Time-Lapse High-Resolution Seismic Surveys in the Mississippi Delta Mudflow Area, Offshore Louisiana

Abstract

ABSTRACT Time-lapse high-resolution surveys are useful for studying rapidly changing geologic features such as the mudflows of the Mississippi Delta, and for assessing how these potentially hazardous features might affect offshore structures. For time-lapse surveys, an initial high-resolution seismic survey is repeated at specified time intervals, following the same grid of tracklines, using the same seismic systems and a precise navigation system. Time-lapse surveys have been used successfully to monitor the active mudflow about 6 miles upslope from Shell Offshore Inc.' s Cognac Platform. The surveys show that mudflow activity has had no effect on the Platform. Time-lapse surveys obtain the most accurate determination of seafloor changes available at the present time. If one compares two or more random surveys in an area, instead of carefully performed time-lapse surveys, the accuracy of the results may be less than the actual amount of change; so results may be meaningless or misleading. Time-lapse surveys can produce results with vertical accuracy of about ±5 feet along survey tracklines. INTRODUCTION Time-lapse, or repeated, high-resolution seismic surveys have been used to study rapidly changing geologic features such as the mudflows of the Mississippi Delta, and to assess how these potentially hazardous features might affect offshore petroleum structures. In a time-lapse survey, a specified grid of tracklines is surveyed with high-resolution seismic systems, using a highly accurate navigation system. The initial survey is repeated at specified time intervals, using the same systems, and following the initial grid lines as closely as possible with a precise navigation system. Data and maps from each survey are compared to determine any surficial geologic changes that may have occurred between surveys. Seafloor changes along survey trackline scan be detected with vertical accuracy of as much as ±5 ft. Time-lapse high-resolution surveys can be used to monitor any rapidly changing geologic feature that could affect offshore structures. Perhaps the most common use for time-lapse surveys at the present time is studying the mudflows of the Mississippi Delta. They are ideal for this type of study because of the great degree of accuracy they provide. Time-lapse surveys have also been used successfully to study and monitor strudel scours and ice gouges in arctic Alaska. The surveys could also be used on other active geologic features such as active seafloor sand waves. DESCRIPTION OF TIME-LAPSE SURVEYS Seismic Systems. The high-resolution seismic systems needed for a time-lapse survey are those producing records that can be used to determine seafloor and shallow subsea floor conditions. These are usually the only geologic conditions that change rapidly enough to detect with time-lapse surveys. A side-scan sonar and a 3.5- or 7- kHz sub bottom profiler would usually be included in a time-lapse survey. A boomer-type system such as the Acousti-pulse is often useful as well.' Other systems may be employed for the first survey to help define the geologic setting, then it may be determined that they are not necessary for succeeding surveys.