Marathon Expedition Leg 15


Thomas Washington Departed Rodman Naval Base, Panama, on 21 May after loading
explosives.  The successful delivery of explosives is due to much hard work by
Bob Stapleford, Bob Lum, Jack Langille, George Shor, and no doubt many others…
many thanks.  Arrived at experiments site early on 27 May and spent the day
filling in Seabeam survey while sister ship R/V Conrad deployed her
multi-channel streamer.  We have completed two expanding spread profiles (ESPS)
using an airgun array on Washington with greater than a 2000 cubic inch capacity
on the ingoing legs and 60 pound explosive charges on the outgoing legs.  Airgun
array has been very nicely synchronized and the seismic source function is
extremely clean.  Seabeam is valuable in conducting this sort of detailed
seismic work.  Atlantis II is also in the area with a diving program just to the
north with Fornari.  (Orcutt)


KGWU Thomas WAshington 031650Z June 1985. We have completed our seismic work at
the initial southern area near 9 degrees north on the Est Pacific Rise. Dure to
excellent weather and hard work we completed the schedules a day early which
allowed three additional expanding spread profiles (ESP) to be conducted.
Following completion of our work in the southern area, with Washington on
Station 5200 m aft of LDGO R/V Conrad. This configuration, called wice angel
reflection profiling *WARP), permits synthesizing an array twice conducted
obliquely to the East Pacific Rise axis during the three day transit to the
final operation area at 14 degrees north. Following warp 10 the two ships
completed three ESPPS using the remainder of the explosives on board. The ESPS
conducted on those just 1-1/2 km off the rise axis provides strong evidence for
the presence of a crustal magma chamber. ESPS only 2-1/2 km off magma chamber is
less than 2-1/2 km. We will depart the northern experiment area on 6 June after
completing several more ESPS and warps. (J. Orcutt)