18 May 12

Seismograph – Type of Seismometers

science.jrank.org

A number of possible arrangements have been designed for detecting the motion of the Earth’s surface in comparison to some immovable standard. Early seismometers, for example, extended Chang’s invention by measuring the amount by which a pendulum attached to a fixed support moved. Today, however, most seismometers can be classified as inertial or strain devices.

In an inertial seismometer, a heavy mass is suspended by a spring from a heavy support that is attached to the ground. When the ground begins to move, that motion is taken up by the spring and the mass remains motionless with reference to the frame from which it is suspended. The relative motion of the frame with regard to the mass can then be detected and recorded.

A strain seismometer is also known as a linear extensometer. It consists of two heavy objects sunk into the ground. When earth movement occurs, the two objects change their position relative to each other, a change that can be detected and recorded. Many variations in the extent design of this system have been designed. For example, a beam of light can be aimed between the two objects, and any movement in the ground can be detected by slight changes in the beam’s path.

A common variation of the strain seismometer is known as a tiltmeter. As the name suggests, the tiltmeter measures any variation in the horizontal orientation of the measuring device. Tiltmeters often make use of two liquid surfaces as the measuring instrument. When an earth movement occurs, the two surfaces will be displaced from each other by some amount. The amount of displacement, then, is an indication of the magnitude of the earth movement.

The StrataVisor NZ has a daylight visible color screen, built-in plotter and is weather and shock resistant. To see our complete line of seismographs, please visit our website: heritagegeophysics.com.

Seismograph – Recording Systems

One of the simplest approaches to the recording of earth movements is simply to attach a pen to the moving element in a seismometer. The pen is then suspended over a rotating drum to which is attached a continuous sheet of graph paper. As the drum rotates at a constant speed, the pen draws a line on the graph paper. If no earth movement occurs, the line is nearly straight. Earth movements that do occur are traced as sharp upward and downward markings on the graph. Since the rate at which the drum rotates is known, the exact timing of earth movements can be known.

In some kinds of recording devices, the moving pen is replaced by a beam of light. Earth movements can then be recorded photographically as the beam of light travels over a moving photographic film. This type of device has the advantage that friction between pen and rotating graph paper is eliminated.

Seismograph – Practical Considerations

Seismographs must be designed so as to take into consideration the fact that small-scale earth movements are constantly taking place. The seismogram produced by a simple seismograph sitting on a laboratory table, for example, would show not a straight line but a fairly constant wiggly line resulting from these regular microearthquakes.

Two methods are commonly used to eliminate this background noise in the detection of earthquakes. The first is to sink the supports for the seismograph as deeply into bedrock as possible. When this is done, movements in the more unstable parts of the Earth’s upper layers can be eliminated. A second approach is to lay out a network of seismographs. The data obtained from this network can then be averaged out so as to reduce or eliminate the minor fluctuations detected by any one instrument.

Seismograph – The Richter Scale

A variety of methods have been devised for expressing the magnitude, or intensity, of earth movements. For many years, the most popular of these has been the Richter scale, named after seismologist Charles F. Richter, who developed the scale in 1935. The Richter scale is logarithmic. That is, each increase of one unit on the scale represents an increase of ten in the intensity of the earth movement measured. An earthquake that measures 6.0 on the Richter scale, as an example, is ten times as intense as one that measures 5.0 and one hundred times as intense as one that measures 4.0.

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12 May 12

Promis-10, Multi-Frequency EM Profiling

Heritage GeoPhysics is pleased to introduce its new multi-frequency electromagnetic profiling system, PROMIS-10, bringing improved productivity with its fully incorporated data logger and its multi-components receiver sensor. PROMIS-10 is the ideal geophysical tool for locating conductive targets such as: fractured zones for groundwater exploration, mineralized veins for mineral exploration. The operator at the receiver fully controls the transmitter through the cable link. After each new reading, the profile curve is updated on the receiver graphic display.

PROMIS-10 transmitter features ten frequencies from 110 hertz up to 56320 hertz and is designed for transmitter-receiver spacings of up to 400 meters.

PROMIS-10 receiver features a three-components sensor allowing to simultaneously acquire data in the Max and Min coupling configurations. The additional perpendicular component provides information on the target strike.

The optimized design of the PROMIS-10 provides unmatched characteristics both for the transmitter (lighter but more powerful) and for the receiver (versatile and easy to handle).

