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\def\figdir{Fig}
\email{brad@geo.stanford.edu}
\author{Brad Artman}
\chapter{Time sections from synthesized shot gathers at Valhall}
\label{chapter:TimeCMP}
\section{Summary}
NMO-stack, using a velocity model generated with active seismic
surveys, was performed on synthesized shot gathers produced from 
the Valhall passive seismic array.  Inline sections show strong events 
with minor to no topography.  The events are completely discontinuous
across the cross-line direction and uninterpretable on time slices.  While
some events are can be tracked between sections, some events appear/dissappear
completely.  While aliasing could be the cause, the apparent dips of these
events are geologically unreasonable.
\par
Various muting and modeling excercises were performed using the kinematics
of the energy from the production facility 40~km away from the array.  Shot
gathers were modeled, windowed and muted, before NMO-stack processing to 
compare to original results.  These exercises show that the direct arrival
and ringing correlation side-lobes are a coherent noise source that gives 
rise to the events that would be believable on 2D sections. 

\section{Introduction}
After synthesizing shot gathers from the passive data collected by the
Valhall subsea array, the traces were sorted, normal moveout (NMO)
applied, and stacked.  A similar processing flow with the same data
was described by \longcite{hohl:1560}.  Several coherent, roughly flat
events are imaged in all time windows over which the raw data were
subdivided for processing.  
\par
Of paramount concern with this
workflow is the presence of the strong direct arrival from 40~km away
from the array investigated in the previous chapter.  Thus, the
origin of the event is not at the location of the source traces
used for correlation to produce synthesized shot gathers.  Therefore,
the kinematics impilicit in sorting to common-midpoint coordinates (CMP) 
are not appropriate for this event.  Thus, the event is a strong coherent
noise source for both sorting and NMO.
\par
Several methodologies were employed to investigate whether the events in the 
stacked sections are actual subsurface reflections or the coherent stack of 
the distant production noise.  Comparative volumes of NMO-stacked data
were generated in an attempt to identify the nature of the events.  Two 
main efforts were employed: Constant water velocity NMO, and
various modeling and muting experiments using the inversion results
described in the previous chapter.  

\section{{NMO-S}tack time sections}
Figure~\ref{fig:events} shows strong coherent events in inline
sections after NMO-stack using a velocity model provided by BP.  The
shot gathers were synthesized with the spectral whitening correlation
strategy explained in the previous chapter.   The left images were
generated  using 9 hours of raw data from February 15, 2004.  The
right images used 5 hours of data 25 hours later.  The two volumes
were processed identically.  The upper rows are sections extracted
from the 3D cube at the central CMP in the cross-line direction and
bandpassed from 10 to 25~Hz. The bottom sections were extracted from
the next cross-line CMP, +150~m.  The shallow geology under the array
consists of very flat, continuous layers.  These NMO-stack results have no
continuity on time-slice panels, and the events change radically in
character and timing between adjacent cross-line CMP locations.  Some
events appear with no correlation between cross-line locations.  Other
events are correlable between the two sections, though move approximately
0.25~s.  Assuming that the events are aliased in the cross-line direction,
this is an apparent velocity of 600 m/s which is too slow to be 
believable.  The strength of the events is also suspicious as the 
shot-gathers shown in the previous chapter contain no apparent energy at 
small time other than the strong event from the distant production 
facility.
\par
\activeplot{events}{width=\textwidth}{NR}{NMO-stacked inline sections from
  the center (left) of the Valhall array, and (right) the neighboring
  cross-line CMP (+150~m).  Left and right columns were processed
  identically using data collected 25 hours apart in February,
  2004.} 
Figure~\ref{fig:water} used the data from the beginning of the
recording period, left column of Figure~\ref{fig:events}, and a
constant water velocity for the NMO correction.  The two panels
correspond to the cross-line sections displayed above.  Coherent
events are present, but have troubling characteristics again.  There
is still no consistency of events in the cross-line 
direction.  Figure~\ref{fig:waterslice} is a time slice through the
same data cube at 0.4~s.  There is no consistency in the cross-line
direction, while individual lines along the CMP-X directions show the
continuity associated with the clear events in Figure~\ref{fig:water}.
\par
\activeplot{water}{width=\textwidth}{NR}{Inline sections corresponding to the
  data on the left of Figure~\ref{fig:events} using constant water
  velocity for NMO.}
\activesideplot{waterslice}{width=2in}{NR}{Time slice, 0.4~s, through the
  NMO-stacked cube using water velocity.  No continuity exist across
  the panel, while individual lines in the CMP-X direction show some
  continuity.} 

