GG 304L: Solid Earth & Planets Lab
LAB 9: Seismic Velocity Structure of the Kualoa Park Mound
Due Monday April 16
Now you are ready to apply what you have learned about using Linray to reveal the
shallow P-wave speed structure beneath the mound we surveyed at Kualoa Beach Park.
First, you will pick first arrivals, then you will fit your observations and generate 1-D Pwave speed profiles for each shot, and finally you will combine each of your four profiles
to construct a 2-D cross-section of P-wave speed beneath the mound. The diagram below
illustrates the geometry of our geophone arrays and the locations of our four shots. The
numbers shows coordinates in feet of our tape measure (solid line).
1) Picking first arrivals. Follow the same procedure done in Lab 6 using “PickWin”.
Please include a print out of the data and your picks.
2) Fit picks with computed ray paths and
arrival times to determine P-wave speed (Vp)
versus depth (z) for each shot.
Use
“linray_fit.m” to fit your travel-time versus
distance picks. If you picked what I picked in the
records you should be able to make out at least
two, or possibly three distinct slopes in the travel
time-distance plots for each record. This suggests
the presence of two or three layers jumps in Vp.
As you found from the previous lab, you can
simulate a jump in Vp between two layers with
three velocity layers in Linray. To simulate two
jumps between three distinct layers you will need
to specify 5 layers in Linray.
I strongly
recommend that you start with fitting the arrivals
from the shallowest layer first, and then work
your way down the Vp versus z profile
(corresponding to greater shot-receiver offsets and
arrival times). Please include plots of your final
fits to the data. Also, remember to save your Pwave speed profiles.
3) Estimating the 2D P-wave speed structure beneath our tape measure. Now that
you have determined the best-fitting 1-D Vp versus z profiles for each of your shots, you
can combine them to generate a 2D (x-z) model of Vp beneath our mound.
(a) Each file of your four velocity profiles will contain two columns: depth (z) in the
first column and P-wave speed in the second. First, you simply need to assign an xcoordinate to Vp - z pair.
(b) From the ray paths diagram you can see where the rays are turning with respect to
the shot location. The turning points are where the rays are “sensing” the earth’s
velocity. Therefore you can estimate the x coordinates (with respect to our absolute
coordinates as defined by the tape measure) along your 1-D z versus Vp profiles.
(c) Finally, combine all the x coordinates of your four profiles in to a single array (or
vector, say Xall), all your z coordinates from your four profiles in to another vector (Zall),
and all your Vp values from each profiles in to a vector (Vall). Then use “interp2” to
interpolate from these arrays to a regular grid in x-z beneath our tape measure.
“Lab9_interp2.m” will give you some hints.
Look forward to conducting a magnetic survey of the mound on April 16h!!