Recent Advances in Shallow Seismic Reflection Processing

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ABSTRACT Digital seismic data acquisition systems have recently become available to supersede the analog equipment commonly used in very high resolution (better than 1 meter) shallow marine seismic reflection surveys. These systems have been successfully tested and used both in the domain of subsurface sedimentology and in that of site surveys for operational needs. Due to the specific nature of very high resolution seismic reflection data, new processing methods and tools are studied and customizedfrom other application fields. Companion computation of synthetic seismograms is carried out and compared with field measurements. Data provided from site surveys by ELF-Aquitaine serve as a basis to demonstrate the abilities of digital processing and its contribution to the use of geotechnical surveys. Acquisition is performed using the DELPH1 system which allows for digital acquisition and real-time processing, as well as cost-saving storage of the data for further reassessment. Lines of particular interest are post-processed on engineering workstations through SITHERE software. This last package also allows for easy development, testing and implementation of new methods and algorithms, of which finely tuned versions can then be integrated in the DELPH1 package for field processing. 1 INTRODUCTION Marine seismic profiling is widely used to provide information about sub-sea soil structures on a range of different scales and resolutions. At the most sophisticated level, it may involve multichannel streamers, multiple sources and very dedicated digital recorders ([1]). Digital multichannel systems are increasingly used for oil prospecting and to predict gas hazards in site surveys. Yet, many of the very high resolution seismic systems currently in use for coastal studies are still based on analog equipment, especially in the cases of mineral resource assessment, environmental monitoring and site surveys. A surprising amount of work can be done with singlechannel, analog systems. Digital acquisition systems have been recently developed, by IFREMER ([2]), ELICS ([3]) and others. The availability of digital data has initiated the development of micro-seismic processing techniques, leading to significant improvement of the seismic images of the first 100 meters, and to better appraisal of drilling hazards and of soil conditions. Some of these techniques have been presented in [4]. More are detailed here, with special emphasis on the prospects for geotechnical application opened by the comparison between synthetic seismograms, calculated from measured soil data, and the actual high resolution seismograms. ELF-Aquitaine has conducted several site surveys using these new digital techniques. Data acquisition has been performed using the DELPH1 system ([3]) which allows for digital acquisition and realtime processing. Lines of particular interest were post-processed using SITHERE software ([4]). This modular, workstation-based software allows for an easy development and testing of new algorithms, which can then be implemented in the DELPH1 system for field processing. 2 DIGITAL ACQUISITION ELF-Aquitaine has used for several site surveys the DELPH1 digital acquisition system. This application to site survey is described here. 2.1 Delph1 specifications DELPH1 hardware consists of an AT/386 processor and an 80387 co-processor.