Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Lecture 2 Wave Types and Travel Paths - Example Applications

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So - to see additional detail in the ground motion - to measure the fractional motion - you need to increase the dynamic range of the recording system. The engineering seismograph we demonstrated in class today is restricted primarily to the shallower applications since events that have traveled great distances will have very small amplitude (less than 1on the scale of ±

Liner and Liner, 1995

GPR Profile E = mc 2 Velocities are in the 1 foot per nanosecond range - i.e seconds.

Liner and Liner, 1995 Processed GPR profile

Liner and Liner, 1995 GPR data (right) compared to the outcrop exposed beneath the survey line.

Miller et al Seismic and GPR methods both record waves that have been reflected from subsurface interfaces. In the one case (GPR) these waves are electromagnetic (and much faster), in the other (Seismic) they are acoustic or mechanical waves.

In general V R <V L <V S <V P But this is not strictly true. The Love wave is a surface wave and its velocity will be equal to the shear wave velocity in the upper medium. The Love wave like the Rayleigh wave is also a dispersive wave. That means that the deeper extend of the Love wave usually moves more quickly with the shear wave velocity of that deeper medium. Shear waves in the beneath the surface layers are generally much faster than those at the surface, so in application, the shear waves that we are concerned with generally have higher velocity than the Love waves.

Love waves also tend not to be recorded in a conventional seismic survey where the interest is primarily in the recording of P-waves. The geophones used in such surveys respond to vertical ground motion and so generally do not record the side-to-side vibrations produced by the Love waves. Rayleigh waves produce large vertical displacements and are a significant source of “noise” to those interested in recording the deeper reflection events.

Wave Packet

Regional View - Vibroseis Line Intermediate scale -higher frequency, higher resolution, weight drop line. Detailed view with sledge hammer. High frequency - high resolution.

How do waves move from one place to another?

Miller et al. 1995

INTRODUCTORY RAY TRACING EXERCISES (I -VI) GENERAL INSTRUCTIONS 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 (I - VI). In addition to constructing the time-distance plots,

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

Reading Assignment Read over derivation of direct and refraction travel time equations ( pages 57 through 61). Read over the derivation of the reflection travel-time equation (pages ).