Marine-FTG®

Full Tensor Gradient Marine Data Acquisition

(by John Watkins, Scott Hammond, Bell Geospace Inc.)

Introduction

Acquisition of Full Tensor Gradient (FTG) data requires the operator to consider the unique nature of this high frequency, small amplitude measurement. As in any quality measurement, the need for a high signal to noise ratio dictates that noise be held to an absolute minimum. System deployment is the first important factor in assuring a successful FTG survey. In addition there are several key parameters that are used during acquisition to minimize noise and therefore maximize the S/N ratio. To insure an efficient survey, the results of these efforts must be monitored in real time to make sure that the S/N requirements are being met. In addition this paper will discuss the unique calibration procedures required for FTG surveying as well as the license restrictions dictated by working with such a highly sophisticated system that was only recently declassified.

Survey Equipment and Deployment

The heart of the acquisition system is the FTG instrument itself. This system, manufactured by Lockheed Martin, is installed as close as possible to the center of pitch, roll and yaw of the survey vessel being utilized. The instrument is 3' X 3' X 4' and weighs approximately 500 pounds. To maximize efficiency, portability and ease of deployment, the system with all of it's peripheral equipment has been containerized so that the integrated system can be set on the deck of a vessel, welded in place, and collecting data within hours. Peripheral equipment includes; a system control and monitoring computer, a survey planning and tracking computer, GPS and DGPS systems, satellite communication systems, and data processing and archival systems. When required a multibeam echosounder system is also available for Swath bathymetry measurement and correction of free air gradients. The containers are set up to accommodate a 2 man crew.

Acquisition Parameters

Acquisition parameters are dictated by several factors including water depth, target depth, geologic concerns and the type of imaging problem being modeled. In general a grid of data is obtained which allows optimum survey line spacing and cross tie spacing that will allow sufficient tie points for line leveling (described in a separate poster session) and a short enough time interval between common point surveying. For this reason large survey areas are usually divided into more manageable "chunks" of 100 to 200 blocks which can later be merged together. Line spacing is typically 2km by 2km in deep water and can be much closer as water depth becomes shallower. Due to the fact that data is sampled at 128 Hz, survey speed is not an issue as FTG surveys are usually acquired as fast as the vessel will go. The survey speed is further dictated by sea states as there is a limit to the amount of acceleration that the system can compensate for.

Real-Time Processing & Quality Control

The FTG system is designed with live time status indications and fault alarms to indicate hardware malfunctions. FTG data is recorded in 800 MB files (about 2 hours of data) for archival and Quality Assurance checks. The operator processes each data file and the software program displays noise values, acceleration values, platform performance, individual instrument data plots, environmental conditions, positional accuracy, and presence of data gaps that are monitored to meet strict criteria. In the event that a survey line is substandard, it is identified immediately and will be repeated to ensure optimum quality of data. Abbreviated data is sent via satellite to the Bell Geospace Technical Department in Houston for further analysis. As the survey nears completion, the data is processed to determine if extra survey lines may be required to highlight particular areas of interest.



Calibrations

The stabilised platform and the individual instruments that make up the FTG require calibration both dockside and at sea. Calibrations are performed at initial installation and periodically, usually monthly, while in operation. Dockside calibrations consist of aligning the platform gyros and accelerometers, and determining bias and scale factor values for the 3 Gravity Gradiometer Instruments (GGI's) and the Gravimeter (GMA). The operator initiates the calibrations when the dockside vessel conditions are most benign and they are archived for quality evaluation. At sea calibrations to the GGI's improve the performance of the instruments in dynamic conditions to minimize the effects of the vertical accelerations. While on survey, the operator monitors system status and will initiate calibration when necessary.

Conclusions

Many people have asked why FTG is acquired on a dedicated vessel. As this paper shows, this is an exciting new technology with unique requirements that will insure a top quality "prospect level" interpretation tool. As accompanying papers show, FTG is far different from standard gravity and to treat it the same would diminish its value and leave the interpreter with far less value than the tool is presently capable of delivering. Through continued research and engineering advances we will work to find ways to provide this information in an ever improving, more efficient manner which will allow the client to gather more data cheaper and in more difficult environment.