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StrataGem Electromagnetic Imaging System

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 magneto-tellurics 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 iis 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.

Features:
  • 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.

Configurations:

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

Specifications:

IMAGING  SYSTEM:

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 Process:

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

OPTIONS:

Seismograph channels for  integrated seismic measurements

Available with 12, 24, 36,  or 48 channels.

Magnetic Sensors:

Low-frequency 0.1 Hz to 1  kHz Sensors
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