\email{brad@geo.stanford.edu}
\author{Brad Artman}
\chapter{Time sections from 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/disappear
completely.  While aliasing could be the cause, the apparent dips of these
events are geologically unreasonable.
\par
Various muting and modeling exercises 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 Chapter~\ref{chapter:TimeShot}.  With
a single strong source dominating the data, so far away from the
array, the synthesized gathers did not reveal obvious subsurface
information that conform to the kinematics of surface data.  Therefore,
the event is a strong coherent noise source for both sorting to CMP
coordinates 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.  
\par
Muting the direct arrival from the shot gathers before NMO-stack
can provide a 3D volume that does not suffer contamination from the
distant cultural noise.  These volumes contain a few plausible events
that may represent subsurface structure.  The volumes are not very
convincing however that the goal of producing subsurface information
has been achieved.
\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 top images were
generated using 9 hours of raw data from February 14, 2004.  The
bottom images used 5 hours of data 25 hours later.  The two volumes
were processed identically.  The left columns are sections extracted
from the 3D cube at the central CMP in the cross-line direction and
bandpassed from 10 to 25~Hz. The sections on the right column were
extracted from the next cross-line CMP, +150~m.  The dissimilarity
between the results produced with data differentiated only by when
they were recorded leads immediately to the conclusion that the events
seen in the sections are artifacts rather than subsurface reflections.
\par
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 (dis)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.  The strength of the events
is also suspicious as the shot-gathers shown in the previous chapter
do not contain obvious geologic events at these times.
\par
\plot{events}{width=\textwidth}{NMO-stacked inline sections from
  the center (left) of the Valhall array, and (right) the neighboring
  cross-line CMP (+150~m).  Top and bottom rows 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, top row of Figure~\ref{fig:events}, and a constant
water velocity for the NMO correction.  The two panels correspond to
the same cross-line sections.  Coherent events are present, but have
troubling characteristics again.  There is now some 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.  The panel shows some
uniformity of polarity,  which is indicative of flat events.  The
parallel events at early time in Figure~\ref{fig:water} are all at
integer multiples of 0.048~s, which is the time through the 70~m water
column.  Since NMO was performed with water velocity, multiples are
flattened in this volume.  It is impossible to determine if these
events are multiples or correlation side-lobes in this case.
\par
\plot{water}{width=\textwidth}{Inline sections corresponding to the
  data on the left of Figure~\ref{fig:events} using constant water
  velocity for NMO.}
\sideplot{waterslice}{width=2in}{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 odd events
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 three correlated shot
gathers from the Valhall array.  The first is a modeled gather
synthesized by convolving a wavelet with the forward modeled time picks
for the distant production noise described in
Chapter~\ref{chapter:TimeShot}.  The first trace is the zero-offset
trace.  The second gather was synthesized from passive data and muted
to 0.32~s after the arrival time of the event in the top 
panel.  Notice that there are significant correlation 
side-lobes that remain in the section.  The bottom panel shows the
application of a short time window around the arrival of the noise
event to the synthesized gather in the center panel.
\par
\plot{modldat}{width=\textwidth}{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} shows 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.  Inspecting the
bottom two panels from the bottom up, there are many similar features,
that share characteristics: location, length, dip, and curvature.  The
steeply dipping events on the right section are not present in the
left, but almost all other main features are.
\plot{evnttest}{width=\textwidth}{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 
the muted shot gather generated with the first 9 hours of passive data
(top Figure~\ref{fig:modldat}) with one, bottom panel, from data
collected 25 hours later.  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 co-parallel energy, but
this time does a much better job at removing most of 
it.  Figure~\ref{fig:evnttest2} compares the NMO-stacked sections
created with this second volume of data with and without the mute to
the direct arrival.  Most of the strong flat events have been
eliminated.