Features:

  • Ten frequencies:110, 220, 440, 880, 1760, 3520, 7040, 14080, 28160 and 56320 hertz
  • Coil separation: Any value between 10 and 400 meters thanks to digital compensation of primary field
  • Reference cable: Lightweight unshielded two twisted conductors teflon cable with kevlar thread, for wide operation temperature range and higher strength
  • Standard lengths: 50, 100, 150, 200 and 400 meters. Other length on request (between 10 and 400 meters)
  • Intercom: Voice communication through the reference cable

Receiver specifications:

  • Receiver EM sensor: choice of 1, 2 or 3 axis coil assembly
  • Parameters measured: in-phase and quadrature components of the secondary magnetic field (in percent of primary)
  • Tilt sensor: measurement over 2 or 3 axis
  • Display: graphics LCD 128 x 64 pixels
  • Data storage: 1 Mbyte Flash memory (optional 32 Mbytes)
  • Power supply: NiMh 4.8 V – 4 Ah battery
  • Operating temperature range: -40 to +70 °C
  • Weight: 3 kg with one component sensor, 4 kg with three components sensor

Transmitter specifications:

  • Transmitter coil: optimized for wide frequency range, light (8 kg)
  • Tilt sensor: measurement over 2 axis
  • Transmitter console: remote-controlled by receiver through reference cable, or operator-controlled with four keys keyboard
  • Console weight: 3 kg
  • Display: LCD with 4 lines of 16 characters each
  • Power supply: NiMh 12 V – 24 Ah battery in belt pack (3.5 kg) providing more than one day of operation in most conditions

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For more information about magnetometer, cesium magnetometer, and seismograph, visit our website cesium magnetometer.

For more information about geo magnetometers, ground magnetometer and resistivity meters, visit our website geo magnetics.

For more information about geo resistivity, geo hydrology, and cesium magnetometer, visit our website resistivity meter.

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4 May 12

ES-3000 Solution Seismograph

terraplus.ca

Looking for a lightweight underground imaging tool but unwilling to spend a bundle? Need an ultra-portable recorder, but don’t want to give up on features? Look no further!

If you are a geoscientist doing teaching or research, or just need an exploration seismograph to find bedrock, the ES-3000 is for you. The system includes ESOS data acquisition software and the ES-3000 seismodule connects directly to your PC via the Ethernet port – no additional hardware or drivers are required.

The ES-3000 operates from your laptop loaded with the ESOS data acquisition program.

The ES-3000 system comes with all the data analysis software you need to do comprehensive refraction surveys to find bedrock, the water table, where to rip, or even outline gravel deposits.

The ES-3000 comes with a 3-year warranty.

Features:

  • Find bedrock, depth-to-water, faults and fractures.
  • Ideal for engineering, construction, roadbuilding, and teaching.
  • Best quality data: automatic settings, make no mistakes.
  • Lightweight (8 lbs/3.5 kg) and low-power.
  • Easy interface: no complicated drivers, plugs directly into your PC Ethernet port.
  • Includes analysis software to give you quick answers in the field and reports for your client.
  • Reduced noise and cost: data transmitted from ES-3000 to host computer by digital cable.
  • Optional software for blast and vibration measurements, earthquake monitoring.

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For more information about magnetometer, cesium magnetometer, and seismograph, visit our website cesium magnetometer.

For more information about geo magnetometers, ground magnetometer and resistivity meters, visit our website geo magnetics.

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28 Apr 12

Learning How to Use the Geometrics StrataView Seismograph

engr.uconn.edu

The Strataview is a fully functional seismograph suitable for commercial surveys. It differs from the most advanced commercial systems in that the connection between the geophones and the recording system is analog, consisting of a 2-wire connection for each geophone (or 6-wire for the 3 component phones). More advanced systems digitize the signal at the geophone and transmit digital data via a 2-wire computer network to the recorder. For example, for our 48 channel StrataView system, 96 wires run along the cable, two for each channel, while a fully digital system would only need 2 along the main cable. This saves in weight and complexity. For example if a raccoon eats through your cable, you only need to patch 2 wires, instead of 48. And if you need to carry cables in a back-pack, the lighter cables are better.

The StrataView system has cables for 48 vertical phones. Use the phones and the appropriate cable (the one with take-offs grouped in threes) to conduct a simulated experiment in the lab. Use the wooden blocks to hold the geophones, and conduct an experiment to measure the seismic velocity (wave speed) in the concrete floor, and/or air. Geophysicists usually use the word ‘velocity’ rather than ‘wave speed’ when talking about how fast waves travel. Students new to geophysics recognize this as a misuse of the word ‘velocity’ which in physics means a vector. In geophysics it means a scalar, and to distinguish the terminology, when one refers to the vector motion of individual particles, a geophysicist says ‘particle velocity’.

The basic experiment is to lay out the geophones in a linear array, generate a wave (both seismic and sound), and record the signal as it propagates across the array of receivers. The ray paths are essentially all horizontal lines connecting the seismic source to the geophones. Though the geophones are not intended as pressure sensors, they do generate a small signal as the sound wave passes by them.

