Geophysical data acquisition - OGS

three wells in the Blake Ridge, offshore southern United States. Here, a strong
‘BSR’ marks a transition from a hydrate rich sediments zone above, to a free gas
bearing sediments below. This transition is reflected in the velocity profile with a
boundary at 4150 ms between a high velocity region (1670 m/s above) and a low
velocity one (1500/1600 m/s below) (Figure B).
A velocity structure obtained by prediction can be translated in gas hydrate and
free gas concentration structure. We used the method proposed by Tinivella
(1999). The concentration is estimated by fitting the theoretical velocity to the
experimental P/wave velocity (derived by the prediction of the VSP in our case).
The discrepancies between the inverted velocity profile and the velocity for water
filled marine sediments are interpreted as due to the presence of the gas hydrate
(where positive anomalies are present) and free gas (where negative anomalies are
present). Figure C shows the distribution of the two phases: positive values (red
colours) are the gas hydrate concentration, while negative concentrations (blue
colours) are related to the percentage of volume occupied by free gas.
A
VSP velocity Section
B
A) Position map of the
study area. Wells
location and seismic
line position are shown
in the close-up
window.
B) VSP velocity panel.
C) Distribution of the
two gas phases;
positive values (red
colours) are the gas
hydrate concentration,
while the negative
concentrations (blue
colour).
C
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