OHMMAPPER

​

​

Q. What is the depth of investigation? 

​

A. The depth depends on the length of the array, which is determined by the dipole length and the distance from the transmitter dipole (Tx) to  the receiver dipole (Rcv).  For instance with receiver and transmitter dipoles of 5 meters and a Tx-Rcv separation of 10 meters, the depth of investigation is 4 meters.  If the Tx-Rcv separation is increased to 20 meters the depth is 6 meters.  A general rule of thumb is the maximum depth of investigation will be about one-fifth (1/5) of the total array length. The maximum separation obtainable in any given site depends on the site ground resistivity.  In very resistive areas such as frozen ground or permafrost the separation could be up to 60 meters using 20 meter dipoles for a depth of 20 meters.  In very low resistivity sites, like highly conductive clays, the greatest separation might be 15 meters with 5 meter dipoles for a maximum depth of 5 meters.

​

Q. What are its limitations?

​

A.  The biggest limitation is limited depth of investigation.  In most sites the maximum depth will be between 6 to 12 meters.  Like all resistivity instruments it will not get good data over long metal structures parallel to the direction of the survey and within the depth of investigation.  For example if the depth of investigation is 4 meters and you are over a buried metal gas line which is at 2 meters depth, or surveying next to a railway track 2 meters away, you will not get reliable data.

​

Q. What are its advantages over traditional galvanic resistivity meters?

​

A.  There are several advantages:

​

i. The greatest advantage is the speed at which a survey can be completed relative to a traditional resistivity survey using metal stakes in the ground.  At an average walking speed 3.6 km/hour (2.2 miles/hour) a 1 km line with data sampling every meter can be done in less than 20 minutes.  The amount of time to do multiple lines is greater due to the time it takes to turn around and start another line.  However it is still a great time savings over using metal stakes which would take many hours.

​

ii. Since the OhmMapper is capacitively coupled it can be used over concrete, snow, paved surfaces, bare rock, and other areas not practical for planting metal stakes.

​

iii. Because there is no ground contract required there is no contact resistance problem.  The survey can be done over ice, dry sand, caliche, etc. without concern for contact resistance problem.

​

iv. The receiver is tuned to a very specific AC transmitter frequency so it is virtually immune to powerline interference and natural telluric current noise.

​

Q. How long will it take to cover XXX km, miles, square meters, feet, etc.?  In other words, if I am going to bill for a set number  of days how many days is it going to take  me to do the job?

​

A. To survey a 200 m x 200 m plot with data density of 1 reading/meter and line spacing of 5 meters would require a tow speed of 3.6 km/hour over 41 profile lines.  That would 8.2 km of line-kilometers surveyed.  At a conservative estimate of 30 minutes per kilometer that would be slightly over 4 hours of acquisition.  With a turn-around time of 10 minutes per line that would add another 7 hours for a total of 11 hours.  The survey could be done in 2 days.  This is under the assumption that a multi-receiver system is being used and lines do not need to be repeated.

​

Q. Does it use GPS, and if so what kind?

​

A. The system is designed to collect GPS data along with the resistivity measurement during the survey.  It requires a standard NMEA output from the GPS.  The software uses the $GGA string.  If the OhmMapper G-858 console is used a serial output from the external GPS is required.  If it uses a Windows device an external GPS with any interface to the PC is acceptable.  If the data logger is an Android device the internal cell phone or tablet GPS can be used.

​

Q. Where can it be used?  For example, how well does it work on ice, in water, on paved surfaces, over permafrost, inside a building, under power lines, on rocky surfaces?  Where should it not  be used?

​

A. The OhmMapper can be used over paved surfaces and concrete as long as the concrete does not contain rebar. It can be used on any type with soil and geologic structure such as sand, exposed bedrock, permafrost, snow, and caliche.  Because it is an AC electrically coupled system the receiver has a very narrow acquisition bandwidth that filters out power line interference so it can work under power lines.  It is designed to work on land but can be dragged through a puddle or shallow stream.  It is water resistant with an engineering specification to withstand temporary submersion to 3 meters.   

 

For a depth of investigation of 5 meters the array is 25 meters long.   It needs to be towed along a profile line in order to create a 2-D depth section. That means it needs a profile line length of 40 meters or more to be effective.  It is not recommended for surveys in confined spaces of less than 50 meters.

​

Q. Can it detect tunnels and caves, and if so how deep?

​

A. Yes, it has been used successfully to detect and map shallow tunnels, caves, voids, and cavities.  With a sample density of approximately 1.25 meters it is unlikely to accurately position targets smaller than 1 meter in lateral dimension.  Towing the OhmMapper at speeds of around 1 km/hr will increase sample density and increase the likelihood of detecting shallow targets 50cm in dimension.   Detecting voids and cavities deeper than three meters requires them to be larger than one meter.