Strataview - Stand alone system for reflection or refraction, supports 12-64 channels, includes internal PC with hard drive. Fully weatherproof system with easy, menu driven design. To see our complete gallery of equipment, please visit us at heritagegeophysics.com.

Use the following directions to set up the experiment.

  • Establishing connections
  • Connect 12V battery to Geode via yellow power cable.
  • Connect the Geode to the network card on the laptop computer via the Geode interface
  • (Belden Data twist) cable at the output terminal of the Geode. Connect the geophone cable to the Geode at the terminal indicated by a geophone icon. Connect the hammer/weight drop to the Geode via the Amphenol twist-lock cable at the terminal indicated by a hammer icon. Turn on Geode by depressing green power button. A blue LED next to the power button should now blink in 3 second intervals to indicate the Geode is in standby mode.

Beginning a survey

  • Open Multiple Geode OS icon.
  • Under Survey tab select New Survey.
  • Select an appropriate survey name and line number that will aid in remembering the survey. Under the System tab select Set Date/Time/Units and specify the time and unit preference. Under the System tab select Trigger Options. Trigger Holdoff should be around 0.2 seconds, Arm Mode should be Auto and Geode Number should be 1.

Geometry

  • It is not necessary to enter geometry position in order to do a survey, but is required if planning on using the analysis functions in the Answers menu.
  • Under the Geom tab select Survey Mode and choose the appropriate option. Under the Geom tab select Group Interval. This refers to the spacing between geophones.
  • Select an appropriate interval.
  • Under the Geom tab select Group/Shot Locations. This menu shows the layout of the line with shot and geophone locations. Key in a Shot Coordinate of zero in the box on the upper left corner of the screen. Using the down arrow key, move to the Geophone Coordinate box of the first geophone and enter an appropriate value. Move the cursor to the right one box and the software will automatically recalculate the geophone position based on the initial geophone position and the Group Interval as specified in the Geom tab. Values in the Interval box can be entered manually in the event there is an obstruction in the field that requires adjustment to the geophone spacing. If ENTER is accidentally pressed, just return to this menu.
  • Selecting the Shot Coordinate box and then using the arrow key to scroll along the line displays shot coordinates relative to geophone locations.
  • Under the Geom tab select the Phone Increment. This is used to increment the value of the geophone positions each time the data are saved, most typically in reflection surveys.
  • Under the Geom tab select Shot Increment. This is used to increment the value of the geophone positions each time the data are saved. This is not widely used in most refraction surveys, so 0 would be recommended as well.

Acquisition

  • Under the Acquisition tab select Sample Interval/Record Length. For class room demo surveys, 31.25 microseconds sampling and a record length of 0.050 seconds is recommended. Leave the Delay at 0 and select OK. For small surveys, use .250 seconds sampling and a record length of 0.200 seconds.
  • Under the Acquisition tab select Stack Options. The Auto Stack feature is recommended, and a higher Stack Limit should be specified in noisy areas. Geometrics recommends 5-7 stack in noisy areas. Unstack Delay should be 0 seconds. Check the Display immediately box, make sure it is turned on.
  • Under the Acquisition tab select Specify Channels. Verify that all channels in the Use section are set to DATA. This makes all channels active.
  • Under the Acquisition tab select Preamp Gains. 36db is recommended.
  • Under the File tab select Storage Parameters. It is recommended that the Next File Number be set to a four digit number (ex. 5001). It will increment automatically with each subsequent save. Disabling Autosave and checking Save to Disk will ensure that the data can be written to a preferred location.
  • Under the File tab there is the option to save data in SEG-2, SEG-D and SEG-Y format. SEG-2 is recommended.
  • Now select the Do Survey tab from the main menu bar. Select Noise Display. In the noise display mode, signals from the geophones are displayed as wiggle traces on the screen in a waterfall display. Use the up and down arrow keys to adjust the noise monitor sensitivity, until small excursions are visible on the traces. Stomp your foot on the ground next to each geophone and you should see a noise burst on the traces generated by the geophones closest to your foot. A trace that has considerably more excursions than the others may indicate a bad or noisy geophone or may indicate that the geophone has been improperly implanted. Traces that are straight lines indicate a disconnected geophone or a break in the cable. The level of background noise can be measured quantitatively using the scale factor displayed in the upper left-hand corner and noted in the field records.
  • The next step is to generate the seismic wave and record it on the instrument. If not already selected, choose Arm/Disarm under the Do Survey tab to arm the source. The arm/disarm status is displayed on the lower, left-hand side of the screen.
  • The hammer/weight drop blow should be timed for quiet periods. If using a hammer, the hammer switch should be on the top side of the handle, away from the ground. If an error occurs before the last blow in a multiple-stack shot, it is possible to retry that shot by selecting Clear Memory from under the Do Survey tab.

