Our research at the Mardi Gras Shipwreck is guided by a number of specific questions:
At 4,000 feet deep, the sheer weight of the water (over 3,500 pounds per cubic inch) at the Mardi Gras Shipwreck site would crush a human body. As a result, all of the work will be carried out by robotic Remotely Operated Vehicles (ROV’s) using a range of specialized equipment tailored for performing tasks under these conditions. The equipment is a combination of off-the-shelf vehicles and tooling adapted from that in common use in the offshore oil and gas industry and specialized tooling specifically engineered for the needs of this project. In addition to high-resolution cameras and lighting systems complementing imaging and positioning sonar systems, the project will employ two ROV systems. The work will be performed from onboard the 256-foot long Toisa Vigilant, an ROV support vessel that will serve as the scientists’ work platform, living quarters, and laboratory during the duration of the project.
Veolia Environmental is contracted to provide the Perry Triton XLS-17 for the project. This ROV is a work-class, 150 hp ROV system with a payload capacity of 550 pounds (reserve). The XLS-17 features a heavy lift tether-management system (TMS) and is rated to 9,800 ft (2,987 m). It is fitted with a Shilling seven-function T4 and five-function Rig Master Manipulators. The system is completely fiber optic and supports up to eight cameras. TAMU has worked closely with Veolia representatives to design and incorporate specific tooling to be used with this ROV; this tooling will allow the ROV to conduct the necessary archaeological objectives.
The Sub-fighter 7500 ROV is a Sperre AS vehicle constructed specifically for the Mardi Gras Project and is the property of the Department of Oceanography at Texas A&M University. While it has a limited payload capacity of 11-17 pounds (5-8 kg), it is equipped with two cameras (one of which is high-resolution), as well as four 250-watt lights. The primary purpose of this vehicle is to document the work of the Triton XLS-17. The 7500 is also equipped with a five-function manipulator and is capable of picking up small artifacts in the event the Triton is temporarily non-operational.
Unlike archaeological projects on land, every single task performed on the seabed during the Mardi Gras Shipwreck Data Recovery Project will be documented on video. For the success of the project, it is important to document the artifacts on the seabed with as much resolution as possible. Once removed from the seabed, the only information allowing the archaeologist to match the artifact to its number (along with the lifting basket allocation information) will be the visual file. For this reason, visual documentation is of crucial importance and will be provided by high-resolution digital still cameras with high-intensity lights.
Special Tools to be used on the Mardi Gras Shipwreck excavation include:
Large Artifact Retrieval Tools (LART’s) are being constructed for the recovery of large artifacts and features. They were specifically designed for the Mardi Gras Shipwreck Project by Perry Slingsby Engineering in Houston and are being manufactured under the direction of Veolia Environmental Marine Services. The two halves of the LART can be closed at a controlled rate by means of hydraulic rams. Once deployed and lowered to the seafloor, the Triton XLS-17 will position over the artifact to be retrieved, and the hydraulic connection will be established. The Sperre 7500 may be used to monitor positioning and closing operations. Once the artifact is closed within the LART, it will be brought to the surface using its own winch and cable. The artifacts will remain in the LART’s until they are carefully removed at the Conservation Research Laboratory (CRL) in College Station, Texas. Precautions will be taken to minimize any possible damage to the artifact by packing each shell with burlap prior to transportation from Port Fourchon, Louisiana, to College Station, Texas. These clam-shell-like devices are admittedly experimental and will only be employed after careful consideration of a number of factors (sea-state, present condition and stability of the feature, visibility, clearance around the feature, etc.) seem to suggest a reasonable chance of success. In the event a decision is made not to use this method, strapping, boxes constructed prior to the cruise, and additional material for modifications will be onboard the research vessel as a method of recovery. A third alternative will be to sample the contents of the feature, record it extensively, and leave it in place if it is determined that it will be impossible to recover the feature without substantially damaging it.
One of the most critical aspects of archaeology is the precise location of artifacts in the context of the site. By carefully recording every artifact’s position on the seafloor, we should be able to start to see patterns emerge that will define specific use areas, such as the captain’s quarters or the common seaman’s berths. One of the greatest challenges of working in 4,000 feet of water will be in mapping the artifacts on the seafloor and positioning the ROV. UTEC Survey Inc. will provide survey support, which will include acoustic positioning and geo-referenced video support. Basic positioning of the ROV’s will be provided employing a Kongberg Ultra-Short Base-Line (USBL) system, which has a typical accuracy of 0.5 percent of water depth. In order to minimize the USBL error as a function of the water depth, UTEC will deploy a seabed transponder known as “relative USBL.” The system is based on navigation software that examines the location derived from normal USBL observation and error correction provided by the fixed transponder on the seabed, thus maintaining system and environmental errors to a minimum on the remote transponder attached to the ROV. In addition, Kongsberg will provide a system known as the High Accuracy Inertial Navigation System, or HAINS. The result will be a more stable and precise reading of the USBL signal, which will increase the accuracy of all measurements taken.
Prior to removing artifacts, we hope to learn as much as we can about the site by non-destructive means using remote-sensing instruments like sonars, multi-beam fathometers, and subbottom profilers. This will help us to plan the excavation.
