Gas hydrates, bottom-simlating reflectors (BSR)

Two views of the same portion of the methane-hydrate stability zone. (a) High-resolution image based on a survey operated at 250 to 650 Hz and (b) image resulting from conventional seismic survey operated at 10 to 80 Hz. Notice that a coherent BSR is apparent from conventional seismic data but not from the high-resolution data.

The compressional velocity of pure hydrate is believed to be similar to that of ice, but the exact value has not been agreed upon. The acoustic velocity of a hydrate-bearing layer is also believed to be high, higher than in fluid-filled sediment. As a result, the contact between a hydrate-rich layer and a gas-filled layer can act as a prominent seismic reflector. These reflectors, which occur at the base of the hydrate zone, are known as bottom-simulating reflectors (BSR) because they parallel the water bottom-sediment reflector. So BSRs result from the compressional velocity contrast between the gas hydrate-cemented zone and the underlying sedimentary zone with pores containing water or water and free gas. As the shape of BSRs tends to track the shape of the sea bottom, sometimes BSRs appear in seismic data as multiple reflections. Therefore, proper multiple attenuation is required to ensure that BSRs are not distorted by the multiple-attenuation process or that they do not end up being interpreted as multiples.

Like the values of the acoustic velocity of hydrates, the characterization of BSRs is also an open question. Early investigations suggest that BSRs are identifiable in conventional seismic data (ranging from 5 Hz to 70 Hz) but not in high-resolution data, in which frequency can go up to 650 Hz. These observations suggest that BSRs are not a well-defined interface but rather a set of small-scale heterogeneties whose averages depend on the frequency content of the source signal. Understanding seismic propagation through small-scale heterogeneities may perhaps help in the identification of BSRs, which leads to the identification of gas hydrates. (Adapted from Wood et al., 2002.)

Other Images in Chapter 1

An example of the sailing path of a marine vessel in a towed-streamer survey

Images - Chapter 8 An example of the sailing path of a marine vessel in a towed-streamer survey. Note that the time for turning from one sailing line to another is about nine hours for vessels carrying streamers that are 10 km long. The dotted line indicates the turning legs of the sailing path.

This figure illustrates a typical sailing path of a 3D survey; the vessel travels back and forth, shooting and collecting data along many parallel lines, resulting in seismic data generated along lines 25 to 50 meters apart. Note that it takes about nine hours to turn from one sailing line to another for a vessel carrying 10-km-long streamers. Today, data are recorded even when the vessel is turning.

A shot diagram

Images - Chapter 7 A display of source and receiver distribution of a 2D seismic line in the so-called shot diagram. The rows corresponding to common-shot gathers and columns to common-receiver gathers. The diagonal is the zero-offset section, and all the other lines parallel to the diagonal are common-offset gathers (also known as common-offset sections). The lines perpendicular to the diagonal are the CMP gathers.

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

Interference noise

Images - Chapter 7 An illustration of interference noise in seismic data before and after. This figure shows the stack of seismic interference noise contaminated shots from another line in the Gulf of Mexico. Interference noise is clearly visible. Attenuation of seismic interference noise can be achieved by the use f-x prediction filters. Courtesy of Western Geco.

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.