The STRATAGEM EH4 is the result of a joint R&D/Marketing agreement between Geometrics, Inc. and Electromagnetic Instruments, Inc. (EMI). The STRATAGEM provides high resolution electrical conductivity imaging of the subsurface for depths between 10 meters and 500 meters. Conductivity information is calculated from measurements of surface electric and magnetic fields along a series of profiles.
The STRATAGEM obtains depth information at one site setup by measuring signals over a wide frequency range. Deep structures (more than 100 meters) are imaged using source fields provided by background magnetotellurics signals or conventional CSAMT sources. For shallow features, a new low power portable transmitter is used to supplement the background field signals.
The STRATAGEM system records orthogonal electric and magnetic fields which are processed to provide tensor impedance measurements for interpreting complex 2-D structures. The recording and display units is compatible with the Geometrics' StrataView multi-channel seismic system. If the user wishes to be able to record multi-channel seismic data and electrical conductivity imaging data at the same field location, combined STRATAGEM/StrataView system are available in one compact unit.
- 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 for data collection. Typical data collection times of 10 minutes per station and instrument set up time of 10 minutes per station.
- One-dimensional and Two-dimensional Field Plots: Stratagem allows viewing of 1D soundings and 2D sections in the field.
- Built-in Thermal Printer: Built in thermal printer can print hard copy of time series, signal amplitude, signal phase, coherency, apparent resistivity, resistivity curves, and depth and frequency sections.
- Exploration as Shallow as 10 Meters and as Deep as 1 km: Actual depth of investigation will vary with the ground resistivity, signal quality, and whether low-frequency options are employed.
- Complete Data Sets: Stratagem data file store complete time series, cross-power data, spectral amplitudes, scalar resistivity, tensor resistivity, phase, and coherency.
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 V DC battery.
The illustration 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 dipole. For example, the X-direction E field is measured between Ex0 and Ex1 as shown in the diagram. The response of the E-field sensors is amplified and filtered by the Analog Front End and transmitted to the Stratagem console for analog-to-digital conversion and digital signal processing..
The standard configuration provides for acquiring data in the range of 10 Hz to 92 kHz. The low-frequency configuration will allows the use of frequencies to as low as 0.1 Hz and thus greater depth of investigation.
The standard configuration uses a small, portable controlled-source antenna with a magnetic moment of approximately 400 A-m2. For work in highly resistive areas there is an optional large-loop antenna available of approximately 6,000 A-m2
- Operating Principle: Natural and Controlled Source Tensor MT
- Frequency Range: 10 Hz to 92 kHz
- Transmitter: Model TxIM2 with Vertical Loop Antenna with Frequency Range of 1 kHz to 70 kHz. Antenna Moment of 400 Amp-m2 from Two Perpendicular Vertical Loop Antennae Each 4 m3. Operates on 12 V DC.
- Electrical Sensors: Four Model BE-26 Buffered Active High-Frequency Dipole 26-Meter Cable with Four SSE Stainless-Steel Electrodes
- Magnetic Sensors: Two Model BF-IM Magnetic Field Sensors (10 Hz to 100 kHz) with 10-Meter Cables
- Analog Front End: One Model AFE-EH4 Unit for Analog Signal Conditioning. Couples 2 Electric and 2 Magnetic Channels to the Data Acquisition Package.
Data Acquisition Package
- Channels: Four (2E, 2H)
- Hard Disk: 1.2 Gbyte or greater
- Analog-to-Digital: 18 bit
- Digital Signal Proces: 32-bits floating point with bandwidth of DC to 96 kHz
- Display: Liquid Crystal VGA
- Plotter: Built-in 4" (11 cm) wide plotter
- Power: 12 V DC, 40 Ah
- Operating: 0 C to +50 C in rugged portable/waterproof case
- Magnetic Sensors:
Low-frequency 0.1 Hz to 1 kHz Sensors
Part 1: Receiver
Part 2: Transmitter