- Email: [email protected]
Optical methods in Earth Sciences
Optics and Lasers in Engineering 37 (2002) 87–89
Editorial
Optical methods in Earth Sciences Mankind progress was started by systematic observations of Earth and celestial rhythms. As soon as the technology of the first modern optical instruments became available, they were applied to astronomical and geodetical measurements. Modern science itself originates from the Copernican revolution based on new observations and interpretation of Earth and celestial bodies motions. The revolutionary telescope of Galileo Galilei represented the first ‘scientific’ instrument in the modern sense of the word, aimed to test a previous theory and eventually to put the foundations for a new one. Galileo’s optical instruments put the basis of geography and cartography, solving problems like the determination of longitude and the detection of the cause of gravity variations with latitude. The advent of the laser started a new era in optical science, allowing the development of new methodologies and instruments based on coherent radiation. Despite the strong link between optics and Earth and planetary sciences of past times, laser applications in modern Earth Sciences have been rather limited. They have been mainly restricted to the field of geodesy, with laser telemetry for electronic distance measurements (EDM), some laser strain-meters, and interferometric absolute gravimeters. Some sporadic applications of optical techniques have also involved seismic data processing and filtering, now completely overcome by computer analysis. In very recent times, the use of InSAR interferometry for differential geodetic measurements has become widespread, and laser interferometry started to be commonly used. The increasing demand for telecommunication technologies, in the last few decades, has been sustained by the tremendous progress of optoelectronics, fibre optics and integrated optics. New perspectives of application in many different fields have been thus opened. The wide availability of low-cost, compact optical devices, allows to design new instruments for environmental applications. In this context, a crucial role has been played by diode lasers with output wavelengths from 760 to 2000 nm. These lasers have generally a distributed feedback (DFB) design enabling for single mode emissions with output power of milliwatts. They are currently used as sources in laser-based spectrometers to monitor trace-gas concentrations. Their characteristics of room-temperature operation and compatibility with telecommunication-grade optical components are being exploited to develop portable instrumentation for several applications, including environmental monitoring, factory-process control, bio-medical diagnostics. However, nearinfrared diode lasers probe overtone or combination vibrational bands of simple 0143-8166/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 4 4 - 0
88
Editorial / Optics and Lasers in Engineering 37 (2002) 87–89
molecules, which are orders of magnitude weaker than fundamental transitions occurring in the mid-infrared. Laser absorption spectroscopy would gain in sensitivity, precision and accuracy if the laser wavelength moves to the mid-infrared. This is one of the main reasons of interest towards the generation of coherent and tunable radiation with longer wavelength. In recent years, difference frequency generation in periodically poled crystals has emerged as a convenient technique to produce laser radiation between 3 and 5 mm. At the same time, a great innovation in photonics dealt with a different way of producing coherent radiation in semiconductor heterostructures that led to the invention of quantum-cascade lasers. Despite their CW operation at cryogenic temperatures, these lasers might open interesting perspectives in sensing applications because of their excellent performances in the mid-infrared, in terms of spectral purity and emitted power. That may give rise to a new generation of portable optical sensors. Apart from these very recent developments in coherent sources, a major role for optical sensing is played by more established techniques like light detection and ranging (LIDAR) or Fourier transform spectroscopy. As can be seen from the papers collected in this special issue, the future for all these devices seems to be an effective integration in more complex systems, due to their complementary features. In the last few decades, Earth Sciences, mainly driven by significant developments in geophysics, have undergone a progressive transition from a qualitative towards a quantitative, rigorous physical approach. In the meantime, problems like catastrophe hazard assessment and forecast, involving the most practical aspect of such disciplines, have been recognied as very complex ones, not affordable by any simple empirical approach. In particular, a modern view of volcano and earthquake hazard assessment must recognise the intimate link between the physical understanding of the basic processes and the monitoring of any change, in the system, which can prove useful to forecast the evolution of the system itself. Geodynamics, seismology, geochemistry and physical volcanology are rapidly evolving towards a deep physical understanding of the source processes and dynamical evolution. The most evident example of a field, in Solid Earth sciences, which requires a fully interdisciplinary approach and maximum technological effort is given by volcanology, which has long been considered the most empirical and qualitative discipline, due to its intrinsic complexity. A fundamental objective of physical volcanology is to describe in detail all the processes, occurring in a shallow magma chamber, driving the transport of magma to the surface, along with the processes underlying eruption dynamics. A critical component of such processes is the interaction between magmatic and hydrothermal systems, both by direct contact and via heat exchange between those systems. The specific purpose of physical volcanology is to develop physical models of these processes and to track their spatio-temporal evolution on the basis of geophysical and geochemical observations. Within this framework, seismology, geodesy, geochemistry, thermodynamics, and quantitative volcanology together constitute a multidisciplinary approach aimed at understanding and modelling the complex dynamics of volcanoes. The same multidisciplinary approach must also be applied to the study of seismic and volcanic processes within the larger geodynamic framework of regional structural features and stress fields.
