Instrumentation and Metrology in Oceanography E-Book

Marc Le Menn

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 657

Veröffentlichungsjahr: 2012

Beschreibung

Through research, physical oceanography aims to solve the numerous problems stated by thermal, optical and dynamical properties of the oceans. Instrumentation and Metrology in Physical Oceanography describes the means used in oceanography to determine physical properties of the oceans by medium of in situ measurements. This book explores the theoretical functioning of sensors and instruments, as well as different practical aspects of using these tools. The content of this book appeals directly to technicians or engineers wishing to enhance their knowledge of instrumentation and application to environment surveillance. Instrumentation and Metrology in Physical Oceanography details the functioning of sensors and instruments used to assess the following parameters in oceanography: temperature, conductivity, pressure, sound velocity, current in magnitude and direction, time and position with GPS, height of water and tide, waves, optical and chemical properties (turbidity), dissolved gas (O2, CO2), pH, nutrients and other dissolved elements. Furthermore, this book also elaborates on the different means used to obtain measurements at sea (boats, drifting floats, moorings, undersea platforms, gliders...) and techniques currently being developed.

Leseprobe

Table of Contents

Preface

Chapter 1. What We Measure and What We Process

1.1. The quantities we want to know

1.2. Linking of essential quantities in oceanography

1.3. Calculation of density

1.4. Bibliography

Chapter 2. Measurement Systems in Practice

2.1. Determining temperature

2.2. Determining conductivity

2.3. Determining pressure

2.4. Determining velocity

2.5. Determining current

2.6. Determining time or measuring frequency

2.7. Determining position and movement

2.8. Determining the height of water

2.9. Determining waves and swell characteristics

2.10. Determining the turbidity or sea water’s optical properties

2.11. Determining various physicochemical properties

2.12. Bibliography and further reading

Chapter 3. Measurements at Sea

3.1. Oceanographic vessels

3.2. Moorings

3.3. Drifters

3.4. Instrumented buoys and underwater platforms

3.5. Bibliography

Chapter 4. Evolutions and other Measurement Concepts

4.1. Other processes for measuring salinity and density

4.2. Acoustic tomography of oceans and acoustic measurements

4.3. The unmanned underwater vehicle: a new means for ocean exploration

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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John Wiley & Sons, Inc. 111 River Street Hoboken, NJ 07030 USA

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© ISTE Ltd 2012

The rights of Marc Le Menn to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Le Menn, Marc. Instrumentation and metrology in oceanography / Marc Le Menn. p. cm. Includes bibliographical references and index.     ISBN 978-1-84821-379-1   1. Oceanographic instruments. 2. Oceanography--Measurement. I. Title.     GC41.L44 2012     551.46028′4--dc23

2012025490

British Library Cataloguing-in-Publication Data

Preface

The ocean is generally defined as “a vast stretch of salt water that covers the greatest part of the globe”. However, the simplicity of this definition must not hide the complexity of this environment, which oceanography attempts to decrypt. More than just a stretch of salty water, it is a place with living rules, where study over takes the biology. Within the science of oceanography, there are fields of study focusing on geology and chemistry, but it is the problems posed by thermal, optical and dynamic properties that physical oceanographers seek to answer.

Numerous models and theories attempt to describe and predict these properties. To initiate and reinforce these, some measurements are necessary. The complexity of this environment and its hostility affect another field: instrumentation. In order to examine the complexity of this environment, specific technological developments are necessary. These developments fall under the field of oceanographic instrumentation.

This work mainly aims to describe the oceanographic instrumentation used to determine the physical properties of the ocean through in situ measurements. With the development of space satellite technology, new systems have appeared that have enabled us to discover these properties, creating a new field (that we will not discuss in this book): space oceanography. Space is a favored place from which we can observe the ocean, as it permits large-scale study.

The instruments loaded on satellites use the properties of electromagnetic waves to measure the thermal, optical and dynamic characteristics of the ocean. Unfortunately, these waves are very rapidly absorbed or reflected by the oceanic environment, so if ocean-predicting models use “spatial” data, the collection of in situ observations remains an essential source of diagnostics to validate the models, and it is always necessary to develop complementary equipment that permits the study of deep water layers.

There are multiple fields of application for these studies. They concern the evolution of fundamental knowledge on the movement of water masses, the creation of thermal or density anomalies, and the coupling between the ocean and the atmosphere that lead to a better understanding of climate change. These also include the development of acoustic technologies that have multiple civil and military applications. The ocean is a favored environment for these technologies; however the propagation of acoustic waves is dependent on its physical characteristics, so the use of tools that permit emission and reception (sonar, echo sounders, Doppler current meters, etc.) must be optimized by measuring the properties of the environment if we want to achieve ultimate accuracies.

These accuracies cannot be achieved without the use of metrology. It is unusual to associate oceanography with metrology, but this is the other field that this book attempts to describe. Metrology is officially “the science of measurement and its applications”, but it is, above all, the science that allows the referencing of measured data, and referencing is an essential part of oceanography when we want to ensure the accuracy and replicability of measurements. Oceanography is probably the area of physics where the requirements of the subject matter are greatest because the acquisition and interpretations of variables are often at the limits of our know-how.

This book, first and foremost therefore, is for instrumentalists and metrologists who want to know more about measurements in oceanography. It is also for scientists who want to gather information on oceanographic instrumentation, and for all those for which this is an area of interest.