\plot{modldat2}{width=\textwidth}{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.}
\plot{evnttest2}{width=\textwidth}{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.}
\par
The right panel of Figure~\ref{fig:evnttest2} shows a stacked section
of data from which the bulk of coherent noise has been removed before
NMO-stack processing.  This volume then represents the best candidate
for finding subsurface structure.  Notice a roughly sinusoidal series
of identifiable events, on the right panel, across the section between
0.0-0.2~s.  This event will be described further in the next figure.
\par
Figure~\ref{fig:Fourth.mute} shows two neighboring slices through the
3D volume of the stacked data produced with the muted shot gathers.  The  
volume has been bandpassed from 8-15~Hz.  The time slice in the right
panel was extracted through the curving events near time zero
mentioned above.  There is a roughly circular feature on the time
slice identifiable as a mostly white ring with a dark center. In
movies progressing down the time axis, these events are
expanding, circular rings emanating from the location of the surface
production facilities.  
\par
\plot{Fourth.mute}{width=\textwidth}{Best 3D time section processed
  from 5 hours of passive data in February, 2004.  Preprocessing
  included correlation, deconvolution, mute, NMO, and stack.}
There is little continuity in the cross-line direction throughout the 
bulk of the stacked volume.  The events that approach the edge of the
array are suspicious since these CMP locations do not have the
complete coverage needed to build reflection gathers as discussed in
Chapter~\ref{chapter:TimeSynth}.  However, there are a few events that
could be real subsurface structure.  There is a fairly
continuous event that can be interpreted across the first panel at
just less than 2~s.  The event is interpretable on the next section as
well, and there is a black-white-black event on the cross-line panel
where the section line cuts through 2~s.  This seems like the most
likely event in the volume to be a genuine reflector.  There is also a
coherent reflection between 6000-8000~m just beneath the time line on
the right panel that is stationary in both sections.  At approximately
the same time on the right edge of the section, this event is again
present.  This event could be associated with the production noise, or
a real subsurface reflection that is interrupted in the middle of the
array. 
\par
Figure~\ref{fig:synNMO} 
uses the synthetic transmission functions from Chapter~\ref{chapter:TimeSynth} 
to analyze the effects of the distant production noise on passive 
data.  The first panel is a transmission wavefield due to a source
below the center of the model domain with the addition of an event
modeled from the analysis of the farfield noise in 
Chapter~\ref{chapter:TimeShot}.  The 
second and third panels are depth migrations produced with
transmission wavefields due to sources below the left edge and center
of the model domain which included the farfield noise 
event.  Chapter~\ref{chapter:DepthSyn} presents the 
migration of the synthetic data in detail, but these images provide
insight to the sections presented above.
\plot{synNMO}{width=\textwidth}{Left: Transmission wavefield from the modeled
  data presented in Chapter~\ref{chapter:TimeSynth} due to a source
  below the center of the domain.  Center: Depth image produced from a
  transmission wavefield due to a source below the left edge of the
  domain.  Right: Depth image produced from the transmission wavefield
  in the left panel.}
\par
The steeply dipping events in the images shown in Figure~\ref{fig:synNMO}
are readily identified in the sections produced with real data 
above (e.g. Figures~\ref{fig:evnttest2} and~\ref{fig:Fourth.mute}).  Also 
note the concave-up features that connect to the steep 
events at the surface.  These noise events are large hyperbolas, centered 
over the location of the subsurface source, that have been folded down 
at the free surface.  These events are not present in images produced 
with out the addition of the farfield noise event (0.75-0.9~s across left 
panel).  Images produced with only the farfield noise event contain only a
strong event at $z=0$~m.  These steep events are caused by the convolution
of the farfield noise event with real subsurface reflections.  Therefore, 
the presence of the bowl-shaped structures in Figure~\ref{fig:Fourth.mute}
and the steep events in Figure~\ref{fig:evnttest2} are indicative that there
is real subsurface information within the passive data that was not well
imaged with this processing strategy (NMO-stack).



\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} spanning a limited acquisition
duration.  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.  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.  
\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 
previously.  Comparing the muted 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 in the
recording effort. 
\par
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.  Several orders of flat
multiples are identified in this volume relating the the 70~m of water
above the geophone array.
\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 very hard to interpret in
both the shot domain and the CMP domain.  Muting the synthesized shot
gathers before NMO-stack to remove the distant production noise
source, produced a much simplified data volume.  A second noise source
was then identified in the volume as expanding rings emanating from
the production facilities contained within the array.  These features
correspond to the local water-velocity events picked on the raw and
correlated data sections in the previous chapter.  The best stacked
volume of data has several intriguing features that are indicative of
subsurface reflectors, but do not provide convincing results that the
passive data recordings can illuminate the subsurface at Valhall.

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