The Stratagem EH4 provides high-resolution two- dimensional images of geological structures by detecting and mapping variations in subsurface conductivity/resistivity
The Stratagem EH4 uses the magnetotelluric (MT) method to measure subsurface conductivity. The magnetotelluric method is based on the fact that the ratio of the magnetic to electric fields (known as the impedance) at a given frequency is constant for a constant resistivity. Natural signal sources, such as lightning activity, can be measured to determine this ratio. Unfortunately, natural signals are sometimes not available at the time, frequency, and amplitudes needed. Stratagem's hybrid-source technique helps overcome this problem. Hybrid source means we use a combination of natural MT signals and man-made transmitter signals. Any available natural background signals are used in the entire frequency band while the Stratagem transmitter is used to provide additional high-frequency signals in the range of 1k Hz to 70k Hz where natural signals are weak. The standard Stratagem can be operated using frequencies from 10 Hz to 100k Hz. The low-frequency option can use signals as low as 0.1 Hz for a greater depth of investigation.
Typical Field Setup
The Stratagem EH4 transmitter consists of a dual-loop antenna, transmitter electronics, and controller. The transmitter provides unpolarized source fields which allow for true tensor measurements of ground resistivities. This provides more accurate interpretation of true resistivities than conventional single-dipole transmitter source signals. The transmitter is powered by a 12 VDC battery.
The illustration below shows a typical configuration of the Stratagem receiver. The magnetic fields are detected with two perpendicular H-field sensors. The electric fields are detected by measuring the differential voltage between the two electrodes of the electric dipoles (between Ex0 and Ex1 for the X-direction E-field, for example). The response of the field sensors is amplified and filtered by the Analog Front End and transmitted to the console for analog-to-digital conversion and digital signal processing.
Fast Data Collection and Instrument Set Up
The operator can select the frequency bands and the number of time series "stacks" for data collection allowing optimization of high/low frequency data collection. Typical data collection time per station is 5 to 10 minutes. The Stratagem can then be moved and set up in from 5 to 10 minutes per station. This means complete setup and data acquisition can be done in 40 to 20 minutes giving from 3 to 6 stations per hour The MT technique means that each station is a complete sounding. In other words, you can do 3 to 6 complete soundings per hour.
You can explore from the near surface to depths as great as 1 km
The actual depth to which a target can be imaged depends on the resistivity/conductivity of the earth at the measurement site and the lowest frequency for which there are reliable data. Depths up to 500 m can be expected with standard sensors (10 Hz to 100k Hz), and up to 1 km with optional low-frequency sensors (down to 0.4 Hz). This is based on the skin depth formula.
You can archive and store complete data sets
The Stratagem EH4 maintains files of the complete time series data, cross-power and spectral amplitudes, as well as full tensor and scalar values of resistivity, phase, and coherency. Inverted depth and resistivity data are saved and can be exported to third-party software. These data files can be used in the built-in Stratagem software and with third-party software tools.
1D and 2D Field Plots
Stratagem lets you view 1D soundings and 2D sections in the field to provide better field quality control and immediate access to resistivity results.
Built-in Thermal Printer
In addition to the LCD video display you can use the built-in thermal printer for field generation of hardcopy for time-series data; signal amplitude; phase; coherency; apparent resistivity; depth curves; and depth and frequency cross-sections.
Combined EM/Seismic in Stratagem/StrataView Combination:
Ask about Geometrics' combined Stratagem/StrataView for maximum economy in geophysical instruments. With the addition of seismic data acquisition and digital signal processing boards, the appropriate seismic connectors, geophones, software, and other standard accessories, the instrument can be operated as a seismograph.
You can have confidence in the instrument
The Stratagem EH4 was developed jointly by ElectroMagnetic Instruments, Inc. and Geometrics, Inc. Since its founding in 1969 Geometrics, lnc. has been a recognized leader in developing reliable, innovative geophysical instruments. Founded in 1984, EMI has been a driving force in advancing magnetotellurics technology and instrumentation.