Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Energy Loss and Partitioning
Tom Wilson, Department of Geology and Geography Look over ray-trace assignments Continue discussions of absorption Reflection and transmission coefficients AVO/AVA – pre-stack attributes 2-layer refraction problem – an overview Basic Seismic Interpretation: identifying stratigraphic sequences and tectonic history Objectives for the day
Tom Wilson, Department of Geology and Geography
1) label all plotted curves, 2) label all relevant points and 3) Note (comment on) basic relationships between events observed in the time-distance plots (see following)
Tom Wilson, Department of Geology and Geography If you wish you can do Exercise II and III using Excel. In exercise II comment on the origins of the differences in the two reflection hyperbola? What is their relationship to corresponding direct arrivals.
Tom Wilson, Department of Geology and Geography For Exercise III, explain the differences observed in the arrival times of the reflection and diffraction observed in the shot record. Why does the diffraction event drop below the reflection?
Tom Wilson, Department of Geology and Geography Back to the Absorption Problem > Review When we set a spring in motion, the spring oscillations gradually diminish over time. In the same manner, we expect that as a seismic wave propagates through the subsurface, energy will be consumed through the process of friction and there will be conversion of mechanical energy to heat energy. We guess the following - there will be a certain loss of amplitude dA as the wave travels a distance dr and that loss will be proportional to the initial amplitude A. i.e.
Tom Wilson, Department of Geology and Geography In order to solve for A as a function of distance traveled (r) we will have to integrate this expression - is a constant referred to as the attenuation factor In the following discussion,let
Tom Wilson, Department of Geology and Geography
Mathematical Relationship Graphical Representation
Tom Wilson, Department of Geology and Geography - the attenuation factor is also a function of additional terms - is wavelength, and Q is the absorption constant 1/Q is the amount of energy dissipated in one wavelength ( ) - that is the amount of mechanical energy lost to friction or heat. The physical significance of
Tom Wilson, Department of Geology and Geography is also a function of interval velocity, period and frequency
Tom Wilson, Department of Geology and Geography is just the reciprocal of the frequency so we can also write this relationship as
Tom Wilson, Department of Geology and Geography Smaller Q translates into higher energy loss or amplitude decay.
Tom Wilson, Department of Geology and Geography Higher frequencies are attenuated to a much greater degree than are lower frequencies. increase f and decrease A 8.7 is 20log(e)
Tom Wilson, Department of Geology and Geography When we combine divergence and absorption we get the following amplitude decay relationship The combined effect is rapid amplitude decay as the seismic wavefront propagates into the surrounding medium. We begin to appreciate the requirement for high source amplitude and good source-ground coupling to successfully image distant reflective intervals.
Tom Wilson, Department of Geology and Geography At a distance of 100 m from a source, the amplitude of a P-wave is mm, and at a distance of 150 m the amplitude diminishes to mm. What is the absorption coefficient of the rock through which the wave is traveling? (From Robinson and Coruh, 1988)
Tom Wilson, Department of Geology and Geography But we are not through - energy continues to be dissipated through partitioning - i.e. only some of the energy (or amplitude) incident on a reflecting surface will be reflected back to the surface, the rest of it continues downward is search of other reflectors. The fraction of the incident amplitude of the seismic waves that is reflected back to the surface from any given interface is defined by the reflection coefficient (R) across the boundary between layers of differing velocity and density. More ways to loose or partition energy
Tom Wilson, Department of Geology and Geography The transmitted wave amplitude T is
Tom Wilson, Department of Geology and Geography Z 1 and Z 2 are the impedances of the bounding layers.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography You can also have a look at AVO.xls linked on the class site
Tom Wilson, Department of Geology and Geography See Ostrander, 1984, Plane-wave reflection coefficients for gas sands at non-normal angles of incidence: Geophysics, vol 49 p
Tom Wilson, Department of Geology and Geography AVA response predicted for shallow strata in the Central Appalachians, Marshall Co., WV
Tom Wilson, Department of Geology and Geography The critical distance and the crossover distance. To determine the crossover distance set the direct arrival time equal to the critical refraction arrival time and solve for X cross
Tom Wilson, Department of Geology and Geography This is a new one, but pretty simple – see 3.2.4
Tom Wilson, Department of Geology and Geography We now have several equations which contain quantities that we can measure directly from the shot record and use to determine layer thickness - h 1. h1h1 ?
Tom Wilson, Department of Geology and Geography Reflection time intercept Refraction time-intercept Crossover distance Critical Distance
Tom Wilson, Department of Geology and Geography The two-layer refraction problem (see 3.3.1)
Tom Wilson, Department of Geology and Geography Time =distance traveled/velocity
Tom Wilson, Department of Geology and Geography For the details see 3.3.1
Tom Wilson, Department of Geology and Geography
CC Snell’s law for multiple layers
Tom Wilson, Department of Geology and Geography The velocity triangle
Tom Wilson, Department of Geology and Geography Expressing trig functions in terms of velocities
Tom Wilson, Department of Geology and Geography The end result, whereand …
Tom Wilson, Department of Geology and Geography What is the critical distance What is the relationship of the reflection from the base of layer 2 to the critical refraction from the top of layer 2
Tom Wilson, Department of Geology and Geography As x gets larger and larger the reflection from the base of layer 2 and the refraction across the top converge
Tom Wilson, Department of Geology and Geography
In the three layer problem the number of possible terms that could potentially be measured directly from the shot record includes - V 1, V 2 and V 3 two reflection time intercepts two refraction time intercepts one crossover distance, and two critical distances After reading chapter 2 consider the following questions and slides for Monday
Tom Wilson, Department of Geology and Geography How would you determine the thickness of layer 2 (h 2 )? From reflection arrivals From refraction events?
Tom Wilson, Department of Geology and Geography What variables can be determined from an analysis of the shot record V 1, V 2, V 3, t i1, & t i2, where the t i ’s refer to the reflection time intercepts
Tom Wilson, Department of Geology and Geography What variables can be determined from an analysis of the shot record V 1, V 2, V 3, t i1, & t i2, where the t i ’s refer to the refraction time intercepts
Tom Wilson, Department of Geology and Geography 1) We have assumed that our layers have successively higher and higher velocity. What happens if we have a velocity inversion - let’s say V 2 is less than V 1 and V 3 ? 2) Another assumption we have made here is that the refraction from the top of the third layer, for example, will actually show itself, and not get buried somewhere beneath the earlier refraction and reflections. This can happen if the 2 nd layer is too thin.
Tom Wilson, Department of Geology and Geography Paper sections for some of these lines will be provided in class Data are from the Barents Sea area and were used in last year’s Imperial Barrel Competition
Tom Wilson, Department of Geology and Geography As you start reading through Mitchum, Vail and Thompson’s paper come back to these sections and think about the sequence identification, internal reflection configuration, top and base reflection interrelationships
Tom Wilson, Department of Geology and Geography General location within the regional surveys shown at right Barents sea lines
Tom Wilson, Department of Geology and Geography
Data are from the Barents Sea area and were used in last year’s Imperial Barrel Competition Barents sea lines
Tom Wilson, Department of Geology and Geography
Problems 2.7 and 2.12 are due today Exercises I-III are due next Monday. The attenuation problem will be due next Monday. Read pages 81 through 100 in Chapter 3. Read “Seismic stratigraphy and global sea level, Part: The depositional sequence as a basic unit for stratigraphic analysis by Mitchum, Vail and Thompson for Monday