Chapter 2. Equipment Overview
2.1 Introduction
The CMG-6TC is an ultra-lightweight seismometer consisting of three sensors in a sealed case, which can measure simultaneously the North/South, East/West and vertical components of ground motion over a wide frequency range. This frequency response is made possible by advanced force-balance feedback electronics. It is ideally suited for rapid, temporary installations in locations with medium noise.
The robust sensor elements are designed so that no mechanical clamping is required. Because of this, the 6TC is ready to record ground movements as soon as you provide it with power and the processor has booted-up. The instrument will operate with up to 20° of tilt (if buried; up to 10° if free-standing) but better results will be obtained if the base is within 3° of the horizontal and installed on a hard surface, well coupled to the bedrock.
Each instrument is delivered with a detailed calibration sheet showing its serial number, measured frequency response (in both the long period and short period sections of the seismic spectrum), sensor output calibration figures and the instrument's transfer function, specified in “poles and zeroes” notation.
2.2 Features
Compact size: 89mm x 160mm
True broadband, three-component, force-feedback instrument
Direct velocity outputs
Lightweight (1.2kg) and waterproof to 1 metre submersion
No mass clamping required – plug in and go
High sensitivity and dynamic range (>135 dB)
Orthogonal instrument with high cross-axis rejection (>65 dB)
The sensor response is completely flat across the entire passband. Its high-gain feedback loop eliminates mechanical non-linearity (the overall measured linearity exceeds 95 dB) and minimizes resonances in the spring system.
Low-frequency vibration modes are carefully avoided in the design. The lowest spurious vibration mode of the 6TC is a barely measurable resonance at 440Hz.
Refer to Appendix B on page 14 for full specifications.
2.3 Build options
The CMG-6TC is supplied in two standard build versions:
Selectable 1, 10, 30 and 60 second low frequency corner, flat to 100Hz
Selectable 1, 30, 60, 120 second low frequency corner, flat to 100Hz
A 50Hz option is available on request.
The responses for each build version are user-configurable. See section 4 for instructions for configuring the responses.
2.4 Ports and Connections
Note: Refer to Appendix A on page 13 for information on connector pin-outs.
2.4.1 Ports
A single 26-pin connector provides power to the instrument and supplies analogue data from the sensors. A TTL-level serial interface is also exposed on the same connector.
The supplied cable has a 26-pin instrument connector at one end and either a 26 way connector or free pigtails at the other (depending on customer requirements).
2.4.2 Breakout box
An optional breakout box is available and comes with all necessary cables to connect the instrument to a power supply, digitiser and hand-held control unit, as required.