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Chapter 11 Structural Interpretation of Seismic Reflection Data

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Chapter 11 Structural Interpretation of Seismic Reflection Data Chapter 11 Structural Interpretation of Seismic Reflection Data Introduction Geology presents us with a basic problem. Because rocks are opaque, it is very difficult to see through them and thus it is difficult to know what is the three- dimensional geometry of structures. This issue is particularly obvious when con- structing cross sections as in the previous chapters.C It is too expensive to drill close- ly-spaced holes in order to constrainD the geometry of rocks in the subsurface and, A B commonly, there are some structures that have no surface expression and thus can- not be projected to depth from surface outcrop. To the rescue comes a geophysical technique for remotely sensing the subsur- face using sound waves. Seismic reflection profiling has been standard practice in the oil and gas industry for more than 50 years and is the most commonly used technique for mapping the subsurface. Some of the most profound structural ob- servations about our planet — thrust belts have decollements, low-angle normal faults exist — are best demonstrated with seismic reflection data. In this chapter, we will give you the bare-minimum background needed in order to begin using this uniquely useful type of data as structural geologists. CHAPTER 11 SEISMIC REFLECTION DATA Echo Sounding Seismic reflection profiling is exactly analogous to echo sounding (Fig. 11.1). Lets examine the simple case of making an echo first to see what the important pa- rameters are. Why do you get a reflection or an echo? You get an echo because the densities and sound velocities of air and rock are very different. If they had the same density and velocity, there would be no echo. More specifically, the P-wave velocity is: E " velocity = V = (11.1) ρ where E = Youngs Modulus and 9 is the density. We tend to think of velocity in- creasing with density but you can see that in Equation (11.1) density is in the de- nominator. There are a few rock types that have high velocity but low density; the most common one is salt. The acoustic impedance of a material is its density times the velocity of sound in the material, ρV. The reflection coefficient is: amplitude of reflected wave ρ V − ρ V " reflection coefficient = R = = 2 2 1 1 (11.2) amplitude of the incident wave ρ2V2 + ρ1V1 This is what tells us how strong the reflection will be. If you were in Yosemite Val- ley making echo by shouting at the granite walls of the valley, the reflection coeffi- cient, R 3 0.999944. In other words, almost all of the sound is reflected back at you ρair, vair ρrock , vrock a very small Figure 11.1 — Anatomy of amount of an echo. It is the density sound and velocity contrast continues across the air-rock inter- into the rock face that produces the echo. most sound is reflected back to the listener rock wall MODERN STRUCTURAL PRACTICE "238 R. W. ALLMENDINGER © 2015-16 CHAPTER 11 SEISMIC REFLECTION DATA (a) in the Earth (b) what we measure ground! time sound was made surface depth st t 1 subsurface time to go down to the ime interface 1st layer and return time to go down to the nd 2nd layer and return 2 subsurface interface Figure 11.2 —(a) We make a sound (red star) on the surface of the Earth, the sound then goes down to different interfaces within the earth and some of that sound is bounced back and recorded on the surface. (b) At the surface we can only measure the time that the sound was made and the time that it takes for the sound to go down to each interface and come back to the surface from the interface, but a very small proportion actually continues into the rock (Fig. 11.1). In seismic reflection profiling, what do you actually measure? If you think about the Yosemite example again, we could measure the time that we made the sound and the time that we recorded the echo. The time difference is a function of the velocity of sound in the air and twice the distance between us and the wall be- cause the sound has to go from us to the wall and come back again. When you make an echo, the source of the sound (your mouth) and the receiver of the sound (your ears) are essentially in the same place. As we will see below, in seismic reflection profiling, the source of the sound (an explosion, a vibrating truck, etc.) and receiver (the geophones) are offset from each other but we process them as if they were in the same place. The above example highlights three important things about seismic reflec- tion profiling (Fig. 11.2): • Measure time, not