Figure~\ref{fig:water} does show that NMO with water velocity 
drastically changes the events that coherently stack compared to
Figure~\ref{fig:events}.  The sections are obviously different.
Despite the fact that the synthesized shot gathers did not capture the
apex of the hyperbola due to the distant noise source, the water
velocity NMO was close enough to destroy almost all the energy in
Figure~\ref{fig:events}.
\par
The inversion described in the previous chapter provides very accurate
prediction of the kinematics of the strong event in the correlated
gathers.  Figure~\ref{fig:modldat} shows two correlated shot
gathers.  The first is muted to the arrival time of the distant noise
source.  Notice that there are significant correlation side-lobes that
remain in the section.  The bottom panel shows a narrow pass-window
selected around the arrival.  A third data set was also generated by
convolving the forward modeled direct arrival with a wavelet.
\par
\activeplot{modldat}{width=\textwidth}{NR}{Two shot gathers used to test the
  origin of the events in Figure~\ref{fig:events}.  Top panel is muted
  to the kinematics of the distant noise source.  Bottom panel is a
  narrow window around the modeled event.}
Figure~\ref{fig:evnttest} are inline time sections exploring the
relationship of the direct arrival from the distant production
facilities to the events observed in Figure~\ref{fig:events}.  The
top-left panel is the same as Figure~\ref{fig:events}.  The top-right
panel used the muted data from the top of Figure~\ref{fig:modldat}.  The
bottom-left panel used the data from the bottom of
Figure~\ref{fig:modldat}.   The final section is the result of
processing the forward modeled shot gathers.  The difference between
the first two panels is nearly impossible to find.  The bottom two
panels also show several similar features.
\activeplot{evnttest}{width=\textwidth}{NR}{Left to right: Inline NMO-stacked 
sections from the center of the array.  Top-left panel processed all
data available after correlation.  Top-right panel processed the 
correlations after application of a mute to the modeled direct 
arrival.  Bottom-left panel used data windowed around the direct
arrival.  Bottom-right panel used a modeled data volume containing
only the direct arrival.}
\par
In contrast to the results above, Figure~\ref{fig:modldat2} compares 
a muted shot gather generated with the first 9 hours of passive data,
Figure~\ref{fig:modldat} top, with one from data collected 25 hours
later, bottom panel.  In contrast to the top panel, the data from a
day later do not suffer as much from the ringing correlation 
side-lobes.  The mute
leaves a small amount of the direct arrival, but this time does a much
better job at removing most of the energy associated with the direct
arrival.  Figure~\ref{fig:evnttest2} compares the NMO-stacked sections created
with the data from 25 hours later with and without the mute to the 
modeled direct arrival.  All of the obvious flat
events have been successfully eliminated.  
\par
\activeplot{modldat2}{width=\textwidth}{NR}{Top:Muted shot gather corresponding
  to the first 9 hours of data (Figure~\ref{fig:modldat} top).
  Bottom:  Muted shot gather generated with 5 hours of data 25 hours
  after those used to create the top panel.}
\activeplot{evnttest2}{width=\textwidth}{NR}{NMO-stacked sections from center
  cross-line location generated from data a day after those used for
  Figures~\ref{fig:modldat} and~\ref{fig:evnttest}.  Muting down to
  the direct arrival (Figure~\ref{fig:modldat2}) successfully
  eliminates all coherent events.}
\section{Discussion and conclusions}
NMO-stack of the whitened, correlated passive data from the Valhall
OBC array produces strong events on inline sections.  Other authors
have interpreted these as geologic information after processing only
2D subsets of the data \cite{hohl:1560}.  Very good imaging from many
active surveys has been carried out at the location that shows the
shallow geology to be very flat and uniform in both map directions.  The 
time sections shown above however have several unbelievable
characteristics that lead me to conclude that these are coherent noise
rather than genuine subsurface information.  
\par
Of paramount importance, the same section produced with the same
processing flow using different windows of the available data
(separated by 25 hours) have dramatically different events.  The OBC
cables are buried approximately 1~m beneath the sea floor.  Thus,
within the first 2 samples (10~m depth), the interval velocity model
has values in excess of 1600~m/s which is much too fast to correctly
NMO the direct arrival that arrives at water velocity (1450~m/s).  By
performing constant velocity (1450~m/s) NMO, all of the spurious
events are removed from the stacked section.
\par
Because the kinematics of the direct arrival in the shot gathers are
well constrained, it was possible to mute, model, and window around
the event before NMO-stack.  Using the gathers generated with the
first section of data, the muted volume was barely distinguishable
from the result produced with the full data.  However, the section
produced with a short time window around the event was very comparable
to that produced by forward modeling the event.  The final observation
that links these two facts was provided by performing similar
experiments with data from 25 hours later.  The NMO-stacked section
produced with the muted gathers had no evidence of the events seen in
previously.  Finally, comparing the shot gathers produced from the two
data subsets showed that there is significant ringing (correlation side-lobes)
present in the gathers produced with the data from early time.  The 
conclusion from these facts is that the direct arrival form the distant 
production facilities is the cause of the events in the stacked sections 
and that the correlation side-lobes lead to coherent stacking of strong 
events that are not related to true subsurface information.
\par
These experiments lead me to conclude that the strong events revealed
by sorting the gathers to CMP coordinates, NMO, and stack are
artifices of the coherent noise event generated by a production
facility 40~km away from the Valhall array.  While there may be
subsurface information in the time domain gathers, it is weak and
distributed amongst the volume to make it uninterpretable in both the
shot domain and the CMP domain.  Simple processing with NMO and stack
does not reveal subsurface information, but rather coherently
preserves an artifact that looks very believable on 2D sections.

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