The energy responsible for the features in the raw data,
Figures -
, are likely related as they
share a locus at the production facilities and are both characterized
by water velocity. I suspect that the noise-train in
Figure
is a collection of aliased repeating events
similar that found in Figure
. Also, the gathers
generated with data showing the diffuse noise-train,
Figure
, have almost identical kinematics to the raw
data showing a crisp water-velocity event, Figure
. It
is possible that some activity on the platforms could sometimes be
impulsive in nature or more drawn out. In either case, the correlated
gathers do not show convincing evidence that the energy penetrates the
subsurface after the direct arrival is recorded. Forward modeling the
direct arrival to the receiver stations shows that water velocity
events are aliased close to the source with this array. This analysis
of the raw data also shows that the predominance of observable energy
shares a single source location.
Correlating the raw data to produce synthesized shot gathers was
always performed on 12 second records. These individual results were
summed from 1-2500 times to generate some approximation to active
seismic shot gathers in accordance with the theory of interferometric
imaging. The gathers produced with a single 12 second record,
Figure , or 15 hours of data correlated in 12 second
sections, Figure
, show an identical strong event
with velocity 1450 m/s arriving from the Ardmore field 40 km to WSW. Less
obvious events are also present in the gathers with locus at the
Valhall production facilities in the center of the array. These
events are also characterized with water velocity.
It seems counter-factual to attribute the source of this energy to a
distant production operation when major production, drilling, and
injection activities within the array have not generated
events that match the power and bandwidth of this most obvious
arrival. The British Department of Trade and Industry
describes the Ardmore development. It differs from the
Valhall operation in several important ways. The Ardmore facility
has no permanent structures, nor is it connected to an export
pipeline. Instead, two vessels are permanently moored over the field:
One contains production facilities, the other is used to store liquids
for periodic transport to shore. This operation has no way to handle
produced gas and must therefore flare gaseous production to the
atmosphere. I believe this is the most significant difference between
the two operations, and that the gas flare is the most likely sonic
source for the energy recorded at the array.
With only two main energy sources developing the events in the correlated gathers, the result does not satisfy the equation which relates passive recordings to active data acquisition. Correlating events from a single source simply moves the minimum travel-time captured by the array to zero. Constructive and destructive interference from many events are required to generate events with the kinematics of an active survey. Sorting the gathers to midpoint-offset coordinates yields completely uninterpretable CMP gathers. Similarly, further processing which assumes impulsive sources located at the trace used as source function to synthesize a gather (migration) is therefore inappropriate. However, having identified the locus of the energy in the correlated gathers, the equation describing the kinematics of the event can be used as a mute function or areal source function for further processing.