Data display

  • It is possible to use the Auto Scale Traces function under the Do Survey tab as a first attempt to make the first arrivals of seismic energy clearly visible.
  • The Display menu is used to control how the data looks on the screen. Display Boundary under Shot Parameters can be used to make sure that your first breaks are properly positioned in the display window. Variable Area, Shaded Area, or Wiggle Trace may be chosen under Trace Style as well as Clipped waveforms. Display Gains allows you to change the gain on the traces.

Picking first breaks

  • Go to the Answers menu and select Pick Breaks. Use the arrow keys to move the top of the red line to just above the area that contains the first arrivals. Press the TAB key to toggle control to the other red line and position it just below the first arrivals. Press ENTER and the seismograph will automatically pick the first breaks.
  • You will be offered the option of manually editing the picks. Move the picks up or down on the display using the arrow keys. Press ENTER to save your picks. Your pick file will be saved with the same name as your data file, but with the extension ‘.bpk’.

Saving your data

  • Select Do Survey from the main menu and select the Save option.

Printing paper copies

  • Make a paper copy by using the Print option in the Do Survey menu or choose Shot
  • Parameters in the Print menu to Expand or Compress the record.

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For more information about magnetometer, cesium magnetometer, and seismograph, visit our website cesium magnetometer.

For more information about geo magnetometers, ground magnetometer and resistivity meters, visit our website geo magnetics.

For more information about geo resistivity, geo hydrology, and cesium magnetometer, visit our website resistivity meter.

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20 Apr 12

Geo-Magnetic Surveys

geo-sense.com

Magnetics are used to detect subsurface elements due to spatial variations in the total magnetic field.

Residual magnetic fields can be isolated from regional measurements, indicating the existence of typical subsurface bodies. Using a magnetic gradiometer multisensor enables efficient detecting of shallow subsurface bodies. In this method, the total field is measured simultaneously at two elevations by using two sensors on a pole separated by a fixed distance. The difference in magnetic intensity between the two sensors divided by the distance between them is the vertical gradient. This technique reduces interference from solar magnetic storms and regional magnetic changes. This type of survey is used to locate faults, mineralized zones, and underground structures associated with ore deposits, as well as oil fields.

Magnetometer surveys are rapid and efficient. Magnetic surveys can be used to locate buried ferro-magnetic objects (barrels, drums and storage tanks) in order to complete site assessment investigations. Detection depends on the amount of magnetic material present and its distance from the sensor. A single steel drum can be detected at burial depths up to around 6 meters. Burial depth can be estimated from magnetometer data collected using the gradient method.

The most common uses for magnetic surveys are:

  • location of buried tanks and drums
  • geological mapping
  • mineral exploration
  • archaeology

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For more information about magnetometer, cesium magnetometer, and seismograph, visit our website cesium magnetometer.

For more information about geo magnetometers, ground magnetometer and resistivity meters, visit our website geo magnetics.

For more information about geo resistivity, geo hydrology, and cesium magnetometer, visit our website resistivity meter.

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13 Apr 12

Syscal Kid Switch 24

terraplus.ca

Specially designed for shallow resistivity imaging, the Switch 24 features an internal switching board supporting 24 electrodes. Two cable strings with 12 electrode take outs each are connected to the back of the meter. Software is provided to automatically perform roll along surveys.

The all new user interface is quickly absorbed so that a wide range of users from archaeologists, to civil engineers, to geologists, can easily take advantage of the Switch 24′s capabilities. Select the type of array, enter your electrode spacing, and tell the instrument to start. When sampling is complete move the cables down the line and start again. During data upload chose the profiles to be merged, create a final data file, and send it to Surfer for pseudo section display, or to your imaging software for modeling. Surveys are quickly completed, even with a single operator.

Syscal Kid Switch 24 resistivity imaging system for depths to 25m. Designed for shallow ground water surveys, archaeological surveys, and geologic mapping. For more information please visit us at heritagegeophysics.com.

Resistivity anomaly over an ancient Roman Tomb (2D inversion of Wenner Data). Electrodes were spaced at 0.5m intervals. Total time to set up equipment, collect data, invert data, and print results was just over an hour.

General Specifications

The Syscal Kid features what is called auto ranging. This means that the instrument automatically sets the correct output voltage based on the level of the measured signal. No more fussing with output voltage selections to get the best measured signal. The Kid also features automatic SP compensation, but does it better than most. What is unique to all of the Syscal meters though, is that during stacking, with every third stack, SP is again measured and reapplied, so that any drift during the stacking process is corrected. The result is a very accurate measurement, even in the presence of significant noise.

