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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. How much does it cost?

A. There are several different options in hardware, spares, and training for the Stratagem. Please contact Geometrics or your local Geometrics representative for a quotation.

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. Please see /files/mtvstem.pdf.

Q. What is the low-frequency option for and how much does it cost?

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.

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