Active seismic experiments (Rice, UTIG, Memphis, FUNVISIS)

From April to June 2004 we collected 6000 km of marine reflection data, the 4 MCS/OBS/onshore-offshore
profiles and the MSC/ OBS profile described previously. The onshore-offshore/OBS profiles extend from the Venezuela Basin to the southern edge of the fold and thrust belt system, while the OBS wide-angle profile extends from the Venezuela Basin across the extinct Aves Ridge, the active Lesser Antilles Arc, the Trinidad-Tobago high, and into the Atlantic. Alan Levander coordinated the entire active source seismic investigation from Venezuela. We also acquired a number of secondary margin perpendicular reflection profiles, and an east-west composite profile along the length of the Leeward Antilles arc.

The marine reflection data were acquired with the R/V Ewing’s 6 km, 480 channel digital streamer, firing a tuned 20 element airgun array at 50m shot intervals. Reflection data fold is nominally 60. The data were field processed through time migration aboard the Ewing, using velocity analysis at 5 km or larger intervals. Some data were lost due to marine mammal sitings, which normally required that source intensity be reduced until the mammals left the survey area. The only notable data gap lies between the 64W and 65W profiles, where we lost a second crossing of the active El Pilar fault C-13 system due to cetacean activity. In all we acquired about 95% of our desired program. Paul Mann and Dale Sawyer were the chief scientists aboard the Ewing.

A total of 169 OBS deployments along 5 key transects were made from the R/V Seward Johnson II during the active-source field program. This was one of the largest field programs ever supported by the OBSIP group, and was the first major field program utilizing the WHOI D2 OBSs. Recovery rate was 96% for the WHOI D2 OBSs (122 deployed, 117 recovered) and 98% for the Scripps LC2000 OBSs (47 deployed, 46 recovered). 161 of the 163 recovered OBSs acquired full data. The OBSIP and shipboard personnel were outstanding, and allowed us to deploy and recover instruments much faster than expected. 3 students and 3 Venezuelan observers assisted in watchstanding duties. Shot files were e-mailed from the R/V Maurice Ewing letting us to create SEGY files for the majority of the OBSs at sea (2 OBSs required shore-based data download because of instrumentation problems; SEGY files from these instruments was delivered within 2 months of the cruise). Overall data quality for the project is excellent. Christeson was chief scientist aboard the R/V Seward Johnson II.

Three of the onshore-offshore/OBS wide-angle profiles and the TRIN profile were shot twice, once at 50m and once at 150m shot intervals, primarily to minimize water borne noise in the OBS recordings, but also to maximize night time recording of airguns by the land stations. (Each profile required approximately 48 hours of shooting, Venezuela being near the equator, has ~12 hours of night). The land profiles also recorded two explosive sources each, one near the southern end of each profile and one near the profile center. The explosive sources were funded by Venezuela. Land data were recorded by 550 Texan seismographs (Reftek 125s) with 4.5 Hz geophones deployed at ~350 sites. [The 350 Texans with 32 Mbyte memories were doubled at sites to permit recording ~48 hours at 8 msec, the 200 with 64 Mbyte memories were deployed singly]. Instrument spacing was generally 250-350 m near the coast with spacing increasing to 500-1000 m to the south. This arrangement provides adequate receiver density for wide-angle migrations of the intermediate offset data, which samples the structures in the transition from ocean to continent, while still recording traveltime information to relatively long offsets. Indeed, some airgun recordings are clear to ~335 km offset, and offsets to greater than 200 km are common. The Texan data were parsed, transcribed to SEGY format with geometry, and timing corrections at Rice following the experiment. The data set will be delivered to the IRIS DMC in January 2006. Zelt and Magnani managed the onshore-offshore recording operations, M. Schmitz (FUNVISIS) managed the land explosion teams. Parts of the 64W, 67W, and 70W onshore-offshore profiles were acquired as low-fold reflection surveys in late 2004- early 2005 by FUNVISIS at part of GEODINOS. At least one of these profiles will be processed and interpreted at Rice, by a Venezuelan student beginning Ph.D. studies at Rice in January 2006.

Rice student A. Beardsley and PASSCAL personnel installed 15 seismic recorders on the ABC Islands, Los Roques and Margarita Island to record the Ewing’s airguns. The NAMS and FUNVISIS provided secure sites and helped install instruments. Despite the relatively high noise levels endemic to island recording, the island data have provided a dataset suitable for 3D crustal tomography, which we describe below.