Groundroll

Shot record showing extreme groundroll obscuring reflection events.

A significant amount of generated seismic energy is trapped in the low-velocity layer in the form of surface waves which travel horizontally along the earth's boundary.

These surface waves (ss described in Chapter 3, there are two types of surface waves: Love and Rayleigh waves. In petroleum seismology, we are interested only in Rayleigh waves) which are known as groundroll, spread out from a disturbance like the ripples seen when a stone is dropped into a pond. This figure shows a shot gather of real land data. We can see that surface waves appear as coherent events which can completely cover the desired reflected data. Notice that groundroll alignments are straight, just like direct waves, but they have much lower apparent velocities. The theory of wave propagation on the free surface of a semi-infinite elastic homogeneous solid, developed by Lord Rayleigh in 1885 and described in Chapter 3, shows that groundroll propagates with velocity V_R = 0.92 V_S, where V_S is the shear velocity.

In general, a groundroll builds up and decays slowly, and its energy is high enough to obscure a significant number of reflections.

An interesting observation that we can make is that when the source is located below the low-velocity layer, a groundroll energy is small compared to the case in which the source is located in the LVL or at the surface. Actually, this is one of the advantages of a buried explosive source (when explosives are buried below the LVL) over the vibroseis source, which is located at the surface. So in general, groundroll energy generated by the explosive source is not as high as that generated by vibroseis.

The need to drill holes to reduce groundroll when using explosives is also a disadvantage in terms of turnaround time. The drilling of shot holes is very time-consuming.

Other Images in Chapter 7

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.