Reflectivity Gathers

I pick several spots along the BSR to examine the amplitudes that correspond to the BSR reflection. The corresponding gathers can be seen in Figures and . The first panel of Figure represents a gather from the continuation of the BSR at 28 km distance, the second one shows a gather from 33 km distance. The third panel at 37 km distance represents the part of the BSR that appears highly discontinuous. The gathers displayed in Figure are from the strong parts of the BSR at 42 km, 44 km and 46 km, respectively. The reflection events that should correspond to the BSR are indicated by an arrow.

 

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Figure 7 Reflectivity gathers showing the assumed BSR reflections at lateral distances of 28 km, 33km and 37 km.

 

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Figure 8 Reflectivity gathers showing the BSR reflections at lateral distances of 42 km, 44 km and 46 km.

In the case of clearly separable reflection events from the top and bottom of different layers, the BSR should bear a wavelet shape similar to the seafloor but with reversed polarity. The gathers along the continuation of the BSR (see the left and middle panel in Figure ) do not show a strong, negative BSR reflection, but instead a wavelet shape that appears to be indicative of thin layering (tuning). Depending on the time-thickness of the interfering events, the wavelets of both reflections are being superimposed and influence both shape and amplitude in the recorded reflection Yilmaz (1987). Figure shows that the BSR in this region between 25 km and 35 km lateral distance appears to be wedged against the flat reflector beneath the BSR, which is still dimly visible. This can yield a superposition of both reflection events. The P- and S-impedance contrasts across the flat layer beneath the BSR indicate that this flat reflection might be the transition from gas- to brine-saturated sediment. This would suggest that the BSR between 25 and 35 km lateral distance might be underlain by a thin free gas zone.

The right panel of Figure shows a gather representing the part of the BSR which appears highly discontinuous (see Figure between 35 km and 44 km). There is no clearly identifiable negative BSR reflection, but it appears to be an increasingly positive event. Among the factors potentially contributing to this drastic disappearance are: disturbance of the hydrate stability field, fractures and thin layering.

The gathers between 42 and 46 km, as displayed in Figure , correspond to strong BSR reflections and show clearly identifiable BSR wavelets. However, the gathers at 42 km and 44 km distance also appear to display some interfering effects with the layering underneath the BSR. Comparison with Figures and shows that the location of these gathers (42 km and 44 km) coincides with strong underlying structure. At a lateral distance of 46 km, the gather in the right panel of Figure shows a clear BSR response that has increasingly negative amplitudes with increasing offset. This BSR amplitude behavior is consistent with the BSR amplitude characteristic observed in this region by Lee et al. 1994 and Katzman et al. 1994.

The structure and thin layering intersecting with the BSR can cause a strong distortion of the original BSR amplitudes. Therefore, the amplitudes and resulting impedance contrasts have to be evaluated carefully.