Chapter 2. Introduction
The Güralp 3ESPCD is a full-featured three-axis digital seismometer consisting of three sensors in an ultra-lightweight case, which can measure the north/south, east/west and vertical components of ground motion simultaneously, and a built-in digitiser.
Note: The Güralp 3ESPCD is available with one of two alternative built-in digitisers: the Güralp DM24 and the Güralp CD24. The DM24 version offers greater functionality at the cost of increased power consumption. This document describes the DM24 versions of the instrument. For CD24 versions, please see MAN-C3E-0005.
Each sensor is sensitive to ground vibrations in the frequency range 0.003 Hz to 50 Hz, a broadband frequency response made possible by advanced force-balance feedback electronics. Because of this wide response range, the 3ESPCD 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. Its small size and low weight make it an ideal choice for rapid deployment of low-noise installations.
The 3ESPCD will produce the best results if it is mounted 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.
The seismometer unit is self-contained apart from its 12 V power supply. Once levelled and centred, it will begin operating automatically. Its output is digitised within the sensor by the internal DM24 digitiser, which provides data streams in GCF format. The instrument can be connected to a PC's serial port or to a Güralp Systems data module using the cable supplied. The digitiser also controls the instrument's centring and mass locking processes.
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.
2.1 Sensor response
The 3ESPCD 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) or 0.0083 Hz (120 s). Alternatively, a hybrid response function may be provided. See section 7.1.1 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.