 

Q. What are the purpose and advantages/disadvantages of using multiple receivers instead of a single receiver?

 

A. In order to do a depth section you need to have data at multiple depths.  The depth is determined by the transmitter-receiver separation.  With a single receiver you need to make multiple traverses with different Tx-Rcv separations along your profile line.  For example for a four-layer depth section with a single receiver you would need to tow the OhmMapper up the line, then change the separation and tow it back down, then change it again back up, and once again down the line again.  That is still much faster than pounding stakes into the ground.   With four receivers in the array each receiver measures at a different depth.  With a single traverse up the line you have a complete 4-layer depth section.  Using multiple receivers makes the survey much more efficient at the cost of a greater initial investment.

​

Q. Can it be used to detect utilities?

​

A. Generally other tools that are specifically designed to locate utilities are preferable to the OhmMapper for utility location.

​

Q. Where can it not be used?

​

A. The OhmMapper is not recommended for the following:

 

i. On concrete or other materials containing rebar.   It should be at a distance greater the depth of investigation over a long, parallel metal structure such as a metal gas pipe line or a railroad rail along the path of the profile line.

​

ii. In confined areas where there is not at least 40 meters to maneuver.

​

iii. When the depth of investigation is over 20 meters, even in highly resistive areas, or when the depth of investigation is over 8 meters in sites of less than 20 Ohm-meters (high clay content or saline groundwater for example).

​

Q. How difficult is it to use the instrument and run the software, and do I need training to learn how to use it?

​

A. Data acquisition and data interpretation are straight forward for people experienced with the collection, processing, and interpretation of geophysical data.  However, it is always much more efficient to get training before going in the field with a new technique and new instrument.

​

Q. What sort of vertical and horizontal resolution can I get, i.e. what size and type of targets can I resolve?

​

A. Horizontal resolution depends on the data density.  The instrument measures once per second, so at 3.6 km/hour (1 meter/sec) the data density is 1 reading/meter.  At 1.8 km/hour it is 1 reading/50cm, and at 7.2 km/hour it is 1 reading/2 meters.

 

Vertical resolution depends on the separation between the centers of the receiver dipoles.  At 2.5-m receiver-receiver separation vertical resolution is approximately 50 cm.  At 5 meter separation it is 1 meter.  Of course the ability to resolve a target depends on there being a resistivity contrast.  Resolving a 10 Ohm-meter target in a 100 Ohm-m background is easier than resolving a 90 Ohm-m target in a 100 Ohm-m background.  It is not possible to resolve a 100 Ohm-m target in a 100 Ohm-m background.

​

Q. How difficult is it to manually tow the OhmMapper, and can it be towed with an ATV or other vehicle?

​

A. It is easy for a healthy, fit person to tow the OhmMapper on flat dry ground or pavement.  It is more difficult on grass or other surfaces where more friction is present.  Towing the OhmMapper up hill is, of course, more difficult.  The OhmMapper can be towed with a vehicle.  However, a tow adaptor is required that will release the array in case a receiver cable is snagged on something in the tow path.   A connector can be broken if it is snagged and towed by a vehicle but the tow adaptor is not required when it is manually towed because a person cannot exert enough force to break the connector under normal operating conditions.

​

STRATAGEM EH-4

​

Q. What depth of investigation can be achieved with the Stratagem?

​

A. The Stratagem is a frequency domain instrument using the technique known as hybrid-source AMT (audio magnetotellurics).  It uses a combination of natural telluric currents in the ground and signals from a controlled-source transmitter.  The range of depth is determined by the frequencies available and by the apparent resistivity of the sounding site.  The lower the frequency is the greater the depth of investigation, and the higher the resistivity the greater the depth.   The frequency bandwidth of the Stratagem receiver is 10 Hz to approximately 70 kHz.  At 10 Hz in 10 Ohm-meter earth the depth of investigation is approximately 350 meters.  At 10 Hz in 100 Ohm-m earth the depth is approximately 1,100 meters. 

​

Q. How long does it take to set up and do a sounding?  In other words, how many soundings can I do in a day?

​

A. With an experienced crew and contiguous stations it will take 20 to 30 minutes per station to do a complete sounding.  In practical terms 15 or 16 stations per day are possible. 

​

Q. Do I need training?

​

A. The Stratagem is a hybrid-source audiomagnetotellurics instrument using a combination of both natural fields and a controlled-source transmitter.  This is a new technique for most people.  Although the setup, data acquisition, and processing are relatively straight forward it is recommended that before going to the field a basic training course should be done.  Prior knowledge before starting a survey will save time and avoid potential errors in the field.

​

Q. What software is available to process and interpret the data?  How much does it cost?

​

A. The basic software for data acquisition, editing, processing, and interpretation are bundled with the cost of the Stratagem system.  There is no additional cost of this software and updates are available on the Geometrics website.

​

Q. Can I use my own MT processing and interpretation software with Stratagem data?

​

A. The data file format and method of converting time series to spectral data are described in the manual.  Following the procedure should allow experienced users to read the Stratagem data into their own program.

