Ninth international conference on - Marum

Abstracts of posters 111
consisted of a standard sequence to produce stacked sections and post-stack Kirchhoff depth migration sections
by using the interpolated and smoothed stack velocity fields. Seismic reflection profiles are characterized by the
presence of remarkable Bottom Simulating Reflector (BSR) showing reverse phase with respect to the sea
bottom and a very high-amplitude. To better define the seismic character of the hydrate bearing sediments, we
selected parts of two seismic lines to perform a detailed velocity analysis by using iteratively the pre-stack depth
migration. Then, we translated the velocity field in terms of gas hydrate and free gas concentration.
The selected part of the line RC2901-734 is 20 km long. This seismic line is located in the southern sector, close
to 44°S. The frontal part of the prism is characterized by an intense active accretion, realized trough thrust faults
ocean-ward vergent, as evidenced by some anticline folds. A strong and continuous BSR was recognized and it
is about 200 m depth (Fig. 1).
Note that the BSR is shallower than expected in correspondence to the structural high; this feature can be
explained supposing a lateral variability of the geothermal gradient.
The final velocity model allowed us to recognize the high velocity layer (1700-2200 m/s) above the BSR
associated to gas hydrate presence and the low velocity layer (1200-1400 m/s) associated to free gas presence
(Fig. 1). We detected a reflector below the BSR that we interpreted as the base of the free gas layer (BGR). We
estimated average concentrations of 12% and 1% of volume of gas hydrate and free gas respectively. We
detected strong lateral variability of concentration and thickness in the free gas layer.
Fig.1: Velocity model and pre-stack depth migrated section of a selected part of seismic line RC2901-734 (see
position map). A strong BSR is presents; above it, a weak reflector was interpreted as the BGR. Note, above
BSR, the high velocity layer associated to the gas hydrate occurrence and, below the it, the low velocity layer
associated to free gas occurrence.
Along the seismic line SO161-44 line, we selected a segment 25 km long, located close to 39°S. The continental
slope is characterized by small slope basins located in correspondence of paleo-thrust and normal faults. The
BSR is discontinuous and deeper than the selected part of the RC2901-734 line, and it reaches 500 m of depth, as
expected.
The deformational style on the accretionary prism may influence the pressure and temperature conditions, which
allow the gas-hydrate stability condition. We associated the accretional process variability along of the Chilean
continental margin with the BSR features and the gas hydrate occurrences.