Texas A&M University has acquired detailed, high-resolution multi-beam bathymetry from a survey conducted by C&C Technologies, Inc. in May 2005 using an Autonomous Underwater Vehicle (AUV) employing a Kongsberg EM2000 multi-beam swath system. The survey was run at a 4-m (13-ft) line spacing over the shipwreck with the sensor at 10 m (32 ft) off the seafloor. This system produces an extremely high resolution, accurately-positioned 3-D image of the seafloor that can be used to groundtruth the photomosaic mapping proposed for the 2007 season.
Prior to removal of artifacts or overburden from the site, a survey will be run using an Omni Technologies, Inc. (OTI) parametric subbottom profiler mounted to the Triton ROV. Running at a 15-kHz bandwidth centered at a 30-kHz difference frequency, up to 9 m (29.5 ft) of penetration in silty sands has been achieved. The PFRS system comes with an acquisition and playback software package. Acquisition can be performed entirely autonomously or in real-time with user control. The playback software provides a profile screen with controls similar to a VCR. A rich set of functions provides the ability to review a survey, annotate and perform screen captures. The narrow 3° beam may allow the imaging of buried features, hull structure, or large artifacts, which will aid in planning the investigation.
Mapping the site will be done by capturing a minimum of four high-resolution, orthogonally-corrected photo-mosaics over the course of the project. To accomplish this, the Triton ROV will be deployed with scanning sonar, high-resolution video cameras, and digital still cameras. Digital still and video imagery will be acquired at an ROV flight altitude of 4 m (13 ft), which will provide a swath coverage of 4.5 m (14.7 ft). In order to achieve coverage with a 60 percent overlap, transect lines spaced approximately 3 m (10 ft) apart will be flown. A coarse mosaic will be created the same day that the mosaic survey takes place.
After finishing the coarse mosaic and loading it in Site Recorder 4 as a base layer, the process will start to achieve a better resolution image. The first step for obtaining a high-quality mosaic will be to remove camera distortion and parallax.
A controlled recovery of artifacts exposed on the seafloor at present, as well as of artifacts that may become exposed as a result of the surface collection or clearing of hull components, will be started as soon as the remote-sensing data are analyzed.
Artifacts observed on the seafloor include:
Site Recorder 4, developed by 3H Consulting, Ltd., was selected as the main software system for recording, registering, and cataloguing artifacts from fieldwork through the conservation process (http://www.3hconsulting.com/). The software is a fully-integrated GIS designed by archaeologists for use in maritime archaeology. Site Recorder 4 can manage thousands of artifacts, drawings, photographs, video clips, documents, and geophysical data files that can be linked together for analysis and interpretation. Artifacts will be cataloged by their functional class and material of composition, photographed before and after recovery, plotted on the site map and assigned a unique artifact record number.
A minimum of six sediment samples will be collected with push cores (1 m x 7 cm diameter [3.28 ft x 2.75 in]) at various points on and near the shipwreck for the purpose of determining sediment type and basic properties. Immediate field analysis of select sediment samples will help determine the amount of slumping that might be anticipated to occur during the removal of artifacts.
Wood samples will be collected from in situ elements of the shipwreck. Recovered artifacts also may be sampled in the conservation laboratory to assist with identification. Hull components sampled in situ will be selected to minimize adverse impact to the site. Consideration will be given to their position, location, and archaeological context. An attempt will be made to collect wood samples from major elements of the hull where the use of differing types of woods might be anticipated, such as the stem or stern posts, keel, keelson, frames, planking, wales, or treenails, assuming such elements survive.
If ballast is encountered, a representative sample will be collected in sufficient quantity to identify the parent material and possible source.
Excavation will be limited to clearing areas around the large artifacts and features to aid in their recovery, to more fully expose elements of the hull already partly exposed (most of the port side) for documentation, or to investigate possible features or artifacts revealed by the subbottom profiler. One goal of this work would be to determine the position of mast steps and other features that might lead to an estimation of the size and rigging of the vessel. Excavation will be carried out using a small venturi suction dredge attached to a pump mounted on the Triton ROV. Excavated material will be exhausted through a 0.6 cm (¼ inch) mesh screen mounted on the aft end of the ROV to catch small finds inadvertently suctioned off the seafloor. The greatest challenge in conducting excavation on the site is the fact that there is very little to no current to carry off the fine silt that will be injected into the water column. This will result in an immediate loss of visibility that will impair any recovery, mapping, or photography effort for hours until the silt settles. As a result, excavation will be carefully planned and executed according to prevailing conditions at the time.
The Conservation Research Laboratory (CRL), under the direction of Dr. Donny Hamilton, will conduct all conservation of recovered material. Methods will vary with the type and number of artifacts collected. Recovery is anticipated of a series of encrustations that most likely will contain degraded iron-artifacts, along with wood samples, samples of sheathing, ceramic artifacts, glass artifacts, etc.
Once conserved and properly labeled, artifacts will be delivered to the Division of Archaeology of the Louisiana Department of Culture, Recreation, and Tourism for permanent curation and future study.