Objectives:

1)  To assess multiple ping sub-bottom profile (SBP) data collected from R/V New Horizon on 
Monica Kohler's Sept. 2011 Southern California research cruise. 
2)  To compare the .keb data produced in Knudsen's SounderSuite PostSurvey and .sgy data
processed through SIOSeis.  
3)  To start a collection of SBPs off of the San Diego Harbor in order to compare and evaluate
different settings, systems, and installations.  

Definition of Envelope:

The envelope is the instantaneous amplitude.  It is  formed by first creating a complex trace,
where the real part is the original trace and the imaginary part is the Hilbert transform 
(90 degree phase shift).  The next step is to take the complex modulus (square root of the 
sum of squares of the real and imaginary) of each sample.  The envelope contains only 
positive numbers, and has lost its phase information, but it is much lower in frequency 
and can be displayed in a more readable form for a geologist/geophysicist. 

Figures:

Both figures represent envelope SBP data.  The top image was created in SIOSEIS, where the
.sgy correlate file was first made into an envelope, and then processed according to the 
steps below.  The bottom image is the Knudsen PostSurvey .keb file, which is recorded as
an envelope.

SIOSEIS Processing Steps:

Make Envelope from Correlate Files: (using mkenv example from sioseis.ucsd.edu website)
Swell: dampens effects of ship heave in heavy seas by computing the average water bottom 
time of a group of traces, then shifting the middle trace of the group by the difference 
of the middle trace water bottom time and the average water bottom time
Mix: running, weighted sum of adjacent traces
Filter: frequency domain bandpass filter from 20-1000Hz
Automatic Gain Control (AGC):  amplitude normalization (modification); agc calculates the 
average absolute value of each sample, which is used as a multiplier applied to the entire 
window by dividing the average by an output level.
Mute: zeroes the beginning of selected traces; used to silence the noise preceding first 
arrivals - in this case the noise in the water column.  In this example, the water bottom 
was selected using the process wbt peak pos which chooses the time associated with the 
largest positive amplitude of each trace.  
Time to Depth: transforms the time domain traces to depth domain traces using a defined 
velocity function (in this case, a fixed average velocity of 1475m/s).

Discussion:

The multiple ping provides faster data feedback in deep water, resulting in higher spatial 
resolution.

SIOSeis has more extensive processing capabilities than PostSurvey.  In this case, there 
was significant noise above the water bottom (the dark line just below the surface) which 
was muted out in SIOSeis.  The SIOSeis plot displays more detail below the water bottom,  
as well as two multiple reflections, which only faintly show up in the PostSurvey plot.  

Credits:

Data obtained from Sept. 2011 R/V New Horizon Cruise; Principal Investigator: Monica Kohler, UCLA
SIOSeis (http://sioseis.ucsd.edu)
SIO Shipboard Geophysical Group: Lee Ellett, Jay Turnbull, & Bridget Hass