The German Federal Institute for Geosciences and Natural Resources (BGR) has carried out marine reflection seismic cruises (SO81, BGR92 and BGR99) on the active continental margin of Costa Rica for a better understanding of the geological setting and the gas hydrate occurrence in this area. Nowadays, gas hydrates are drawing more attention because of their contributions to geological hazards and climate change, as well as its potential as a future energy resource. One of the objectives of the survey BGR99 was to gather supplementary data for a detailed seismic study on the occurrence and properties of gas hydrates and BSRs in the study area. In order to achieve this, BGR has acquired long-offset seismic data for high-resolution seismic studies (5250 m long streamer, 420 channels, 12.5 m group interval, sample rate 1 ms, shot distance 25 m). The activities of BGR include reservoir investigations, structural studies, comparative studies to understand the origin of the gas and the BSRs.

　　The convergent continental margin of Costa Rica is an area known to have large amount of gas hydrates. The appearance of gas hydrates is very heterogeneous. Continuous BSRs are observed between the upper and lower continental margin southwest of Nicoya Peninsula. However, no BSRs are observed in the area of ODP Leg 170 at the north of the so-called fracture zone trace, presumably because of the unusual low heat flow in this region. However, in the study area the BSRs are characterized by patchy occurrence with strong lateral variation in reflection amplitude at the southeast of Nicoya Peninsula, indicating focused fluid migration along deep-reaching faults.

　　The high-resolution seismic data has been used for the pre-stack amplitude variation with angle (AVA) analyses. The implementation of an effective medium theory accounts for the local properties of shallow marine sediment in allowing the modeling of synthetic AVA responses as a function of gas hydrate concentration and free gas saturation at the BSR. From the results, we can conclude that after careful data processing there is the possibility to detect free gas, and to quantify free gas at low saturations. From this, areas with strong BSRs off shore Costa Rica are characterized by bulk free gas saturations of up to five percent. The results also indicate that the contribution of gas hydrate to the observed angle-dependent reflection coefficient is negligible and can hardly be resolved and quantified from AVA.