Sub-Sea Seismic: Acquisition and Data Analysis procedures
Authors: Sonneland L. , Reed K. , Navrestad T. 
Published: October 2008

INTRODUCTION

The sub-sea sensing technology has been around for some time. The recently developed SUMIC (sub-sea seismic) technique [1] has pointed out some new applications in exploration and production. This will have an impact on how marine seismic acquisition and seismic data analysis are performed. Current procedures will have to be modified to be able to honour the new requirements imposed by the SUMIC concept.

Our paper will highlight how these procedures have been modified to accommodate SUMIC. In particular how inclusion of elastic data might increase the confidence in Iithology prediction and reservoir description from seismicdata.

In sea-bed sensing cost-effectiveness is an important objective. This normally implies a trade-off in sensor deployment efficiency versus sensor coupling quality. InSUMIC there is no compromise on the sensor coupling quality. This is key since the coupling quality to the seabed is crucial with respect to the fidelity of the sampled elastic wavefield. Compared to existing ocean-bottomcable technology, the SUMIC technology represents a significant improvement of coupling quality.

In the implementation of the SUMIC acquisition procedure it has been of importance to cater to both an add-on SUMIC experiment and a combined SUMIC experiment with conventional marine acquisition. In the first scenario, existing seismic data is supplemented with SUMIC data toenhance the Iithological predictability relative to the existing dataset. In the second category there are, of course, no compatibility constraints with respect to existing data.

ACQUISITION PROCEDURE

The SUMIC operation is normally carried out using one source-vessel and one recording-vessel. The receiver spread is deployed from the recording vessel. There is an umbilical cable between the vessel and the spread. The cable is used for towing the sensor-spread as well as for power and communication between the spread and the vessel (figure 1). The recording vessel has to be equipped with a Dynamic Position System and have installed a Remote Operated Vehicle (ROV) System.

The receiver spread consist of units of four component sensors. With the SUMIC method we are measuring P and S-waves using hydrophones and three component geophones. The number of sensor units and the distance between the units can be varied. (Based on the Nessie 3*, recording system the limitations at the present is 250 sensor units and 1000 channels per spread-cable system).

The three component geophones, orientation will have to be known relative to the line direction and to have aproper sampling of the wavefields. This is achieved by having a inclinometerand a compass in each sensor-unit. The orientation of the sensor-units during planting is not important as the orientation for the individualgeophoneswill be calculated in the software of the recordingsystem. This results in time-saving during sensor deployment. The sensor spread is deployed along the planned receiver line at the sea- bottom. The Dynamic Positions System interfaced to a surface navigation system (Differential GPS) will be used to bring the vessel and thespread into the planned position. In previous operations an USBL (Ultra Short Baseline) underwater positioning system have been used.