The Integration of Deepwater Geohazard Evaluations and Geotechnical Studies

Abstract

Abstract The integration of high-resolution geophysics, geohazard evaluations and geotechnical engineering has been termed an "integrated geoscience study". This paper focuses on deepwater Gulf of Mexico examples of integrated geoscience studies. Methods to evaluate geohazards and the usefulness of those studies to geotechnical investigations are discussed. Examples illustrate how integrated geoscience studies have been used in the design of offshore foundations. INTRODUCTION High-resolution geophysical data have been used for many years in geohazard studies to develop an understanding of potential hazards and constraints to offshore operations. Recently, these studies have been extended to the geotechnical engineering community to assist with solutions to problems in several areas. Some of these include the development of sampling and testing plans for soil boring investigations, the interpretation of data from site specific geotechnical investigations, the development of site specific design properties from non-site specific geotechnical investigations, and a better understanding of geological processes which might affect foundation designs. The primary key to the usefulness of high-resolution geophysical data to the geotechnical engineering community depends on the resolution of the data, the development of a geologic model consistent with that data, and an understanding of the geological processes resulting from that model. The integration of high-resolution geophysics, geohazard evaluations and geotechnical engineering is called an "integrated geoscience study". Its purpose is to develop geotechnical parameters for design and to assess, if necessary, the geological hazards and constraints given that geotechnical knowledge. Geohazard Evaluations In US waters, geohazard evaluations are required to meet MMS requirements to assess shallow hazards to ensure that exploratory and development operations are conducted with a minimum risk to human life and the environment'. Similar studies are required around the world. While generally prescriptive in their requirements, these studies can serve to guide geotechnical engineering studies related to sitting structures on the sea floor. The usefulness of geohazard evaluations to geotechnical engineers depends on obtaining high quality geophysical data and developing an interpretation in which the data fits a geological process. Geohazard Survey Methodology. While there is a common government requirement, companies have different practices to fulfill the regulations. Shell's present deepwater practice, for example, is not to obtain high-resolution geophysical data unless an evaluation of the 3-D exploration-level geophysical data indicates sufficient complexity to warrant a deep tow or high-resolution 3-D study. The exploration-level 3-D data set is first subset to represent the first two to three seconds of data to reduce the volume of data to be processed. The subsetted seismic data are enhanced using high frequency enhancement and whitening methods that are well known in the industry. Another technique to enhance the data set is termed the Short Offset method. As discussed by Cowlard2, this method uses near normal incident data volumes and demands that the data position be honored, precise time corrections applied, the absolute true amplitude preserved and the number of near traces reduced to the minimum possible number. By not binning the data and preserving trace location, seismic events are not smeared. The data are migrated using a Kirchhoff migration algorithm.