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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:

1. 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, but it is still a great time savings over using metal stakes, which would take many hours.

2. 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.

3. 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.

4. 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?

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. How much does it cost?

A. There are many different configurations for the OhmMapper. Please contact Geometrics or your local Geometrics representative for a quotation.

Q. What are the purposes 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:

1. 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.

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

3. 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.

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