Resolution

The vertical resolution of seismic reflection data depends on the wavelength of the reflected wavelet. It is commonly assumed that without special processing (see e.g. waveform inversion below), 1/4 of a wavelength can be resolved. Assuming a Vp of 1700 m/s (which is typical for sediments in which gas hydrates may occur), and a dominant frequency of 50 Hz (which is at the higher end of conventional seismic surveys), the dominant wavelength is 34 m, implying a vertical resolution of about 8 m. For data with dominant frequency of 400 Hz, in contrast, the dominant wavelength is only 3 m, with resolvable layer thickness of about 1 m (e.g., Fig. 3). The limits of vertical resolution are immediately visible when wiggles from reflections appear to overlap each other. Horizontal resolution is defined by the Fresnel zone, which depends on reflector depth, frequency and velocity (Sheriff and Geldart, 1995). For a velocity of 1700 m/s, a reflector depth of 1000 m, and a frequency of 50 Hz, the Fresnel zone is about 130 m. Thus changes in BSR properties over lateral distances of less than 130 m cannot be resolved. This is often not considered when interpreting seismic data.

DTAGS surface-towed

o 400 Distance (m) 400 800

Fig. 3: Conventional seismic reflection profile (right) and DTAGS deep-towed data (left) over ODP Site 997 on the Blake Ridge, after Wood and Ruppel (2000). Data are stretched to depth using velocities measured in the borehole. The BSR is at -460 meters beneath the seafloor (mbsf). Note that the BSR appears as a continuous reflection in the conventional data, whereas it appears to be "shingled" in the higher-frequency DTAGS data. Also note the much better resolution of near-vertical faults in the DTAGS data.

DTAGS surface-towed o 400 Distance (m) 400 800

Fig. 3: Conventional seismic reflection profile (right) and DTAGS deep-towed data (left) over ODP Site 997 on the Blake Ridge, after Wood and Ruppel (2000). Data are stretched to depth using velocities measured in the borehole. The BSR is at -460 meters beneath the seafloor (mbsf). Note that the BSR appears as a continuous reflection in the conventional data, whereas it appears to be "shingled" in the higher-frequency DTAGS data. Also note the much better resolution of near-vertical faults in the DTAGS data.

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