Seismic data interpretation, common error & solution.pptx

Seismic data interpretation
Common error & Solutions

1. By: Sharad Kumar Mishra, Geophysicist 1 Seismic data interpretation Common error & Solutions

2. By: Sharad Kumar Mishra, Geophysicist 2 Seismic waves: Types and their Character Seismic waves can be classified in to two classes: Body waves & surface waves Body waves which propagate through the rock matrix and further can be subdivided in to two classes as P wave & S wave) as per their particle motion in the rock matrix during its propagation. Surface waves which travel along the surface of the medium. Velocities of P- and S-waves ( Vp and Vs) are determined by several aspects of a material called elastic constants (or moduli). Velocities of surface waves are governed mainly by the shear modulus of materials. S waves are transverse waves which involve movement of the ground perpendicular to the velocity of propagation. They travel only through solids, and the absence of detected S waves at large distances from earthquakes was the first indication that the Earth has a liquid core. S waves travel typically 60% of the speed of P waves. They are typically more damaging than the P waves because they are several times higher in amplitude. The waves which move the surface up and down are called Rayleigh waves and are sometimes described as "ground roll". Waves whose amplitude of motion is parallel to the surface are called Love waves. Rayleigh waves travel at roughly 90% of the speed of the S waves. Love waves involve the motion of the ground side-to-side, perpendicular to the propagation velocity. They usually travel slightly faster than the Rayleigh waves. Love waves cannot exist in a uniform solid, and can only occur when there is a general increase of S- wave velocity with depth.

3. 3 Physical laws that governs the seismic velocities: Body waves are reflected and transmitted at interfaces where seismic velocity and/or density change, and they obey Snell's law. The velocities of P- and S-waves are given below in terms of the density (ρ) and elastic coefficients of a material: Vp = √((K+4/3G)/ρ) Here K (bulk mod.) & G (Mod. of Rigidity) both are always positive, Therefore Vp > Vs Vs =√(G/ρ) By: Sharad Kumar Mishra, Geophysicist

4. 4 Characteristic of seismic waves: Body waves are reflected and transmitted at interfaces where seismic velocity and/or density change, and they obey Snell's law. By: Sharad Kumar Mishra, Geophysicist

5. 5 Regional effect on velocity & density function Where ρ : density of rock Vp : P wave velocity Vs: Shear wave velocity μ: lame constant Acoustic waves ( P wave & S wave) in subsurface are affected by density of rock matrix in following ways: There are a few more general rules to the velocity ranges of common materials: o Unsaturated sediments have lower values than saturated sediments. o Unconsolidated sediments have lower values than consolidated sediments. o Velocities are very similar in saturated, unconsolidated sediments. o Weathered rocks have lower values than similar rocks that are unweathered . o Fractured rocks have lower values than similar rocks that are unfractured . By: Sharad Kumar Mishra, Geophysicist

6. 6 Regional velocity & density function in the subsurface K & G are correlated with density ρ in such a way that elastic wave velocity increases with depth in subsurface. How ever when brine replaces gas in a rock , the density increases without any increase in shear modulus, and shear velocity drops. Thus when other factors are similar, velocity varies inversely with density. Vp = √((K+4/3G)/ ρ) By: Sharad Kumar Mishra, Geophysicist

7. Common mistakes during seismic data analysis 7 seismic sections that resemble geologic cross-sections, geologists and geophysicists not experienced in seismic interpretation are often greatly tempted to read geology more or less directly from the seismic section. In this process they ignore few crucial things , like in complex geological setup rapid changes in lithology or velocity, or irregular surface or near-surface conditions, serious errors may result from the literal interpretation of seismic sections. Pitfalls associated with velocity occur because seismic data are presented in travel time rather than depth. 2. Pitfalls associated with geometry occur because reflections from a three-dimensional space are plotted in a two-dimensional section. 3. Pitfalls associated with recording and processing occur because all recorded events are not of geologic origin, and improper processing can mask geology. Courtesy:   https://doi.org/10.1190/1.9781560802365.ch1 By: Sharad Kumar Mishra, Geophysicist

8. 8 As the seismic wave passes through the fast layer like salt dome, volcanic intrusion, sills or dike incased between low velocity sediments we find pull up underneath it. 1.Common pit falls in interpretations due to velocity: By: Sharad Kumar Mishra, Geophysicist Pay sand Here in this seismic section first well A was hydrocarbon producer from the pay sand. Two developments wells B & C, which appears to be structurally up at pay level with respect to oil producer well A, were drilled but after drilling they were found structurally down with respect to well A and both are water bearing. This is only due to low velocity anomaly caused by channel fill sediments just above the pay level at well-A. Low velocity channel fill sediments

9. 9 a geological model of a normal fault and its seismic response are illustrated. Theoretically, due to the downward relative movement of the hangingwall , intervals with quite different interval-velocities are juxtaposed, which has important consequences on the seismic response. The footwall reflectors below the fault plane (area of a lateral velocity changing) will be pulled down, since, at same level, the velocity interval in the hanging wall is smaller. By: Sharad Kumar Mishra, Geophysicist 1.Common pit falls in interpretations due to velocity: Due to the lateral changes in velocity as we pass across a fault plane we will have distortion in the gathers causing mis -stacking and distortion of the reflectors, creating a shadow area around the fault.