depth, • The time recorded is round trip or two-way time, and • To get the depth, we must know the velocity of the rocks. MODERN STRUCTURAL PRACTICE "239 R. W. ALLMENDINGER © 2015-16 CHAPTER 11 SEISMIC REFLECTION DATA 3 km/s 3 km depth 6 km/s 6 km 1 s time 2 s 3 s 6 km horizontal reflector Figure 11.3 — Top: geologic section showing a slow velocity sedimentary basin on the right hand side and continuous high velocity material on the left side. Bottom: a time section showing the distortion produced by the laterally varying velocity. Velocities of rocks in the crust range between about 2.5 km/s and 6.8 km/s. Most sedimentary rocks have velocities of less than 6 km/s. These are velocities of P-waves or compressional waves, not shear waves and most seismic reflection sur- veys measure P- not S-waves. Because we measure time and not depth, although seismic reflection profiles resemble geologic cross-sections, they are not. They are a spatially distorted picture of the earth because rock velocities vary, both laterally and vertically. To illustrate impact of laterally varying velocities, consider the case depicted in Figure 11.3. This case is commonly encountered in rift provinces where sedimentary basins al- ternate with older, higher velocity rocks in mountain ranges. The horizontal inter- face at 6 km depth looks like it has a step in it on the time section at the bottom be- cause the sound waves travel more slowly to the 6 km interface on the right hand side than they do on the left hand side. This is just one of many types of artifacts for which the structural geologist/seismic interpreter needs to be aware! MODERN STRUCTURAL PRACTICE "240 R. W. ALLMENDINGER © 2015-16 CHAPTER 11 SEISMIC REFLECTION DATA Common Mid-Point (CMP) Method In the real earth, the reflectivity at most interfaces is very small, R ≈ 0.01, and the reflected energy is proportional to R2. Thus, at most interfaces ~99.99% of the energy is transmitted and 0.01% is reflected. This means that your recording system has to be able to detect very faint signals coming back from the subsurface. An additional complication is that, because the source usually involves a lot of en- ergy, it must be offset from the receivers. Data Redundancy and Signal to Noise Ratio The standard strategy for dealing with very weak signals is to increase the signal-to-noise ratio. If you measure something many times, the signal in which we are interested should add together constructively (because it is the same every time) whereas the random noise should add together destructively (because it is dif- ferent every time). st (a) 1 Shot source receivers (geophones) reflecting interface Figure 11.4 — Cross one ray through point sectional geometry of a nd (b) 2 Shot standard common mid- point seismic reflection survey where the sound is emitted (a “shot”, reflecting the fact that dynamite used to be reflecting interface the most common two rays through point source) at three differ- rd ent stations. (c) 3 Shot reflecting interface three rays through point MODERN STRUCTURAL PRACTICE "241 R. W. ALLMENDINGER © 2015-16 CHAPTER 11 SEISMIC REFLECTION DATA The geometry by which this is achieved is shown in Figure 11.4. Each of the three panels corresponds to one “shot” (i.e., one episode of making noise at a sta- tion on the surface). The black dot, and each point on the reflector with a ray going through it, is a common midpoint (CMP), sometimes referred to as a common depth point (CDP). Notice that there are twice as many CMPs as there are sta- tions on the ground (where the geophones are). That is, there is a CMP directly underneath each station and a CMP half way between each station (hence the name “common midpoint”). As the source is advanced in the direction of the pro- file, each midpoint on the reflector of interest gets sampled multiple times. In a complete survey, the number of traces through each midpoint will be equal to one half the total number of active stations at any one time (not including the ends of the lines where there are fewer traces and assuming that the source moves up only one station at a time). The number of channels in the recording system determines the number of active stations. Most modern seismic reflection surveys use at least 96 (and sometimes as many as 1024 channels or more), so that the number of traces through any one CMP will be at least 48. This number is the data redundancy, of the fold of the data. For example, 24 fold or 2400% means that each depth point was sampled 24 times. Sampling fold in a seismic line is the same thing as the “over-sampling” which you see adver- tised in compact disk players.
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