  • Output Voltage = 200V (400 Peak to Peak)
  • Output Current = 500mA
  • Rechargeable Battery or External 12 V
  • Memory = 1,400 Readings
  • Input Voltage = ± 2.5V, Protected to 200V
  • Input Impedance = 22MOhm
  • Resistivity Range = 0.001 to 10,000 Ohm meter
  • Accuracy = 1%
  • Weight/Size = 3.0Kg, 23 x 18 x 23cm

Standard Components

Syscal Kid console, instruction manual, data transfer cable and software, 2 cable strings with 12 take outs each, 24 metallic electrodes and connectors.

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For more information about magnetometer, cesium magnetometer, and seismograph, visit our website cesium magnetometer.

For more information about geo magnetometers, ground magnetometer and resistivity meters, visit our website geo magnetics informational site.

For more information about geo resistivity, geo hydrology, and cesium magnetometer, visit our website resistivity meter informational site.

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7 Apr 12

Geophysical Methods in Civil Engineering: Georesistivity Methods

Abstract from “Geophysical Methods in Civil Engineering: Practical Applications”, by Emilio M. Morales and Mark K. Morales

Geo resistivity methods fall into the category of Vertical Electrical Surveys which sends electrical current into the subsurface. The resulting electrical resistivities are then measured and correlated and compared with various soil types and water bearing aquifers to yield layering or stratification information as well as identify other layer properties. Two commonly used methods are the Schlumberger Electrode array (Shown below) and the Wenner Electrode array. The former method is more popular for use in well or aquifer surveys.

The Schlumberger Method

The Schlumberger array uses four electrodes: two of which serve as the current electrodes and the other two for potential electrodes. The current electrodes are represented by AB and the potential electrodes by MN. Electric current is introduced into the ground using AB electrodes and the potential difference is read using the MN electrodes. Initially, lengths of AB and MN are set to two meters and one meter, respectively. As the measurement progresses, AB expands from the sounding center at the spacing interval of factor of square root of two, i.e., 1, 1.4, 2, 2.8, etc., keeping the MN constant. However, as the length AB increases, electrical voltage drops considerably. The manufacturer of the instrument has prescribed a minimum voltage of five millivolts when conducting resistivity measurements. To keep voltage above the set minimum voltage, MN has to be expanded as well. In order to detect discrepancy for the reading when MN is expanded to a new length, duplicate readings are taken for the same AB but with different MN values.

Readings from the instrument are raw resistivity data. Actually, they are in the form of volt/current ratio, having a unit of “ohm”. These resistivity raw data are multiplied by a geometric factor unique for every set of AB and MN which is taken from the formula:

Geometric Factor = (π /MN) [(AB/2)-(MN/2)2] where π is 3.1416.

The resulting values when the readings are multiplied by this factor will now be the apparent resistivity. The usual field procedure is to plot the computed apparent resistivity at logarithmic scale paper to gain initial view of the resulting curve. This is undertaken prior to interpretation or at the sounding site to preclude unwanted curve, which results when errors are committed in readings and in distances set up.

The interpretation of the measured values is facilitated through the use of a built in computer software and signal processor within the instrument. Resistivity sounding interpretation software was used for database management and sounding interpretation including plotting of sounding curves.

Case Study: Application of Georesistivity

An industrial plant had to boost groundwater capacity as the existing wells are proving inadequate.

It was originally suspected that the Existing wells would not meet future demands of the Factory.

The four production deepwells were barely adequate to meet the demands of the manufacturing facility although the deepwells are spaced far apart and not competing with each other. The wellscreens were set at the middle of a deep medium yield aquifer at approximately 200 meters below existing NGL.

The site was subjected to Vertical Electrical Survey (VES) using an Electric Georesistivity Equipment.

The results were very surprising, as the VES pointed to a shallow but otherwise very promising aquifer which was consistently bypassed by the previous deep wells drilled and resulted in a new program for Groundwater development to exploit the shallow aquifer which has remained an untapped groundwater resource.

The results of the VES pointed to a very promising highly permeable and very shallow water bearing layer which has been consistently bypassed in all the existing well developments. This shallow aquifer can increase the yield from these existing deep wells by using two well screen settings instead of one by at least 2.5 X.

Future well settings will concentrate on this shallow aquifer for major development.

The direction points to exploitation of this aquifer layer as well as the lower aquifer with a new well.

The potential total yield is around 50 to 70 cu meters per hour which could significantly boost the future water demand in conjunction with the other wells.

We do not expect significant impacts on the lower aquifer in terms of potential yield when exploiting the upper aquifer as the latter is separated by an impermeable clay layer or an aquiclude.

Conclusion

The Geophysical methods are not intended to supplant borings except in specific cases where information gathered would be sufficient to address the intended purpose/s.

It is hoped that through these practical sample applications, a better appreciation of the capabilities and cost effectiveness of each method can be understood better by the engineering community.