All of the chapters of this book are enriched with practical and theoretical details. Sections include recent relationships for the calculation of the physical properties of sea water, the measurement of certain physicochemical properties (carbonates, fluorimetry, etc.), suspended particulate matter, ways of positioning and probing in water, instrumented buoys, underwater platforms, etc. We also discuss the processes of calibration of these instruments, without which these measurements would not have all the legitimacy we attach to them.

Marc Le MennSeptember 2012

Chapter 1What We Measure andWhat We Process

1.1. The quantities we want to know

Measurements made in physical oceanography have two main aims: to improve our fundamental knowledge of the ocean and the functioning of our planet; and to optimize the use of acoustic tools, which also sometimes help us to gain knowledge of the ocean.

The improvement of fundamental knowledge is integrated with the more general topic of climate change. A global research program was launched in 1979 to try and model this evolution: the World Climate Research Program (WCRP). The WCRP is funded by the World Meteorological Organization (WMO) and UNESCO. To maximize the efforts of different countries with regard to oceanographic measurements, in 1982 the American National Science Foundation launched another program called the World Ocean Circulation Experiment (WOCE). In 1989 it created the WOCE Hydro-graphic Program Office, whose aim was to coordinate, supervise and ensure the quality of measurements taken. To ensure the quality of data collected, it was requested that “the standards to approach in terms of accuracy and reproducibility for the one-time survey, are to be the highest possible under current measuring techniques”. With “repeat surveys, these standards must be approached sufficiently closely to achieve the appropriate regional goals”.

There are several applications of this. The data collected will serve to determine the long-term evolution of ocean circulation. To be able to detect small changes over time, it is necessary to make measurements with low resolution and high reproducibility. The data must be comparable from one country to another and in a common format. Therefore, the measurements should be performed with great accuracy and be comparable to common references. Finally, it should be noted that at great depths the thermal stability of water masses is huge, so to detect small changes over time and space and in order that centers or institutions other than the one that carried out the measurements can verify such changes, the measurements must be made with high resolution, high reproducibility and high accuracy.

In 2003, the WOCE program was relayed by an intergovernmental initiative called the Global Earth Observation System of Systems, which aimed to provide the means to overcome the lack of observations concerning key factors that influence the Earth’s climate. In Europe, another initiative pursuing the same objectives also emerged called the Global Monitoring for Environment and Security (or GMES).

When using acoustic instruments, the specifications are less stringent. Oceanographic measurements performed using these specifications aim to determine the propagation speed or velocity of sound in water either locally or along an acoustic path. It is the temporal and spatial variability of physical characteristics of the environment and the lack of accuracy of velocity references that limit the accuracy of measurements, and that reduced the requirements on the measurement of influence quantities in comparison to those displayed by the WOCE program.

1.1.1. Velocity and density

Whether the measurements are performed to gain environmental knowledge or aim to use acoustic instruments, they have a common point: the measured quantities are the same. Indeed, oceanic circulation largely depends on the densityρ of water masses (the ratio between the mass of a sample and the volume it occupies). Similarly, velocity c is dependent on density. In adiabatic conditions, it is defined by,

[1.1]

where p corresponds to the pressure exerted by the acoustic wave, which results in rapid oscillations in density throughout the medium.

It is well-known that the density of a fluid depends on its temperature, pressure and composition. With variation in density, the velocity will also depend on these parameters. Temperature and pressure are therefore the primary variables to be measured.

The composition of sea water varies locally depending on the quantities of dissolved salts. These amounts change the salinity. Fochhammer introduced this concept for the first time in 1865. Salinity is a chemical parameter of sea water, and a characteristic of water masses, but its definition has continued to evolve since the creation of the concept, as it is a difficult parameter to measure in absolute terms.

The first accurate definition of the salinity of sea water dates from 1902 and was produced by Forch, Knudsen and Sörensen. It follows a protocol of measurement that they had developed showing that:

“salinity is the quantity of solid material in grams contained in a kilogram of sea water, having converted the carbonates to oxides, bromide and iodide ions having been replaced by their equivalents in chloride, the organic materials having been oxidized.”

This is the definition of absolute salinity, Sa, which is expressed in g.kg−1. From this definition, we see that it is not only dissolved salts that are measured but all dissolved matter, so the definition of absolute salinity, carried on to present day, is as follows:

Due to the difficulty of implementation, to date this definition has not been applied in routine measurements. To circumvent these difficulties, a Practical Salinity Scale was defined in 1978 (see section 1.2.3). This scale, which is called PSS-78, calculates a practical salinity S from the simultaneous measurement of temperature, pressure and electrical conductivity in a sample of sea water. The third common variable to be measured is therefore electrical conductivity. It is used to calculate the value of salinity that occurs in density and velocity calculations.

The PSS-78 is based on conductivity measurements — and thus on the speed at which ions move — and does not take into account all dissolved materials, hence biases exist between practical salinity Sp, which we can measure, and absolute salinity. These biases affect the calculation of density. To illustrate this point, let us take the simple example (cited in the TEOS-10 guide), where a small amount of pure water is exchanged with an equivalent mass of silicate compound that is mainly non-ionic. If this exchange takes place at constant temperature and pressure, the value of conductivity will also remain constant, while the absolute salinity and density will have increased. Conversely, if we replace a mass of silicates with the same mass of salt (NaCl), the absolute salinity and density will in principle remain unchanged, while the conductivity and therefore practical salinity will increase.