Page 116 - Acquisition and Processing of Marine Seismic Data

2.4 SPECIFIC ACQUISITION TECHNIQUES 107

high-frequency events and smaller earthquakes, after shooting. This requires the shot data to be
and their deployment periods are relatively extracted from node’s continuous data record.
shorter. Long period OBSs can record a broader The reference between the shot and recording
range of frequencies and mid-range earth- is the shot time, which should be precisely syn-
quakes, and their deployment periods are chronized with the node’s internal time.
relatively longer. There are a number of different OBN systems
available worldwide today manufactured by
2.4.1.3 Ocean Bottom Nodes different vendors (Fig. 2.62):
In the last decade, autonomous and cable-
• Trilobit (CGG Veritas)
connected ocean bottom nodes (OBN) directly
• Z100, Z700 and Z3000 (Fairfield Nodal)
beneath the production platforms have been
• CASE Abyss and Manta (Seabed
introduced. These nodes are either connected
Geosolutions)
to each other and to the recording vessel by a
• OBX (OYO GeoSpace)
transmission cable, or record the data into their
internal recording units. Cabled systems can stay Trilobit nodes from CGG Veritas have been
at the sea floor for a longer period than autono- successfully used for surveys in the North Sea
mous nodes, which have less endurance due to and Gulf of Mexico. They are in operation with
their limited internal power supply, with a typi- continuous data recording up to 104 days at a
cal battery life of more than 30 days. Remotely maximum operating depth of 3000 m. Fairfield
operated vehicles (ROVs) are used to accurately Nodal offers three different node models for
locate and retrieve the nodes on the seabed, shallow, mid and deep waters, Z100, Z700,
which also enables a visual inspection of the and Z3000, which have maximum operating
nodes during the deployment and it becomes depths of 100, 700, and 3000 m, respectively.
possible to locate the nodes very close to the plat- The Z100 and Z700 model nodes can record data
forms or seabed pipelines. for 30 and 60 days, respectively, while the Z3000
Since the deployment requires costly ROV can be fully operational for 180 days. The OBX
operations and takes considerable time, the system from OYO GeoSpace is an autonomous
nodes are deployed in a relatively sparse grid node for 4C acquisition, offering 500 h of contin-
withaseparationbetweentheneighboringnodes uous recording at a maximum depth of 3500 m.
of approximately 400 m, and a source vessel gen- Seabed Geosolutions, a joint venture between
erates the seismic signal at the sea surface. The Fugro and CGG, produces CASE Abyss and
recording within the nodes is continuous, and Manta nodes, both of which are operational at
passive seismic monitoring is done before and 3000 m maximum water depths.

FIG. 2.62 Different ocean bottom node devices. (A) CASE Abyss (Seabed Geosolutions), (B) OBX (OYO GeoSpace), and (C)
Z700 (Fairfield Nodal).