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To get additional information on cesium magnetometer and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

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To get additional information on ground magnetometer and other instruments such as magnetometer, seismograph and more, please visit our resistivity meters informational site.

To get additional information on geo resistivity and other instruments such as magnetometer, seismograph and more, please visit our resistivity meter informational site.

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29 Mar 12

Geophysical Methods and Techniques for Ground Based Investigations: Seismics

hgiworld.com

Seismic methods measure acoustic velocities (the speeds at which sound waves propagate) in earth materials. There is a direct relationship between acoustic velocity and material density (dense materials propagate sound faster than loose materials); therefore, seismic velocities can be interpreted to determine the subsurface conditions below a survey area.

Seismic refraction

The seismic refraction method is based on the measurement of the travel time of seismic (sound) waves refracted at the interfaces between subsurface layers of different velocity. A seismic wave is introduced into the subsurface via a shot point using explosives (blank shotgun cartridge), hammer blow, dropped weight or an elastic wave generator. Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface at a series of receivers (geophones) spaced at regular intervals. Beyond a certain distance from the shot point, known as the cross-over distance, the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival). A seismograph records the travel time for the energy to travel between source and receivers. In most refraction work only the first P-wave arrivals are recorded, providing depth information of interfaces. However, techniques can be used to record the arrival of the shear (S-) waves, which provide additional data about engineering properties of the subsurface media.

Seismic refraction methods rely on the tendency of acoustic velocities to increase with depth, which can make it insensitive to low velocity layers in the subsurface. Travel times are interpreted to compute velocities of, and depths to, materials at various interfaces. Refraction data are presented as cross-sectional plots representing P-wave path(s), velocities and depths to various interfaces.

Seismic refraction has many applications. In geotechnical engineering and mining applications, it is used to determine depth to and rippability of bedrock for design and cost estimates. Groundwater applications include mapping bedrock channels, identifying faults and fracture zones, and delineation of geologic boundaries to constrain hydrogeologic models.

Seismic reflection

Reflection seismic methods are similar to the refraction method described above; travel times are recorded for an induced seismic wave reflected from subsurface interfaces to reach an array of geophones placed at known distances from the source. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these interfaces. Since we are recording reflections the seismic waves travel a much shorter distance in the subsurface. Consequently the seismic waves possess higher frequencies, potentially leading to higher resolution. Seismic energy is introduced using the same techniques as for the refraction surveys, and reflected travel times to various interfaces are recorded on a seismograph.

Travel times are a result of seismic velocities of all subsurface materials between the surface and a particular interface, but not their differences; therefore reflection techniques can be used to find depths to less dense materials beneath denser strata. Further, because reflected waves occupy a shorter horizontal distance, the reflection method can reach greater depths with less energy than the refraction method. However, seismic reflection is more sensitive to interference and so the method is not always suitable at noisy sites.

Seismic reflection surveys are commonly used for groundwater investigations, faults studies, landslide investigations, and resource assessments.

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To get additional information on cesium magnetometer and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

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To get additional information on geo resistivity and other instruments such as magnetometer, seismograph and more, please visit our resistivity meter informational site.

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22 Mar 12

Geophysical Methods and Techniques for Ground Based Investigations: Resistivity

hgiworld.com

Resistivity is a volumetric property measured in ohm-m that describes the resistance of current flow within a medium. Its inverse, conductivity in Siemens/meter, describes the ease by which current will flow through a medium.

The earth’s resistivity is a function of soil type, porosity, moisture, and dissolved salts. The resistivity method detects and maps changes or distortions in an imposed electrical field due to heterogeneities in the subsurface.

Data are acquired systematically along a survey line by transmitting electric current (I) into the earth and recording the resulting potential (V) at increasing distances away from the current electrode. In this manner, a cross-section of apparent resistivity values is constructed of the subsurface.

Measurements are typically acquired using the pole-pole array configuration. The term “pole-pole array” refers to a particular arrangement of four electrodes that are used to transmit current and receive the potential voltages. An electric field is established by applying electrical power (I) between two electrodes (transmitting pair or Tx). Electric potential (V) is measured by sampling received voltages using a data acquisition card connected to two additional electrodes (receiving pair or Rx). One of the electrodes of the receiving pair (Rx) and one electrode of the transmitting pair (Tx) is used as a remote grounding reference. The remotely-located reference electrodes must be located far enough from the active survey area that they do not influence the local electric field. A “transfer resistance” value (V/I) is obtained by dividing the electrical potential (V) by the applied electrical current (I). These two remote electrodes remain in a fixed location while the two “roving” electrodes are moved within the survey area. The remote electrodes have minimal influence on the readings, so, we use the term “pole-pole” to indicate that we are only using two active electrodes.

