Guralp Systems Limited
MAN-040-0001 Güralp 40T User's Guide

Chapter 2. Introduction

The Güralp 40T is a lightweight seismometer consisting of three sensors in a sealed, stainless steel case, which can measure the north/south, east/west and vertical components of ground motion simultaneously.

The 40T has a rugged, waterproof stainless steel design for ease of installation. The lightweight sensor elements are designed so that no mechanical clamping is required. Because of this, the 40T is ready to record ground movements as soon as you provide it with power. In addition, the sensor does not have to be levelled or centred as long as the base is within 3° of horizontal. For the best results, however, you should install where possible on a hard, near-horizontal surface well coupled to the bedrock.

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 Response options

The standard frequency response for a 40T is flat to velocity from 60 seconds (0.017 Hz) to 100 Hertz.

Standard alternative frequency response options are:

Other frequency response options have been manufactured in the past. Please contact us if you require a difference frequency response.

If you do not require high-frequency data, a low-pass filter may be installed at a frequency (below 50 Hz) that you specify.

Standard 40T instruments output signals representing ground velocity on three pairs of balanced differential lines. An option is available which provides a second, parallel set of outputs at higher gain. The high-gain outputs have a sensitivity nominally ten times higher than the standard (low-gain) outputs.

2.2 Remote nulling option

A small number of 40T instruments were produced with motor-driven potentiometers in the nulling circuitry. This meant that the instrument could be nulled remotely by applying voltages to the motors via pins in the main connector.

2.3 Digital nulling option

A small number of 40T instruments were produced with an automatic nulling process, known as "digital nulling". The nulling potentiometers in these instruments were solid-state units under microprocessor control. See section 3.4.4 for more information about this function.