Strategies to Incorporate Pre-Development High-Resolution AUV Surveys Acquired for Archeological Requirements into the Project Lifecycle and Workflow
Authors: Monsalve Brenda Isabel , McConnell Daniel Russell , Worthen John A. , Landry Laura 
Published: July 2010

Archeological regulations issued by the Minerals Management Service (MMS) in 2005 require the major elements of a development-type seabed and foundation zone survey be acquired in order to detect significant historical artifacts over large areas of the Gulf of Mexico in advance of any seafloor disturbance (MMS, 2005). Prior to this regulation, these specialized surveys would be acquired only for the planning of a seafloor development, a workscope that requires high-resolution topography and soils information. The effect of the archeological stipulation in the areas affected is to require major elements of these data very early in the exploration-development timeline, rather than during the field development phase. The acquisition and interpretation of pre-development high-resolution data, while fulfilling specific archeological regulatory objectives as its goal, affords an opportunity to integrate the high-resolution data into the efforts by other parallel and potential future users of the data. Parallel users include shallow hazards interpreters that have the task of assessing seafloor conditions, topography, and geologic conditions in advance of the exploration and development wells, and future users would include the post discovery site development team, should the prospect develop into a commercial discovery. Specialized high-resolution deepwater surveys have a high cost of mobilization, for this reason, operators should consider implementing incremental modifications in the survey design and include data from other sensors, which are not only important and useful to parallel users in the site hazard characterization, but if the site goes to development, they become critical to future users as well.

Several integrated strategies could be considered during the planning of seafloor engineering level data acquisition, improving workflows for the archeological interpretation and the site hazards characterization, and potentially eliminating expensive resurvey in the event that the site goes to development.

Introduction

As developments moved to deepwater, the MMS waived the requirement for magnetometer and side scan sonar for site surveys. Operators shifted from traditional high-resolution site survey to the use of 3-D exploration seismic data for site assessments and few to none of the deepwater blocks outboard of the shelf slope break had an archeological stipulation. As exploration and development continued, a number of wrecks were subsequently discovered during pipeline surveys and other construction activities. These include the Mica and Mardi Gras shipwrecks in the Mississippi Canyon Area, as well as others. The Mica shipwreck was discovered during a post-installation survey of a newly-laid pipeline when the inspection surveyors determined that the pipeline had been laid directly over an early 19th century copper clad schooner! As a result of these incidents and reevaluations of areas thought to have higher probability for shipwrecks, the MMS in 2005 added all of the the lease blocks in the Mississippi Canyon protraction area, many contiguous lease blocks in northwest Green Canyon, and Viosca Knoll, and isolated lease blocks in Atwater Valley, Lund, Henderson, Lloyd Ridge, DeSoto Canyon, Howell Rook, and The Elbow Protraction areas (Figure 1). In these new regulations, MMS specified that high-resolution side scan sonar, high-resolution subbottom profiler, and a depth sounder would be required for deepwater archeological surveys (NTL 2005-G07). The specification for accurate positioning of the sidescan sonar would require the deployment of a transponder array with a deeptow sidescan and subbottom fish, or advanced inertial and ultra short baseline positioning systems, or other adept survey platforms. The sidescan sonar and subbottom profiler deeptow platform was developed for deepwater foundation zone surveys by major oil companies in collaboration with major geotechnical/geophysical contractors in the early 1990s. While the data acquired were excellent, these surveys were characterized by long line turns and the use of heavy sediment keels that would keep the position of the fish at a constant distance above the seafloor but would, as a consequence, limit the surveys to virgin, undeveloped, areas so as to not snag or entangle on infrastructure. These drawbacks fomented the development of the AUV platform for industry deepwater engineering surveys. These specialized vehicles were primarily designed to deploy side-scan sonar, high-resolution multibeam echosounders, and subbottom profilers. These " robot submarines?? are programmed to run a survey grid but, separated from any tether to the ship, line turns are eliminated. Furthermore since since AUVs fly untethered a set distance above the seabed, they can be navigated and survey in areas of seabed infrastructure (or over potential historical artifacts). The specifications for the archeological survey data acquisition parameters issued in 2005 would allow for deeptow surveys, but offshore engineering survey support technology has moved largely to the AUV survey platform for the reasons described. These two end uses, pre-disturbance archeological surveys and pre-development seabed engineering surveys, in a large part, require the same data. Hence a key concept in this article is to understand that a deepwater archeological survey is acquiring part of a seabed engineering survey in advance. Because of the high cost of mobilization (common to any marine survey but especially these marine advanced-technology surveys), small incremental costs added to the archeological survey will go far to meet the minimum pre-development survey requirements should the prospect go to development. This article is based on information gathered from various professionals working in geohazards and site development capacities. Key concepts are: acquiring additional data beyond the archeological specifications at little to no additional cost, feeding valuable data to parallel users working in site assessments and other users, and eliminating the need to survey twice with small incremental expense. This article presents several common scenarios and describes strategies to add value while addressing the regulatory cultural resource preservation obligations.