Guralp Systems CMG-1T User Manual

CMG-1T

OCEAN BOTTOM

SEISMOMETER

OPERATOR’S GUIDE

REVISIONS

ISSUE ECN DATE COMMENTS

A First 15.05.96
B 24.01.97 Add drawings and photos.
C 14.09.99 Addition of Appendix A

DESIGNED AND MANUFACTURED BY:

GÜRALP SYSTEMS LIMITED

3 MIDAS HOUSE
CALLEVA PARK
ALDERMASTON

READING

BERKS, RG7 8EA

ENGLAND

Telephone: [phone] Fax: [phone]

Operator’s Guide CMG-1T Ocean Bottom
Seismometer

DESIGNED AND MANUFACTURED BY:

GÜRALP SYSTEMS LIMITED

3 MIDAS HOUSE
CALLEVA PARK
ALDERMASTON

READING

BERKS, RG7 8EA

ENGLAND

PROPRIETARY NOTICE

The information in this Manual is proprietary to Güralp Systems Limited and
may not be copied or distributed outside the approved recipient’s organisation
without the approval of Güralp Systems Limited.

Güralp Systems Limited shall not be liable for technical or editorial errors or
omissions made herein; nor for incidental or consequential damages resulting
from the furnishing, performance, or use of this material.

September, 1999, Issue C

Operator’s Guide CMG-1T Ocean Bottom
Seismometer

HOW THIS USER’S GUIDE IS ORGANISED

This user’s guide is sectionalised with each section dealing with a specific topic.

Generally speaking, background material and technical explanations are found in
the later sections, while practical instruction occurs at the beginning. A list of
tables and specifications are found at the end of the manual.

Each section of the user’s guide is kept, as nearly as possible, self-contained and
free-standing so that the sections can be read in any order. General cross­references are provided where necessary, but complicated notation of the
sections and paragraphs is avoided.

A very brief description of the user guides sections are given below but the
contents page provides the titles of each section.

INTRODUCTION: This section summarises the CMG-1T sensor, the

levelling system and the micro-controller which controls
the sensors and the levelling system.

QUICK START: This section gives quick itemised procedures for

unpacking, installing and operating the CMG-1T. The

user can use this section to quickly deploy and
operate the instrument.

OPERATION: These sections make up the instructions to operate the

CMG-1T sensor, with detailed description of the sensor
and operation.

September, 1999, Issue C

Operator’s Guide CMG-1T Ocean Bottom
Seismometer

CONTENTS

SECTION

INTRODUCTION 1 Introduction

2 Description and Explanation of the
Mechanical Sensor System

3 Feedback System Description

4 Force Transducer

5 Levelling Platform

6 Levelling Platform Inclinometers

7 System Input/Output Signal Format
and Connections

8 Seismometer and Levelling Bowl
Control Commands

9 Mounting of the Levelling System

September, 1999, Issue C

Operator’s Guide CMG-1T Ocean Bottom
Seismometer

1. INTRODUCTION

The qualities of Güralp Systems Limited’s broadband sensors (Ref 1) clearly
opened up a new era in ocean bottom and ocean bottom borehole seismology.
A pilot experiment which deployed CMG-3 sensors (Refs 2 and 3) clearly
indicated some of the advantages that can be ascertained from broadband
sensors installed under water. The only disadvantage of CMG-3 as an ocean
bottom instrument is the sensor mass locking mechanism. While the CMG-3
locking mechanism is adequate for most , if not all, conventional installations,
due to increased reliability requirements, a new foolproof patented locking
mechanism has been designed. The CMG-1T sensor uses this unique mass
locking mechanism which virtually eliminates the possibility of sensor pivots or
springs being damaged.

The CMG-1T OBS seismometer consists of three solid body CMG-1
seismometer components. The three component sensors are organised to be
orthogonal to each other. The base plate of the sensors is fixed to the bottom
plate of the levelling dish as shown in the following diagram. The analogue
electronics for the sensors are stacked above the sensors. The OBS analogue
electronics are specially designed to have low power consumption, and the
quiescent current consumption of the sensor system is 26 mAmps from a 12
Volts supply.

September, 1999, Issue C Section 1, Page 1

CMG-1T Ocean Bottom Seismometer Operator’s
Guide

The levelling dish structure has ± 30 degrees of levelling capability, with an
accuracy of ± 0.2 degrees, and stability which is fit to be used as a levelling
platform for a broadband sensor.

Section 1, Page 2 September 1999,
Issue C

Operator’s Guide CMG-1T Ocean Bottom
Seismometer

Individually each sensors outputs can be further zeroes within a range of ± 2.5
degrees. The horizontal sensors are levelled (or mass position output zeroed)
by tilting the sensor bases, in the case of the vertical sensor the sensor boom
position is controlled by the movement of the tip of the main load bearing
spring.

The sensor command functions are controlled with a single chip micro­controller, type H8 (Hitachi). The sensor function lock, unlock, centre and
levelling bowl functions are all initiated with command words instructed
through the system serial communication port.

A two axis inclinometer is used to measure the tilt of the sensor levelling bowl
in the North/South and East/West directions. The inclinometer is interfaced to
the micro-controller which ensures levelling of the sensor levelling bowl with a
single English command “LEVEL”.

A two axis inclinometer is used to measure the tilt of the sensor base in the
North/South and East/West directions.

The commands to operate the seismometer control functions, the digitizers and
the data from the digitizers is communicated with a serial communication line.

In the case where an internal digitizer is used, the sensor outputs are digitized
with a 24-bit digitizer and the data packets are time stamped at the source. A
complete block diagram of the ocean bottom sensor is given below.

September, 1999, Issue C Section 1, Page 3