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 CD24 digitiser.

Each sensor is sensitive to ground vibrations in the frequency range 0.017 Hz (60 seconds) 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, tightly coupled to the underlying bedrock. After levelling and orienting the case, you can perform accurate adjustments internally by sending the instrument control signals. This mechanism allows it to compensate for a tilt of up to 4° from horizontal.

The seismometer unit is self-contained apart from its DC power supply and an external GPS receiver, which serves as a precision timing source. Once levelled, powered and centred, the instrument will begin operating automatically. Its output is digitised within the sensor by the internal CD24 digitiser, which provides data streams in GCF format.

Data can be streamed over a serial link to a PC or to a Güralp EAM data module using the cable supplied. Optional interfaces allow data to be streamed over wired Ethernet or Wi-Fi. Additionally, data can be stored in internal flash memory for later retrieval to a Güralp FireWire disk, available separately. The maximum of 32 GB of storage will retain well over eighteen months of 100 sample-per-second triaxial data.

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 Options

2.1.1 Sensor response

The Güralp 3ESPCD has a standard frequency response which is flat to velocity across a frequency range from 60 seconds to 100 Hz. A version which extends this response down to 120 seconds is also available.

2.1.2 Internal storage

The 3ESPCD can be supplied with up to 32 Gb of internal Flash memory for data storage. The amount you need will depend on the length of your experiment and the sampling rates used. Allow at least 50 MB per day for triaxial, 100 sample-per-second data. This figure scales linearly with sample rate so, for example, allow 100 MB per day for 200 sample-per-second data.

Data from the internal storage are downloaded using a fast IEEE.1394 (FireWire) link to a Güralp FireWire disk, which can then be read to a PC using Güralp's free GCFXtract software.

It is also possible to download small amounts of data over the sensor's standard RS232 data port or, if fitted, over a local area network using the Ethernet or Wi-Fi. interface - but see the following note:

2.1.3 Networking

The 3ESPCD can be fitted with an optional Ethernet port. This allows data to be streamed into an existing network infrastructure either directly or, for example, via a 3G modem.

In addition, your instrument can be fitted with an optional IEEE 802.11b Wi‑Fi wireless interface as well as the Ethernet port. These options allow data flow to be established from autonomous installations with a minimum of setting up.

A full description of the networking options, together with instructions for configuration and use can be found in document MAN‑NET‑1002, available from our website.