U/D decomposition

(a) Two-layer model above a half-space. The events are labeled by ``Dnnn'' and ``Unnn,'' where ``D'' and ``U'' mean that the events are downgoing or upgoing, respectively, below the sea floor, and ``nnn'' indicates the traveltime in ms. (b) Modeled data of pressure, the vertical component of the particle velocity scaled by the P-wave impedance of the sea floor, and the upgoing component of the vertical traction below the sea floor. The last trace shows the effect of free-surface multiple elimination.

The second model here includes layer of thickness 125 m below the water layer. The P-wave velocity in this layer is 2000 m/s. The reflection coefficient of the sea floor is again R_PP=0.45, and the reflection coefficient of the interface below is 0.16. For this model the zero-offset pressure and the vertical component of the particle velocity scaled by the sea floor P-wave impedance are displayed in Figure 9-4b. Observe that this simple model gives a quite complicated seismic response. To analyze the seismograms, we have in (a) sketched the events up to 625 ms. D and U denote downgoing and upgoing events, respectively, just below the sea floor. Computing the upgoing component S₃^(U) eliminates all downgoing events just below the sea floor. The upgoing primary, U225, with a traveltime of 225 ms is clearly present. The water-layer multiples D300, D500, D700, etc., are clearly eliminated. The internal multiple U350 arriving at 350 ms is hardly visible because it has reflected twice at the lower interface with a small reflection coefficient. Even if the model is simple, it has several noteworthy characteristics. Receiver ghosts and reverberations are downgoing events just below the sea floor and therefore attenuated. However, the free surface produces multiples that are upgoing events below the sea floor. Such free-surface-related events like U425, U550, and U625 are sometimes called source-side multiples. They remain as part of the upgoing wavefield S₃^(U). In Chapter 10 we will derive algorithms that eliminate all free-surface-related multiples. Free-surface multiples are events that have at least one reflection at the free surface. The last seismogram in (b) shows the response of the model when all free surface related multiples are absent. Compared to the upgoing field S₃^(U) just below the sea floor the free-surface demultipled seismogram is almost free of multiples. Free-surface multiple attenuation is therefore a better demultiple tool than U/D decomposition below the sea floor.

A second notable characteristic of the model is that the upgoing primary, U225, in (b) is weaker on the pressure recording than on the particle-velocity recording scaled by P-wave impedance.

Recall that this particular scaling of the particle velocity makes the downgoing events just below the sea floor equal in amplitude between the pressure and particle-velocity seismograms. Stated differently, relative to this primary amplitude, the downgoing multiples are stronger on the hydrophone than on the geophone. To see why this is so, we make the following observations. The pressure is recorded from a hydrophone placed just above the sea floor. Since the vertical component of the particle-velocity field is continuous at the sea floor, we may, for the present analysis, without loss of generality, assume that the vertically oriented geophone is also located just above the sea floor. Consider a downgoing multiple of unit amplitude hitting the sensors. Since the sensors are sitting infinitesimally above the interface when they measure the downgoing event, they will at the same time measure an upward-reflected event with amplitude strength equal to the reflection coefficient R_PP of the sea floor. Since the hydrophone sensor is isotropic (insensitive to the wave's direction), it measures upgoing and downgoing events without distinction---that is, the amplitude sum, 1+R_PP. On the contrary, the geophone is sensitive to orientation and measures upgoing and downgoing events with an opposite sign. In this particular case the geophone measures the amplitude 1-R_PP. With R_PP>0, it follows that the downgoing multiple is stronger on the pressure seismogram than on the vertical component of the particle velocity seismogram.

Other Images in Chapter 9

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.