10. 10 On this seismic line from offshore Angola, the pull-down of the yellow marker (bottom of the evaporitic interval) is induced by the lateral change of the interval-velocity created by the normal fault which limits a Upper Tertiary depocenter. Indeed, such a fault put limestones (upthrown block) and shales (downthrown block in juxtaposition. The geological model of a reverse fault and its likely seismic response is depicted. As illustrated, the reflectors of the footwall are pulled-up due to a lateral change of the interval-velocities. By: Sharad Kumar Mishra, Geophysicist 1.Common pit falls in interpretations due to velocity:

11. 11 The seismic response of a mathematical model of a reverse fault, in which the sediments of the hangingwall are denser than those of the footwall, corroborates the hypothesis that the reflectors below the fault plane are pulled-up creating the common illusion of an anticline structure This seismic line from onshore France illustrates a seismic artifact associated with a thrust fault, that is to say, an apparent anticline structure under the reverse fault plane. In spite of the evidence of the seismic pull-up, “ explorationists ” drilled a wildcat on such an artifact thinking that they were testing a large under-thrust structural trap. Actually, in certain basins, as we will see later, there are prolific petroleum traps under reverse and thrust faults, hence explorationists must always test their interpretations by time-depth conversions. By: Sharad Kumar Mishra, Geophysicist 1.Common pit falls in interpretations due to velocity:

12. 12 Note all time-depth conversions corroborate anticline structures below the thrust faults. In this particular example, coming, as the previous line, from the Colombia foothills, a nice antiform structure (that is to say a potential structural trap) is recognized on the pre-stack section. The same potential structure is also recognized on the pre-stack migrated version, just under the fault plane, which should make the interpretation questionable. Finally, the pre-stack migrated depth versions strongly falsify the hypothesis of a sub-thrust structure. Actually, the sub-thrust sediments are undeformed and not shortened. This seismic line through Cuisiana #2A (discovery well), in the Colombia foothills, was drilled by an international consortium composed of BP, Total and Triton, in order to test the anticline structure under upper thrust-faults. However, before drilling, several time-depth conversions corroborated the hypothesis advanced by certain explorationists that the sub-thrust antiform was a real compressional structure and not a seismic artifact induced by the hanging wall. By: Sharad Kumar Mishra, Geophysicist 1.Common pit falls in interpretations due to velocity:

13. 13 On this reef geological model, above a planar limestone sole (light blue), a reef with a compressional wave velocity of 5490 m/s, is laterally bounded by shaly sediments (yellow) with a much lower velocity (3660 m/s), which are overlain by even slower sediments (brown interval, 3050 m/s). The seismic answer of such a model is roughly depicted on the right. The horizon associated with the bottom of the reef shows a significant pull-up. Notice that in the geological model the compressional wave velocity, in the blue interval (limestone with a local reefal development) changes significantly. It is much higher (around 5500 m/s) in the reef than in the surrounding sediments. Such a reef is supposed to be tight. The seismic response of such a model, on the right part of the figure shows that not only the bottom of the reef, but all others markers below are pulled-up. By: Sharad Kumar Mishra, Geophysicist 1.Common pit falls in interpretations due to velocity:

14. 14 (A) with flat lying events the CMP is midway between the shot and the receiver. (B) if the reflector is dipping the CMP does not lie half way between the shot and receiver, but is smeared across the dipping reflector. This causes the velocities between flat lying events and dipping events to be different (taken from Liner, 2002). The lens effects caused by some rock layers such as salt which focuses some raypaths and diffuses others. This is why with subsalt we may have only small incident angles and it can be difficult to do subsalt AVO. 2. Pitfalls associated with geometry occur: By: Sharad Kumar Mishra, Geophysicist

15. 15 On the left are the seismic events before migration with the diffractions. The termination of the seismic reflectors creates a point source and diffractions come off of it. After the migration the diffractions are collapsed and the fault is apparent. One of the classical pitfalls of seismic data with is what is called “bow-tie” features, because they look like “bow-ties”. Before we collapse the “bow-tie” it appears at first that it will be two anticlines but in fact it becomes a syncline. By: Sharad Kumar Mishra, Geophysicist 2. Pitfalls associated with geometry occur:

16. 16 By: Sharad Kumar Mishra, Geophysicist 3. Pitfalls associated with recording and processing occur : Shallow level data gap Poor migration of data due to insufficient offset spread during recording Poor imaging below the sub basaltic layer The data gap during seismic data acquisition produces some artifacts like vertical strip or vertical fault as can be seen in first seismic section. In case of basaltic exposure on the surface, imaging below sub-basalt requires deep attention during processing. This may produce highly fictious images. In the case of complex geology seismic designing prior to data acquisition is also very crucial. In case of insufficient offset and azimuth seismic data may present very unrealistic subsurface image.

17. 17 By: Sharad Kumar Mishra, Geophysicist 3. Pitfalls associated with recording and processing occur : The data gap during seismic data acquisition produces some artifacts like vertical strip or vertical fault as can be seen in first seismic section. In case of basaltic exposure on the surface, imaging below sub-basalt requires deep attention during processing. This may produce highly fictious images. In the case of complex geology seismic designing prior to data acquisition is also very crucial. In case of insufficient offset and azimuth seismic data may present very unrealistic subsurface image. Vertical fault like feature due to data gap at shallow level. Basalt on Surface Poor imaging due to insufficient offset, poor azimuth and less effective energy source