PASISAR - PERFORMANCES OF A HIGH AND VERY HIGH-RESOLUTION HYBRID DEEP-TOWED SEISMIC DEVICE

The Pasisar seismic acquisition system combines a source at the sea su rface and a deep-towed single channel streamer. This unconventional de vice geometry reduces the width of the first Fresnel zone which increa ses the lateral resolution. However. the device acquisition geometry g enerates artifacts on seismic profiles and induces large incidence ang les of the seismic signal. A specific processing sequence must be appl ied to the data to obtain a readable seismic section. Penetration of t he seismic signal depends on the energy of the signal reaching the sea floor and on its incidence angle. Because of smaller source energy. 80 0 Joules Sparker data cannot be acquired in water depth larger than 15 00 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seism ic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution o f any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as sipnal-to-noise ratio obtained with both the Pa sisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep- towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal r eflector.