Uses of Resistivity

  • Heap Leaching

Electrical resistivity surveys are most reliable as a first-order target recognition tool. In this mode, sufficient background data are needed to distinguish the entirety of the target and confirm the extent of its edges. A target will not be identified if the variations in properties of the background material are similar in contrast and scale to those associated with the target. Assuming the target can be identified, the next order of interpretation is the relative degree of target size and intensity. Low resistivity regions may have discernable features that identify relative concentrations within the target. Finally, if data are of exceptionally high quality, i.e., the data are free from significant noise and have been acquired properly, they may be correlated to specific observed phenomena to develop relationships that convert directly geophysical data to hydrogeological or metallurgical data. This scenario requires that empirical models be developed from observed, co-located geophysical and metallurgical data. The empirical models are then used to translate and extrapolate the geophysical data to obtain metallurgical values over the site. Resistivity can be used to locate preferential flow paths of dry areas in the heap.

  • Pond Liner Leak Detection

This method is applied at the pond using a specialized marine cable with stainless steel electrodes at approximately 10 foot spacing to measure an induced electrical field (i.e., electrical voltage). The electric field is established by transmitting electrical current on two electrodes, with one electrode located between the bottom and top liners and the other electrode within the pond itself. The principle of resistivity as a leak detector is that as long as the top liner is tight (no leaks), then no electrical current can flow between electrodes between the liners and in the pond. The resistivity system uses a 12 volt deep cycle marine battery and outputs a low powered signal which does not present any danger to the liner or operators in the pond. If no leaks exist within the top liner, the ensuing electrical field would show a similar response (equi-potential lines) throughout the pond. If some seams of the liner are delaminated, or if small tears or holes exist in the liner, then these areas would allow electrical current to flow through the openings, thus causing anomalous normalized voltage readings centered over these areas. The resulting data is contoured to form a spatial map of the distortions within the pond which are interpreted to result from leaks within the liner.

  • Resistivity for 3D Imaging

A 3D survey is superior to a 2D survey because considerably more data are collected to define the electrical properties of the subsurface. However, 3D surveys usually take longer to acquire and require more resistivity equipment. Resistivity data are collected based on a 3D data acquisition method that makes use of different electrode arrangements. The surface electrodes are distributed across a uniform grid to optimize the inversion models used in the data analysis and interpretation. The significantly larger amounts of data associated with a 3D survey, relative to a 2D survey, makes an optimized geometry crucial to reduce modeling run times and analysis. Further resolution is possible by adding depth electrodes to a surface electrode geometry, whereby electrical current and voltage measurements can be made near or within a target. Depth electrodes have the added benefit of being further from near-surface infrastructure and associated electrical interference and noise.

With the SYSCAL R1 Plus Switch resistivity surveys can be performed very efficiently with one operator only. For more product information please visit us at heritagegeophysics.com.

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To get additional information on cesium magnetometer and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

To get additional information on geo magnetometers and other instruments such as magnetometer, seismograph and more, please visit our geo magnetics informational site.

To get additional information on ground magnetometer and other instruments such as magnetometer, seismograph and more, please visit our resistivity meters informational site.

To get additional information on geo resistivity and other instruments such as magnetometer, seismograph and more, please visit our resistivity meter informational site.

To get additional information on geo hydrology and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

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8 Mar 12

Portable Cesium Vapor Magnetometer – Model G-859 Mineral Mag™

rmsinst.com

This new low-cost Cesium vapor magnetometer system offers the mining/oil/gas survey companies the best total field magnetic survey tool available. Based on the industry standard G-858 MagMapper system, the G-859 incorporates all of the reliability and proven performance in a lightweight survey package with integrated WAAS/EGNOS enabled Novatel™ GPS.

  • Excellent performance: Low Noise/High Sensitivity, best in the industry – 0.008nT/—Hz RMS – and world wide operation.
  • Very fast: Log mag and GPS at up to 5 samples per second for economic large area surveys at high sample density.
  • Integrated GPS/Backpack: Includes non-magnetic backpack and Novatel™ WAAS / EGNOS ready GPS.
  • Low AC field interference: Best in the industry for rejecting AC power line grid noise (50/60 Hz).
  • Easy-to-use: Simple setup and rapid in-field map generation with free MagMap2000™ software.
  • Reliability: Our Cesium sensors never need calibration or factory realignment. Designed for extreme ruggedness and reliability.
  • Designed for large surveys Mining/Oil/Gas: This versatile tool is specially designed for large area surveys with 8 hr data storage capacity and two 6 hr battery packs.

Operation

The G-859 Mining Mag uses a graphical interface to make survey design and data acquisition quick and efficient. A “Simple” or “Mapped” Mode uses line numbers and known staked reference points to define the map parameters. Or the user may use the integrated Novatel Smart Antenna™ GPS for mapping positions automatically. Position information may come from an external GPS, from regularly spaced fiducial marks input by the operator or both. At any time, the user may switch to “profile” mode to observe the last 5 data lines as stacked profiles.