Editorial / Optics and Lasers in Engineering 37 (2002) 87–89
89
Not only must modern geophysics rely on the most advanced seismological, geodetic and geochemical methods, to quantify source processes and structures, but it must also develop and improve existing as well as new technologies and methods for accurate in situ and remote monitoring of seismic and volcanic activity. Accurate, continuous in situ detection of gas concentrations and isotopes in seismic and volcanic areas still constitutes a hard challenge, but it can be hopefully afforded by optical methods in a near future. Stress and strain changes at volcanic and tectonic areas are by far recognised as the best indicators of changes in the activity of the system and of its possible evolution towards critical stages. Modern research in seismology and volcanology has pointed out the need for dense networks of strain-meters as the only way to resolve the details and the evolution of source processes. Optical methods allow the development of very dense, low-cost and easy-to-operate networks, with the further possibility, with respect to classical sensors, to cover a much broader range of frequencies. Moreover, the most advanced volcanological research has evidenced that magma movements towards the surface involve a range of frequency, which is intermediate between classical low frequency seismology and geodesy. The possibility to operate dense networks of optical strain-meters covering such frequency bands will probably revolutionise our view of volcanology. Finally, the need for integrated, multidisciplinary arrays, for measuring geophysical and geochemical parameters, can be satisfied by optical technologies as well. This was the scientific framework in which this issue was conceived, as the natural cross point between two different but complementary disciplines undergoing tremendous expansion, one of wide technological application, the other of enormous social and cultural impact. We gratefully acknowledge the Editor in Chief Pramod K. Rastogi for making possible this special issue and Carmen Addeo (INOA-Naples) for taking care of the manuscripts. Giuseppe De Natale Osservatorio Vesuviano-INGV, Via Diocleziano, 328, 80124 Napoli, Italy Paolo De Natale Istituto Nazionale di Ottica Applicata, Largo E. Fermi, 6, I-50125 Firenze, Italy Pietro Ferraro Istituto di Cibernetica ‘‘E. Caianiello’’ del CNR & INOA, Comprensorio ‘‘A. Olivetti’’, Fabbr. 70, Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy Email address: [email protected] Livio Gianfrani Dipartimento di Scienze Ambientali, Seconda Universita" di Napoli, Via Vivaldi, I-81100 Caserta, Italy
Editorial
Optical methods in Earth Sciences Mankind progress was started by systematic observations of Earth and celestial rhythms. As soon as the technology of the first modern optical instruments became available, they were applied to astronomical and geodetical measurements. Modern science itself originates from the Copernican revolution based on new observations and interpretation of Earth and celestial bodies motions. The revolutionary telescope of Galileo Galilei represented the first ‘scientific’ instrument in the modern sense of the word, aimed to test a previous theory and eventually to put the foundations for a new one. Galileo’s optical instruments put the basis of geography and cartography, solving problems like the determination of longitude and the detection of the cause of gravity variations with latitude. The advent of the laser started a new era in optical science, allowing the development of new methodologies and instruments based on coherent radiation. Despite the strong link between optics and Earth and planetary sciences of past times, laser applications in modern Earth Sciences have been rather limited. They have been mainly restricted to the field of geodesy, with laser telemetry for electronic distance measurements (EDM), some laser strain-meters, and interferometric absolute gravimeters. Some sporadic applications of optical techniques have also involved seismic data processing and filtering, now completely overcome by computer analysis. In very recent times, the use of InSAR interferometry for differential geodetic measurements has become widespread, and laser interferometry started to be commonly used. The increasing demand for telecommunication technologies, in the last few decades, has been sustained by the tremendous progress of optoelectronics, fibre optics and integrated optics. New perspectives of application in many different fields have been thus opened. The wide availability of low-cost, compact optical devices, allows to design new instruments for environmental applications. In this context, a crucial role has been played by diode lasers with output wavelengths from 760 to 2000 nm. These lasers have generally a distributed feedback (DFB) design enabling for single mode emissions with output power of milliwatts. They are currently used as sources in laser-based spectrometers to monitor trace-gas concentrations. Their characteristics of room-temperature operation and compatibility with telecommunication-grade optical components are being exploited to develop portable instrumentation for several applications, including environmental monitoring, factory-process control, bio-medical diagnostics. However, nearinfrared diode lasers probe overtone or combination vibrational bands of simple 0143-8166/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 4 4 - 0
88
Editorial / Optics and Lasers in Engineering 37 (2002) 87–89
