Sets of reflected arrivals from individual interfaces are recognizable by their characteristic hyperboloic form. The late –arriving, high amplitude, low frequency events, defining a triangular –shaped central zone which reflected arrivals are masked, represent surface waves (ground roll). In this case, time runs along the vertical axis and distance from the source along the horizontal axis. At each appropriate shot and receiver distance, we have plotted the seismogram (record of ground motion at that location). In this particular experiment, receivers are located at five meter distance intervals. Plots such as these are usually referred to as shot records . The advantage of looking at shot records is that you can see how the time of arrivals varies as distance from the shot varies. This variation in the time versus distance is commonly referred to as moveout . Arrivals with large moveouts dip steeply on shot records. Those with a small amount of moveout dip less steeply. Determining the shape of the travel-time curves versus offset will be our primary task in the refraction seismic method. Thus, although we are recording the time history of ground motion at a number of stations, for the refraction method, the only thing we will be interested in extracting from these records is the time of arrival of the first wave to be recorded at each geophone. For the example shown above, this arrival would be associated with the direct wave for offsets less than 275 meters , and it would be associated with the head wave for offsets greater than 275 meters. As we will show later, determining these times from your recorded seismograms is not always easy.
ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS
Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Exploration: Fundamentals 2
Previous Lecture Huygens's Principle Fermat's Principle Travel-time Graph Estimates of Seismic Velocity Reflected/Refracted waves Model Calculation for simple, horizontal, two layers Ray Paths Snell's law-Critically Refracted Arrival Seismic Refraction Behavior of refracted ray on velocity changes
A ray incident on surface results in 3 reflected and refracted rays. If the seismic velocities in medium 1 are α = 6.5 km/sec , β = 3.8 km/sec , what are the seismic velocities in medium 2? What type of material is medium2? Identify rays #1, #2, #3, #4 as P or S waves. Homework Due to Wednesday 12° 15° 20° 35° #1 #2 #3 medium 1 medium 2
Refracted Ray and Angle The angle of refraction increases as the angle of incidence increases.
Energy Return and Critical Angle A critically refracted wave, traveling at the top of the lower layer with velocity V 2 , leaks energy back into the upper layer at the critical angle ( θ 2 ) Lillie, Whole Earth Geophysics, Fig 3.25
Seismic Reflection Reflection occurs when Z 1 differs from Z 2 , where Z Acoustic impedance which is product of density and velocity V-shaped ray paths for a compressional wave from a source to 6 receivers, reflected from a horizontal interface. Lillie, Whole Earth Geophysics, Fig 3.28 =Z 1 =Z 2 Lillie, Whole Earth Geophysics, Fig 3.28
Reflection equation for a reflection hyperbolae:
These exercises are designed to illustrates some of the basic characteristics of wave propagation in a single layer model use ray-tracing concepts to determine the arrival times of particular events. These exercises require that you construct the time- distance plot for the given model. In addition to constructing the time-distance plots, Due to Next Week INTRODUCTORY RAY TRACING EXERCISES GENERAL INSTRUCTIONS
http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/deriv1.html Single-Layer Model Equations
Be sure to do the following 1) label all plotted curves, 2) label all relevant points, and 3) in a paragraph or so discuss the significance and origins of the interrelationships portrayed in the resultant time-distance plots