Data is collected in up to 5 separate survey files and transferred via high speed RS-232 data link (or USB with converter) to a computer for further analysis and map generation. The full featured graphical data editing program MagMap2000 is provided to allow repositioning, realignment, GPS smoothing, data filtering and interpolation of the data. After editing, the data is formatted in either Surfer for Windows or Geosoft formats for further plotting and analysis.

Speed and Efficiency

G-859 data acquisition offers either continuous (automatic) or discrete station recording. Data quality is uniformly high and lower costs are inherent for most projects due to the high sampling rate of the instrument in continuous mode. This allows the operator to survey an area at a fast pace, covering as much as 10 times more area in a given time period than other magnetometers.

Reliable, Rugged & Low Cost Design

In addition to its speed and sensitivity, the G-859 is also reliable, economical, and easy to use. Electrical connectors on the G-859’s sensor have been eliminated in order to increase reliability and reduce setup time.

The G-859’s internal firmware has been streamlined to include those features important for mining exploration. We are so confident in the improved design of the G-859 that it comes with 2 Year Warranty!

Applications

The concentration of magnetic minerals often varies with geological formation or chemical alteration and can indicate hydrocarbon bearing structures or economic mineral deposits. A primary application of the G-859 Mineral Magr™ is surveying for minerals and oil/gas exploration programs. The G-859 is an excellent instrument for use in academic research and education and can also be used for local environmental studies such as mapping waste sites, locating buried metal drums and storage tanks buried pipelines, wellheads and other sizable ferrous structures.

Digital Quality

The G-859 system produces raw data of the highest quality. Data is digitally recorded in compressed form in high capacity RAM and later transferred to a computer for permanent storage and processing.

Sensitivity, resolution and recording rate of the cesium magnetometer are user selectable as well as mapped survey grid coordinates or GPS position data. The system is ruggedly packaged for extreme field conditions. Data storage is sufficient to record 8 hours of data at the maximum rate of 5 Hz. Battery life is approximately 8 hours powering both the Magnetometer and Novatel Smart Antenna™ GPS.

Basic Software

A basic software package MagMap2000 is supplied as an integral part of the G-859 system and provides:

  • Transfer of the raw magnetometer, base station and other survey data to the client PC
  • Standard corrections for position errors, transients, and time varying errors (diurnal)
  • GPS track plot with adjustable smoothing and independent point editing
  • Repositioning, linear interpolation and format of corrected data into X, Y, Z or Latitude/Longitude ASCII columnar values for use with Surfer for Windows, Geosoft or other client supplied contouring programs.
  • Conversion of GPS to UTM coordinate system using a selected Datum.

In addition to the standard MagMap2000 download and editing software, we are pleased to offer MagPick™, a full featured potential field analysis package with excellent profile and contour map generation capability.

MagPick™ can perform source body inversion, reduction to the pole, upward continuation and a variety of other transforms and gradient extractions. We are also pleased to offer a Windows™ version of CSAZII™, a world total field map, field inclination and sensor orientation program for performing surveys worldwide in
any survey direction.

MagMap2000, MagPick™ and CSAZII™ are available on our website for free download. Manuals for these programs are supplied as internal documents in PDF format.

Novatel Smart Antenna™

Accurate data positions are as important as accurate magnetic field measurements and Geometrics is pleased to include the Novatel Smart Antenna™ as an integral part of the G-859 system. This small light-weight, all-in-one GPS Antenna and electronics package is WAAS/EGNOS ready for -1.5m positioning.

Novatel Smart Antenna™. You can get this and other geophysical equipment at heritagegeophysics.com.

The Smart Antenna™ systems is installed on Geometrics nonmagnetic back pack and carefully screened and degaussed for minimum magnetic interference.

The Smart Antenna™ is designed to be quickly assembled and installed on the backpack, with special mounting studs and a cable wiring harness for data and power distribution. The storage case allows the main components to be stored as a unit providing minimum assembly at the job site. The storage case is a rugged reusable fiberglass and aluminum travel case with handles and wheels for easy transport.

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To get additional information on cesium magnetometer and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

To get additional information on geo magnetometers and other instruments such as magnetometer, seismograph and more, please visit our geo magnetics informational site.

To get additional information on ground magnetometer and other instruments such as magnetometer, seismograph and more, please visit our resistivity meters informational site.

To get additional information on geo resistivity and other instruments such as magnetometer, seismograph and more, please visit our resistivity meter informational site.

To get additional information on geo hydrology and other instruments such as magnetometer, seismograph and more, please visit our cesium magnetometer informational site.

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