Ch 2 - Fundamentals of Seismic Exploration I. Seismic waves and wave propagation view waves as emanating spherically from a point source pebble in water earthquake we need to know about: wave type (surface vs body) factors controlling transmission of energy wave speed
Y = mx + b
where E = slope = Young’s modulus of elasticity
Some rules of thumb on p.18-19, table of velocities of materials
saturated veloc>unsat (pore space filled)
consol sed veloc>unconsol
unwthrd rx veloc>wthrd
unfrac rx veloc>frac
think about these….why do these facts make sense?
Note that velocities fall into broad groups but are non-unique:
500 m/sec - dry unconsolidated
1500 m/sec - wet unconsol
4000 m/sec for sed rx
6000 m/sec for ign rx
return to looking at frequency, wavelength, veloc
Veloc = dist = x f
play with couple examples - given 1000 m/sec veloc, look at 2 diff seismic sources - 10 Hz, 100 Hz - what’s the wavelength of the signal in each of these cases??
Why is this important? Wavelength dictates vertical resolution, ability to “see”detail
Tough to determine what signal is what at the first part of the seismic record…
in gen, looks like the direct wave is the first to hit the geophones out to about 45 m, then the headwave is the fastest
We’ll be coming back to this…for now, get the idea that diff p-wave paths exhibit diff velocities, and the “first arrival” at one geophone may not be from the same raypath as the first arrival at another geophone.
Also, DO note that each straight line segment has a unique slope of time/distance….the inverse of velocity. This shows that these straight line segments represent ray paths with unique velocities.