WHAT IS IT? -----------

This model is loosely based on the geological concept of
velocity profiles.  Seismologists use the travel times of
seismic waves in combination with computer modeling software
to determine these profiles within the earthıs crust and
mantle.  In some instances geologists will purposefully set
off explosions on the earthıs surface in combination with
sensors on the ground to time the speed of the explosionıs
shock wave through the ground.  The principle behind this
technique is that shock waves travel fast through hard rock
and slow through soft rock.  Additionally, waves that hit
the mantle or some other change in rock type at an oblique
enough angle will reflect, and waves that hit at an acute
angle will diffract.

In this model these properties are simplified.  Waves
originate from a single blast source, they go faster through
red rock, and they reflect off of the mantle.

HOW TO USE IT --------------

The mantle is represented by the black strip at the bottom
of the screen.  Soft rock is colored brown and hard rock is
colored red.  To start a new simulation click the "setup"
button.  This will remove all red patches from the
simulation and initialize the screen.  To draw red  rocks
(color 15), use the drawing tools to draw in the graphics
window.  Position the blast source using the "right-left"
switch and the "x-offset" slider.  Set it off using the Set
Blast button.  To make a blind simulation hit the button
titled "Hide Red Rocks".  This will make the red rocks
invisible in the simulation window.  The button "Show Red
Rocks" will make them visible.

The graphs in this simulation are operated using two
controls.  The button "Clear All Plots" clears all of the
pens and "Make Graphs" creates a set of three graphs.

The execution of the simulation occurs in two phases.  The
first wave released goes faster through red rock and is
referred to as a modifiable wave.  The second wave travels
at uniform speed through all materials and is referred to as
an unmodifiable wave.  The y-axis is the return time of the
wave and the x-axis is the horizontal position of the sensor
receiving the wave.  Plot window three is a composite graph
of unmodified return time minus modified return time.  The
composite graph is useful for visualizing subtle differences
between the two graphs.

THINGS TO NOTICE ----------------

The shape of the unmodified wave is a parabola.  The
equation of the graph is: 2 * Square Root ( ( .5 * xcor ) ^
2 + ( Mantel Depth) ^ 2 ).  The composite graph is intended
to illustrate differences between the modified and
unmodified graphs.  Notice that with rectangular shapes
there are multiple critical points on the graph.  Some
portions of the wave may travel through a rock formation
twice.  How does this change the shape of the graph?

THINGS TO TRY -------------

Create a simple shape out of red rocks and hide it using the
"Hide Red Rocks" button.  Have someone else use the model
and ask them to predict what the shape and placement of the
formation is.

Try to find critical points on the graph. Especially in the
case of a square formation there should be some critical
points.  Try to explain these points with the geometry of
the rock formation.

As an investigative technique try creating waves from
different locations around the rock formation.  Use the
"the-pen" slider to fit multiple plots on the same composite

EXTENDING THE MODEL -------------------

Try to create a better system or equation for visualizing
the red rock formations.  To do this make modifications to
the procedure "graphit".

Implement diffraction between rock surfaces in the model. 
Try to imagine how this would change the shape of the

Try implementing different rock types with different
densities using a new rock color.

STAR LOGO FEATURES ------------------

Many of the graphs have dips in them.  These imperfections
are points without data and should be ignored.

The procedure "random-real" uses the shift-right primitive. 
The number 65535 is equal to 2^16 ­1.  Or in binary it is
1111 1111 1111 1111.  The decimal part of a starlogo number
is stored in the low word.  Random 65535  shift-right 16
creates a random number between one and zero.  When added to
an integer it forms a "random-real" number.