SEISMIC SURVEYING  Fig. 1. The complete node unit.


Ocean bottom nodes in operation


 Ocean bottom nodes are used to good effect in situations where accuracy of sensor placement is most important, as in time lapse surveys. Brian Anderson and Christi Todd describe how that accuracy is achieved, using the node, an ROV, and navigational systems.


I


n August of 2010, SeaBird Exploration FZ-LLC completed a base line 4D OBN (ocean bottom nodes) seismic survey for


Chevron North Sea Limited in the Rosebank field west of the Shetlands, using node vessel M/V Hugin Explorer and source vessel M/V Kondor Explorer. ROV operations were provided by FUGRO – Rovtech Ltd, Aberdeen. Te nodes used were SeaBird’s patented CASE Abyss nodes. At the end of the project 765 nodes had been deployed. Two of these were deployed outside of 5m due to trenches across the preplot which appeared to have been caused by a dragged anchor. Te remaining nodes were positioned within 5m. Te mean radial distance of as- laid position from preplot was 1.17m. Te CASE Abyss node system consists


of the following main components: Data Acquisition Unit (DAU); Sensor Unit (SU); Acoustic Modem; Battery Units; Plastic Shell; Aluminum frame.


15 IHSS


Te Data Acquisition Unit (DAU) carries out most of the control and communication tasks in the system. It stores digitized sensor data and status data onto files, contains the reference clock, and the system power supply. Te Sensor Unit (SU) contains the


primary seismic sensors (three geophones and a hydrophone). It also contains a hydrophone pre-amplifier with programmable gain. A dual-axis inclinometer measures the SU verticality with great accuracy. Te SU is connected to the DAU via a short, highly flexible cable, to ensure good acoustic decoupling between the SU and the rest of the CASE Abyss unit. Te Acoustic Modem is connected to


the DAU and offers a control and data link between the individual CASE Abyss unit and the sea surface. By way of the acoustic link one can control the operation of individual CASE Abyss units and retrieve status information and data files.


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