictly required. Plotrefa will interpolate missing elevations as needed. 📥 Step 2: Import the Elevation Data Open Plotrefa. From the top menu, go to:Velocity Model > Import elevation data file Navigate to your file and double-click it to import.📌 Note: There is no default file extension, so ensure your file is visible in the dialog. The elevation profile will now display alongside your data. 🎯 Step 3: Interpret Your Data Once your elevation profile is imported: Proceed with your velocity Modeling or time-term inversion. The resulting velocity Model will be drawn relative to the elevation profile, accounting for topography. Example Outputs: Attachment : seismic_velocity_profile.png | |||||
| How to trim data outside Ray-paths for Seismic Models in PlotRefa | 6 Relevance | 2 years ago | Gretchen Schmauder | Software | |
| You can transfer your SeisImager license from one computer to another using the following guide. SeisImager has a function to erase velocity Model where ray path did not propagate. It is not automatic. Select "Velocity Model", ""Define bottom layer", "Manually". Then click the mouse from left to right to define a line to truncate velocity Model as shown below. Final Result: | |||||
| Magnetometer Survey Planning Considerations | 6 Relevance | 2 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. | |||||
| Magnetic Anomaly Model of a Point Source | 5 Relevance | 2 years ago | Gretchen Schmauder | General Magnetometer Info | |
| The Gammas2.exe is a Windows program that can be used to estimate the amplitude of the magnetic anomaly produced by a steel object that has not been magnetized. In other words, the program assumes that the magnetic anomaly is solely induced by the earth’s field according to the object’s susceptibility. The program uses a susceptibility of 10 cgs units to compute the anomaly amplitude. The estimate produced by Gammas2.exe can be used as a tool to help design magnetic surveys and help interpret survey results. Geometrics no longer updates the Gammas2.exe software, and we offer no guarantee that it will work on your computer. | |||||
| Using a USB/Ethernet Adaptor with ES-3000/Geode and SCS | 2 Relevance | 7 months ago | Randl Rivera | Hardware | |
| Question: Can a USB/RJ45 Ethernet be used with a Geometrics Seismograph and SCS? Answer: Yes it possible to use a USB/Ethernet with Geode / SCS software. We have a “vetted” manufacturer we are now shipping with Geode orders. PN 123-00287 (ADAPTOR, USB-ETH, FOR PCS W/O 10MBPS ETH OR RJ45 PORT, CONNECT BEFORE TURNING ON PC) $60 (check for current price) Some vetted Models that are available on the internet: Cisco USB300M. Other vetted Models:HIRO H50223 USB 2.0*TP-LINK Model UE300 (USB 3.0)Supereal SR9600 (USB 2.0)ASIX AX88772a (USB 2.0)* the TP-Link was the preferred choice The important thing to consider is that the device must have the driver installed and be plugged into a 2.0 or 3.0 USB port when the PC is started. | |||||
| GPS info | 2 Relevance | 2 years ago | Andre Santos | G-864 | |
| What is the Tallysman GPS Model? | |||||
| Do Cesium Vapor Magnetometers Require Calibration | 2 Relevance | 2 years ago | Gretchen Schmauder | General Magnetometer Info | |
| Our cesium-vapor magnetometers do not require periodic calibration in order to maintain the accuracy as described in our published specification when the instrument is operated within specified environmental ranges. Geometrics cesium-vapor magnetometers are manufactured and tested based on the discoveries and the basic designs of Carian Associates (U.S. Patent 3,071,721). This method of total magnetic field measurement relies upon the measurement of the optical absorption of a particular cesium spectral frequency by the cesium vapor enclosed in a small glass cell. This method is similar to those used in the measurement of atomic emission and absorption frequencies using spectroscopic reference cells. The technique thus relies on well-known fundamental quantum mechanical constants for accurate and precise measurement of the magnetic field. As a result, no adjustments to the sensor are needed in order to correct or maintain its accuracy and Geometrics sensor and sensor driver electronics are designed to either work correctly or to not work at all and to report both the strength of the magnetic field as well as the strength of the electrical signal produced by the working sensor. In this way, the signal strength measurement provides a direct indication of the operational state of the magnetometer while it is running and serves to alert the operator if the magnetometer encounters environmental conditions that are outside of its operating range. Occasional maintenance of the instrument at Geometrics facility should be performed when the instrument's internal diagnostics indicate substandard performance as described in the operator's manual. Please contact Geometrics Support for technical advice and additional information pertaining to your specific Model. | |||||
| Can you mix different 4.5 Hz Geophones? | 2 Relevance | 2 years ago | Gretchen Schmauder | General Seismograph Info | |
| An issue that can be encountered with seismic work is when older geophones that are in your kit are no longer available. It is advised to not mix and match phones especially when doing reflection. It is the customer’s responsibility to determine if using the newer 4.5 Hz geophones with older 4.5 Hz geophones will work for their situation, site requirements, etc. When purchasing new geophones from Geometrics, check the response curve for the old geophones you have vs the new ones we sell. Model Information: 4.5 Hz Response Curve_13-105-030D is for the old phones with the discontinued element. GS-ONE LF 2450 specs_Vert_147-00198_[13-105-090D] is the current phones we sell. RTC-4.5hz-395-Spec-Curve_147-00155 [13-205-004.5D] Mixing geophones is not best practice, but they can be mixed so long as you understand the effect on the data when doing so. For surface wave and refraction surveys, it is OKAY, and not a big deal. When performing seismic reflection, mixing same frequency geophones with different response curves is not recommended. Even if not doing reflection, if the results of the data will potentially be used in court of law, or research uniformity is required, then all the geophones will need to be the same. Do your own research as ultimately the decision to either mix or not mix same frequency geophones with different response curves is up to you. Here are the 4.5 Hz geophone datasheets with response curve information: Geospace 4.5 Hz 380 Ohms Geophone datasheet (PN 13-105-030D) RT Clark 4.5 Hz 395 Ohms Geophone datasheet (PN 13-205-004.5D) Geospace 4.5 Hz GS-ONE LF 2450 Ohms Geophone datasheet (PN 13-105-090D) For more information please contact seismicsupport@geometrics.com | |||||
