Chapter 2. Introduction
The Güralp DM24 multi-channel digitiser is a state-of-the-art digital module for seismic data. It can be supplied in several different formats:
A stand-alone DM24 can be installed with existing or new analogue sensors.
Conversely, a modular digitiser is fixed to, and becomes a physical part of, a Güralp seismometer. These can be supplied with new instruments, or retrofitted to Güralp 1T, 3T, 3ESP or 40T seismometers at the factory. A Güralp 5TD is also available, as shown on the right. A modular digitiser benefits from the sensor's isolated, clean and stable environment and allows it to operate at optimum noise levels and precision.
DM24s are also supplied with integrated EAMs (Embedded Acquisition Units) in plastic-cased and metal-cased (aluminium or stainless steel) variants.
For these units, the main interface to the DM24 is via the EAM's web and command line. The manual MAN-EAM-0003, available from the GSL web site, is more appropriate for these units than is this document.
The DM24 can also be supplied in borehole sonde form, with optional strain relief accessories. These units operate identically to surface DM24s, although the connector for the borehole unit is necessarily more compact.
Regardless of your configuration, all three device types - sensor, digitiser, and data module - can be customized to meet your installation requirements.
A built-in Digital Signal Processor (DSP), which can provide simultaneous, multiple sample-rate data-streams at user-selectable rates. Multiplexor streams of data for each component are available at sample rates from 1 to 1000 samples per second.
A precision microprocessor-controlled time-base synchronizes the Analogue-to-Digital Converters (ADCs) and the DSP, while also providing time-stamps for data blocks.
Time synchronization to an attached GPS receiver.
An ARM microprocessor, which formats and buffers data in an on-board 256 kByte RAM ring-buffer.
Efficient data storage and transmission using the Güralp Compressed Format (GCF).
RS232-compatible serial data output at user-selectable baud rates, with automatic modem dial-out facility.
Support for configuration and sensor control, including mass-locking and unlocking, centring and calibration.
Low system power consumption.
Flash EEPROM for program code and filter coefficients.
High-impedance, low-noise, differential-input pre-amplifier modules with (optionally) adjustable gain.
Internal electronic thermometer provides temperature measurements at an accuracy up to ± 0.33 °C, with a linearity of ± 0.5 °C.
Optional Flash memory storage.
Optional IEEE 1394 FireWire interface, allowing you to download GCF data directly onto a compatible disk by simply plugging the disk into the FireWire connector inside the casing, and monitoring progress on the optional built-in LCD screen.
Optional USB interface, enabling fast data transfer direct to a PC or other recording device.
The DM24 can be connected to two Güralp triaxial analogue sensors, or up to six conventional single-axis seismometers, through its SENSOR A and SENSOR B connectors. (Single-sensor and integrated DM24 units have only SENSOR A connectors). Signals from these sensors are digitized to 24-bit precision, initially at 2 kHz. Data are output through the DATA OUT serial port of the digitiser in the flexible GCF format, either to a PC or a Güralp data module, such as a Güralp EAM.