Abstract In the Turbiditic reservoir of the Girassol field, there was a risk of sealed faults. If so, the well pattern would need to be altered with a possible increase in wells. To determine this effect, an interference test was to be carried out between a water injection well and a production well before first oil. The paper presents the design of the interference test and the technology of the Subsea Acoustic Monitoring System. It reviews operational and environmental issues related to testing and concludes with the results with their consequences for the well pattern and subsea layout. Introduction Ten wells were planned to be drilled before first oil on Girassol. The well pattern had to be adapted for possible reservoir disconnection, due to possible sealing effects from faults. Early information derived from the interference testing raised reservoir uncertainty and helped with the optimisation of the field development plan. Geological description of Girassol reservoir The late Oligocene (Chattian) Girassol Turbiditic System B was deposited on the Block 17 outer trend, in parallel with other Oligocene Systems, such as at two other Block 17 finds, Rosa or Lirio. These reservoirs are made of unconsolidated fine to very coarse sands deposited in the Girassol area after passing through the canyon of the Zaire River. The evolution of the Girassol structure is driven by gravity tectonics. Two main stages can be identified:Late Cretaceous to Oligocene is characterized by the development of NW-SE trough bounded by passive salt ridges. Upper Oligocene (Chattian) correspond to the deposition of turbidites along NE-SW channelised Systems, within this NW-SE basin between peripheral salt ridges (ex : Girassol, Rosa or Lirio systems).Miocene to Present corresponds to structural trap formation and fluid charging time. It is characterised by the ‘inversion’ of the former trough, first by progressive strata bending associated with symmetric turtle-back anticline formation and then a second time by the development of marginal roll-over syncline. The, ‘inversion’ and the lateral migration of depot-centres is mainly due to deep salt movement, leading to late folding and faulting of turbiditic channels. The Girassol B System is made of three different turbiditic complexes - B1, B2 and B3. Each channel complex is considered as the fourth order scale of the Girassol architecture: 50-100m thick, few kilometers width and many kilometers long. There are additional thin resevoirs with large extension called sheets complexes. Each Turbiditic complex is separated from the others by regional seals which are correlated at a large scale on Girassol area. The main complex is the B3 Complex targeted by all the first development wells. All these turbiditic complexes were found to be oil bearing(32°API) within a regional structural closure with common OWC. After the acquisition and interpretation of high resolution 3-D (3DHR) seismic data, the Girassol architecture understanding was improved and each channel complex B1, B2, B3 was subdivided into different sequences (or channel story) separated by decametric shale prone abandonment phase which are potentially good vertical barriers (Fig. 1).