molecules, which are orders of magnitude weaker than fundamental transitions occurring in the mid-infrared. Laser absorption spectroscopy would gain in sensitivity, precision and accuracy if the laser wavelength moves to the mid-infrared. This is one of the main reasons of interest towards the generation of coherent and tunable radiation with longer wavelength. In recent years, difference frequency generation in periodically poled crystals has emerged as a convenient technique to produce laser radiation between 3 and 5 mm. At the same time, a great innovation in photonics dealt with a different way of producing coherent radiation in semiconductor heterostructures that led to the invention of quantum-cascade lasers. Despite their CW operation at cryogenic temperatures, these lasers might open interesting perspectives in sensing applications because of their excellent performances in the mid-infrared, in terms of spectral purity and emitted power. That may give rise to a new generation of portable optical sensors. Apart from these very recent developments in coherent sources, a major role for optical sensing is played by more established techniques like light detection and ranging (LIDAR) or Fourier transform spectroscopy. As can be seen from the papers collected in this special issue, the future for all these devices seems to be an effective integration in more complex systems, due to their complementary features. In the last few decades, Earth Sciences, mainly driven by significant developments in geophysics, have undergone a progressive transition from a qualitative towards a quantitative, rigorous physical approach. In the meantime, problems like catastrophe hazard assessment and forecast, involving the most practical aspect of such disciplines, have been recognied as very complex ones, not affordable by any simple empirical approach. In particular, a modern view of volcano and earthquake hazard assessment must recognise the intimate link between the physical understanding of the basic processes and the monitoring of any change, in the system, which can prove useful to forecast the evolution of the system itself. Geodynamics, seismology, geochemistry and physical volcanology are rapidly evolving towards a deep physical understanding of the source processes and dynamical evolution. The most evident example of a field, in Solid Earth sciences, which requires a fully interdisciplinary approach and maximum technological effort is given by volcanology, which has long been considered the most empirical and qualitative discipline, due to its intrinsic complexity. A fundamental objective of physical volcanology is to describe in detail all the processes, occurring in a shallow magma chamber, driving the transport of magma to the surface, along with the processes underlying eruption dynamics. A critical component of such processes is the interaction between magmatic and hydrothermal systems, both by direct contact and via heat exchange between those systems. The specific purpose of physical volcanology is to develop physical models of these processes and to track their spatio-temporal evolution on the basis of geophysical and geochemical observations. Within this framework, seismology, geodesy, geochemistry, thermodynamics, and quantitative volcanology together constitute a multidisciplinary approach aimed at understanding and modelling the complex dynamics of volcanoes. The same multidisciplinary approach must also be applied to the study of seismic and volcanic processes within the larger geodynamic framework of regional structural features and stress fields.
Editorial / Optics and Lasers in Engineering 37 (2002) 87–89
89
Not only must modern geophysics rely on the most advanced seismological, geodetic and geochemical methods, to quantify source processes and structures, but it must also develop and improve existing as well as new technologies and methods for accurate in situ and remote monitoring of seismic and volcanic activity. Accurate, continuous in situ detection of gas concentrations and isotopes in seismic and volcanic areas still constitutes a hard challenge, but it can be hopefully afforded by optical methods in a near future. Stress and strain changes at volcanic and tectonic areas are by far recognised as the best indicators of changes in the activity of the system and of its possible evolution towards critical stages. Modern research in seismology and volcanology has pointed out the need for dense networks of strain-meters as the only way to resolve the details and the evolution of source processes. Optical methods allow the development of very dense, low-cost and easy-to-operate networks, with the further possibility, with respect to classical sensors, to cover a much broader range of frequencies. Moreover, the most advanced volcanological research has evidenced that magma movements towards the surface involve a range of frequency, which is intermediate between classical low frequency seismology and geodesy. The possibility to operate dense networks of optical strain-meters covering such frequency bands will probably revolutionise our view of volcanology. Finally, the need for integrated, multidisciplinary arrays, for measuring geophysical and geochemical parameters, can be satisfied by optical technologies as well. This was the scientific framework in which this issue was conceived, as the natural cross point between two different but complementary disciplines undergoing tremendous expansion, one of wide technological application, the other of enormous social and cultural impact. We gratefully acknowledge the Editor in Chief Pramod K. Rastogi for making possible this special issue and Carmen Addeo (INOA-Naples) for taking care of the manuscripts. Giuseppe De Natale Osservatorio Vesuviano-INGV, Via Diocleziano, 328, 80124 Napoli, Italy Paolo De Natale Istituto Nazionale di Ottica Applicata, Largo E. Fermi, 6, I-50125 Firenze, Italy Pietro Ferraro Istituto di Cibernetica ‘‘E. Caianiello’’ del CNR & INOA, Comprensorio ‘‘A. Olivetti’’, Fabbr. 70, Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy Email address: [email protected] Livio Gianfrani Dipartimento di Scienze Ambientali, Seconda Universita" di Napoli, Via Vivaldi, I-81100 Caserta, Italy