​

Q. What vertical and horizontal resolution can I get?

​

A. Horizontal resolution depends on data density.  If a sounding is done every 50 meters (50-m station spacing) the near-surface, horizontal resolution will be 50 meters.  Closer and more widely spaced stations will result in higher and lower horizontal resolution.  Vertical resolution determines the ability to accurately position in depth the location of a detected target.  For any dispersive potential field method the positioning accuracy will be within 10% of the depth of the target.

​

Q. Can I detect the presence of water?

​

A. Geophysical techniques detect the response of a subsurface geology or object based on excitation of that body by a physical phenomenon such as a sound wave, electromagnetic wave, gravitational field.  These responses are not unique.  In the case of the Stratagem it is the response to an electromagnetic field that is used to create a resistivity map of the subsurface and that is correlated the true geologic structure.  However there is no one value associated with the presence of water.  The measured value in Ohm-meters is determined but the water’s ionic content (salt), saturation level, and the type of geologic structure containing the water.  The map is used to by the geophysicist or hydrogeologist to predict the most likely location of water based on knowledge of local conditions and general hydrogeologic principles. 

​

Q. Can I detect and identify the presence of specific minerals?

​

A. This is the same as the answer above.  Although the presence of mineral bearing structures can be interpreted from the geophysical map it is not possible to know exactly what minerals are present without a priori knowledge of the local mining geology.

 

Q. How  many people  does it take to run a survey?

​

A. Generally three people; one person at the transmitter and two to set up and move the sounding stations.  A survey can be done using only two people if needed but efficiency is lost.

 

Q. If it is an AMT system what is the transmitter for and can I run it without a transmitter?

​

A. The Stratagem transmitter is used as a supplement to natural field signals.  There is a natural field dead zone that can range from approximately 500 Hz to 4 kHz in which natural signals are weak.  The natural fields also tend to be weak above 20 kHz.  The Stratagem transmitter transmits a series of 15 frequencies from 800 Hz to 70 kHz to fill in the gaps in the natural field signal for a more complete sounding.  The survey can be done without the transmitter if natural fields are very strong at the time and place the survey is carried out.

 

Q. How does the Stratagem compare to TDEM?

​

A. You can find a comparison table here.

 

Q. What is the low-frequency option?

​

A. Deeper investigation can be done with low frequencies. The standard Stratagem has a lowest frequency of 10 Hz.  The low-frequency option goes two orders of magnitude lower at 0.1 Hz.  If the depth of investigation is beyond the standard Stratagem it can often be reached with the low-frequency option.  Acquisition requires a set of magnetic coils designed for the low-frequency band, a set of porous-pot electrodes with low-frequency buffers, and low-frequency sensor calibration files. 

 

Q. Do you have a list of consultants who can do the survey and interpret the data for me?

​

A. Yes, we can often refer you to a consultant operating in your area, or if you are from a country outside of the USA or local representative may be able to provide you with the name of a local geophysical consultant.  Please contact us at EMsales@geometric.com.

​

​

METALMAPPER 2X2

​

Q. What does the MetalMapper 2x2 do?

​

A. The MetalMapper 2x2 is a specialized instrument used for the characterization and discrimination of UXO (unexploded ordnance).The instrument uses an array of transmitter coils and receiver cubes to measure the EMI (electromagnetic induction) response of buried metal targets in the ground.Using software inversion algorithms, the data is used to discriminate targets of interest (TOI) from scrap metal.

 

Q. What types of UXO can be discriminated?

​

A. Most metal UXO larger than a 37mm projectile can be discriminated from scrap metal. It can be difficult or impossible to discriminate smaller munitions like 20mm or small arms rounds. The MetalMapper 2x2 cannot be used for anti-tank or anti-personnel landmines, or for other UXO that is non-metallic.

 

Q. What is the depth of investigation?

​

A. The MetalMapper 2x2 can usually detect all UXO targets of interest to depths of 60cm, and larger TOI to depths of 1m or more.

 

Q. What does a typical MetalMapper 2x2 survey involve?

 

A. The MetalMapper 2x2 measures in 2 distinct modes.  A dynamic survey involves collecting data over the site while the instrument moves in a series of parallel swaths.  The dynamic survey then identifies targets which need further characterization.  These targets are then measured during a static or cued survey, where the instrument makes measurements while stationary over the previously identified targets.

 

Q. How fast can the MetalMapper 2x2 survey?

 

A. In clear areas with a trained 2 man crew, the MetalMapper 2x2 can survey about ¾ of an acre per day in dynamic mode or collect 200 cued targets per day in static mode.

 

Q. Can the MetalMapper 2x2 be used as a general purpose metal detector?

 

A. Due to the specialized nature of the measurement and the instrument cost, it is not practical to be used as a metal detector for applications like prospecting or utility location.