The CMG-3T is a three-axis seismometer consisting of three sensors in a sealed case, which can measure the north/south, east/west and vertical components of ground motion simultaneously. Each sensor is sensitive to ground vibrations in the frequency range 0.003 – 50 Hz, a broadband frequency response made possible by advanced force-balance feedback electronics. Because of this wide response range, the 3T can replace many of the instruments conventionally used in a seismic observatory; it also produces true pulse-shape records suitable for modern earthquake mechanism analysis.
The 3T is designed for mounting on a hard, near-horizontal surface well coupled to the bedrock. After levelling and orienting the case, you can perform accurate adjustments internally by sending the instrument control signals. These electronics allow it to compensate for a tilt of up to 3 ° from horizontal.
Once levelled and centred, the 3T will begin operating automatically. It outputs analogue voltages representing ground velocity on balanced differential lines. These voltages can be recorded using a separate logging device or digitizer. For testing and installation purposes, a handheld control unit is supplied which can monitor the instrument's output.
The seismometer unit is self-contained apart from its 12 V power supply. Centring and mass locking can be carried out by sending control signals to the instrument, either through the handheld control unit or through an attached Güralp digitizer.
Each seismometer is delivered with a detailed calibration sheet showing its serial number, measured frequency response in both the long period and the short period sections of the seismic spectrum, sensor DC calibration levels, and the transfer function in poles/zeros notation.
The CMG-3T can be supplied in several forms, besides its standard configuration:
The CMG-3T Compact is internally identical to the standard 3T, but has a different arrangement of components allowing it to fit in a smaller casing.
The 3T can also be supplied in a slimline form factor, with vertically-stacked sensors, suitable for installation in postholes (see Section 2.5, page 19.).
The 3T is also available as a fully-fledged borehole instrument, the CMG-3TB.
Any of these can be supplied with integral digitizers and data modules, allowing the 3T to form a complete, integrated seismic installation.
The standard 3T has a single 26-pin mil-spec waterproof connector for signals, control and power:
As an option, the 3T can be supplied with a 26-way “D”-type connector with standard pin assignments (see section 6.2, page 48.)
Other output connector types may also be supplied, to your own pinout specification.
Sensor control interface
3T instruments can be fitted with an optional RS232 control interface. The control interface can be used to lock, unlock, and centre the sensor masses, and query mass positions. See section 5.3, page 41, for more details.
The 3T can be supplied with a response which is flat to velocity from 100 Hz to any of 0.1 Hz (10 s), 0.033 Hz (30 s), 0.016 Hz (60 s), 0.01 Hz (100 s), 0.0083 Hz (120 s) or 0.0027 Hz (360 s). Alternatively, a hybrid response function may be provided. See Chapter 5 for more details.
If you do not require high-frequency data, a low-pass filter may be installed at a frequency (below 100 Hz) that you specify.