Strongest signals sought in Teal South multi-component seismic coupling

Standard ocean bottom cable (OBC) seismic data acquisition normally involves the recording of only two measurements at each location. Hydrophones measure the small changes in water pressure as a seismic wave passes, and a co-located vertical component geophone measures the small particle velocities (ground motion) associated with the same seismic wave. Measuring both quantities simultaneously allows spurious signals (such as the water column reverberation) to be rejected during data processing, and hence to improve the signal quality.

Multi-component OBC adds two extra components: two horizontal geophones, which are oriented perpendicular to one another. Generally, these two horizontal components will, for ease of fabrication and deployment, be oriented with one horizontal geophone aligned with the axis of the cable (the inline component) and one horizontal geophone aligned perpendicular to the axis of the cable (the crossline or X-line component). Adding these last two components produces multi-component or 4C (four component) seismic data.

4C data is valuable because it contains two entirely different sorts of seismic waves. The hydrophone and vertical component geophones primarily record the standard P-waves, while the two horizontal components primarily record C-waves, converted waves, which are P-waves on their downward journey to the reservoir but reflect as S-waves. The C-waves are especially sensitive to changes in reservoir pressure and the presence of aligned fractures in reservoirs. Additionally, C-waves can be used in some places where P-waves cannot (such as beneath gas chimneys).

Consequently, 4C data is desirable when performing time-lapse surveys for ongoing reservoir monitoring over the life of an offshore oilfield. The 4C data can be collected economically by laying down cables for each repeated seismic survey, or by permanently installing the cables (a more expensive approach). Repeatability of the seismic signal, and hence increased data quality, occurs when the cables can be left in place between surveys. There are still, however, thorny issues of cable security, logistics, economics, and in particular data quality related to multicomponent coupling of the geophones with the water bottom.

Geophone coupling

An important factor controlling the quality of shear-wave data is how well the geophones are coupled to the earth. Sensors must faithfully detect actual ground motions (move congruently with the ground). To accomplish this, they need to be firmly planted. On land, this is not a problem where equipment placement is directly observed, but on the seafloor, we must rely on serendipity to a certain extent because divers are not used to planting ocean bottom cables. One exception is to use a remote operating vehicle (ROV) to bury the cables, but this can be expensive.

When sensors move in a way that doesn't represent actual ground motion from subsurface signals, undesirable characteristics are imparted to the seismic data. These "poorly coupled signals" are typically in the form of slowly decaying oscillations, like the ringing of a bell, which interfere with the signals from our reservoirs at depth. Such responses need to be removed before data can be used for its intended purpose.

There are a number of things that can affect geophone coupling: properties of the earth, and physical properties of the acquisition system or cable. For example, soft mud on the seafloor, which is compliant, can result in a very different response than hard sand.

The mass or density of the acquisition system is very important. Systems that are heavier than the surrounding seafloor sediments will respond in a "sluggish" manner to ground motions, resulting in lower frequency oscillations. The most important property of the acquisition system is the area of contact. When the contact area is small, low frequency oscillations can also occur.

This last property (area) has an important effect on our multi-component data from ocean bottom cables. The in-line horizontal geophone can have a different response than the cross-line horizontal component. The contact width of the cross-line is much smaller than the in-line contact length along the cable. A consequence of the cylindrical geometry of cables resting on a flat seafloor is that horizontal motions perpendicular to the cable can cause an imperceptible amount of "roll." This torsional or rocking motion about the axis of the cable can result in spurious shear-wave signals on the vertical component.

Poor coupling is particularly bad for our multi-component recordings because the two horizontal components won't have an equivalent response from a single reflection. Both must respond in a similar manner to seismic reflections in order for their signals to be combined accurately during processing. Correction algorithms can help in many cases when coupling differences are not too severe, but it is better not to rely on such ameliorations.

Field operations

The Teal South field, located in Eugene Island Block 354 in the Gulf of Mexico, has been used as an experimental test site for comparing three different sensor emplacement methodologies. The experiments took place over spring and summer of 1999. These three approaches are:

• Drape onto seafloor followed by burial (trenching)
• Drape onto seafloor (with no modifications)
• Drape onto seafloor with a sandbag (wrapped around the sensor package).

The phrase "drape onto seafloor" distinguishes this acquisition approach from dragging into place. Dragging-into-place was not tested at Teal South during this experiment.

With the aid of a remotely operated vehicle (ROV), five of the seven cables were buried approximately three ft beneath the seafloor. A 150 hp water-jetting skid was connected underneath the ROV and used for cable burial. A benefit of the ROV post-lay burial was the establishment of the as-built cable tracking positions. The ROV position was tracked acoustically and tied to the ship positioning gear. This allowed the actual installed cable track to be plotted on the cable laying pre-plots.

Data quality

A detailed analysis of the coupling at many frequencies shows some surprising results. For the most part, all three cables have a very similar response except for some subtle differences. At almost every receiver station, the horizontal component data from the buried cables responded better at higher frequencies. This is desirable, since high frequencies provide better resolution images at the target reservoir.

However, burying the cable does not entirely eliminate shear-wave (mode conversion) energy on the vertical component. Although the torsional modes are reduced somewhat, the cross-line component does not exhibit better coupling at all the buried receiver stations. Cable width is still small compared to the cable length, buried or not.

Another interesting result is that wrapping the geophone packages with sandbags does not improve coupling at all stations as hoped. Although the cross-line component is slightly improved compared to no sandbags, the higher profile can aggravate the torsional modes on occasion.

Keeping in mind that all the differences are very slight, the data quality improvements obtained from burying cables in this case may not always justify the added expense. However, additional benefits of trenching also include increased physical security of the cables relative to commercial shrimping (trawling) operations. If the cables are to be deployed for an extended period of time, the combined benefits of better coupling, improved repeatability, and enhanced security could tip the balance in favor of cable burial.

Acknowledgments

Oceaneering provided trenching of cables to the Teal South Consortium pro bono. Data acquisition was provided to the consortium at reduced cost by Western Geophysical. The Teal South Consortium is open to participation by oil companies, service companies, and academic institutions.