A method whereby, in contrast to conventional marine seismic (towed streamer) acquisition, acoustic reflections are recorded by receivers placed on the seabed.

OBC

Ocean Bottom Cable (OBC) Seismic

At a glance

Author : JIM, Mundy Obilor 13th Aug., 2011

Copyright © - All Rights Reserved.

Why OBC?

In addition to P-waves, mode-converted shear waves (PS-waves) which do not travel in water are also recorded. These mode converted records, which are less-attenuated by gas than P-waves have proved to be very useful in seismic imaging.

Fig Source:The Leading Edge, 1999

P-waves are more attenuated in gas than S-waves; S-wave passes through An advantage!

Basic Concepts:

1. Receivers are placed on the seafloor2. Source vessel sails across the

cables/nodes3. Possibility of using more than one

cable-laying vesel 4. Multi-component data (P- and S-

waves, commonly 4C P,X,Y,Z) are recorded and retrieved for QC and processing. Sensors may be permanently deployed for monitoring purpose (OBN)!

The gains, the success stories

1. Eliminates surface-related noise (towing, streamer noise, etc)

2. Data quality improved by using hydrophones and geophones/accelerometers

3. Resolves several imaging challenges confronted by streamer techniques (gas clouds, carbonates, complex structures, …)

Gas Cloud

An example from Tommeliten Field in the Norwegian sector of the North Sea (courtsey of Statoil). Here PS-wave data is compared with PP data for imaging through a gas cloud.

The Leading Edge Nov 1999

The gains from OBC surveys have been very remarkable

An example from Alba field: One of the conclusions is that PS-wave shows high-amplitude top and base sand reflectors, not observed on the P-wave data.

1. Practical in areas where towed streamer operations are difficult (platforms, obstructed areas, etc)

2. Feasible in shallow waters3. Improved resolution (broader

bandwidth)

P-wave S-wave (improved imaging)

Fig.The Leading Edge Nov 1999

The gains, the success stories

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Seabed Seismic has been faced with challenges which are continually been overcome, among which are:

Cost: OBC surveys cost much more than streamer surveys but recently, improvements in acquisition techniques have considerably reduced the average acquisition cost

Long cable requirements: Using more than one cable-laying vessel practically reduces acquisition time increases acquisition rates reduces cost

Processing: software packages and tools adapted to shear wave data are on the increase (bearing in mind the assymptotic nature of shear wave reflections)

Enormous Data size: S-waves travel at about half the speed of P-waves, so a longer record length (sometimes10-12 sec) is required to record the converted wave data depending on target deep. Hence you have large data size, and data management capabilities. These have been overcome with inproved hardware and fast computing technolgies good time management.

The challenges are fading…

Oil and gas companies--- both large and small, are increasingly recognizing the values derived from using multi-component seismic techniques. Indeed, the future is bright.

Investment appears to be on the increase.

The future is bright!

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Teaser check it out!

Author : JIM, Mundy Obilor 13th Aug., 2011

All Rights Reserved.

P- and S- wave reflections

Consider two hrizons A and B, and using the standard PP and PS time-ratio method,

1. Derive the equation shown above which relates the the Vp/Vs ratio as a function of 2-way travel time (Hint: assume a distance d between the two horizons and take Θ and Ø as interface reflected angles for P and S reflections)

2. Calculate the average interval Vp/Vs ratio if time-thickness between the two horizons for PP and and PS data are 2000ms and 3800ms respectively