IODP-MSP drilling and
logging contractor: ESO
Hole: M0028A
Expedition: 313
Location: New Jersey (NW Atlantic)
Latitude: 39°33.9427′ N
Longitude: 73°29.8348′ W
Logging date: June 05, June 16-18, 2009
Sea floor depth
(driller's): 52.42 m DRF
Sea floor depth (logger's): 52.00 m WRF
Total penetration: 668.66 m DSF
Total core recovered: 385.50 m (80.82 % of cored section)
Oldest sediment recovered: Early Miocene
Lithologies: Clays to gravels
|
Date |
Top
depth (m WSF)
|
Bottom depth (m WSF)
|
Spacing (m) |
Open hole survey (m WSF)
|
Through
pipe survey (m WSF)
|
|
16-Jun-09 |
667.94 |
395.45 |
1.83 |
n/a |
PQ:
0-404 HQ:
0-668 |
|
17-Jun-09 |
656.36 |
402.15 |
1.83 |
425-656 |
PQ:
0-404 HQ:
0-425 |
|
18-Jun-09 |
431.53 |
20.05 |
1.83 |
404-432 |
PQ:
0-404 |
In hole M0028A the VSP survey was acquired in three stages. First, the data was acquired through pipe, from the base of the borehole to seafloor. As the quality of the data in the pipe was variable, it was decided to repeat the survey in the lower interval open hole. Finally, VSP data was collected following complete removal of the HQ pipe. On reaching the base of the PQ pipe (~396 m SWF) the tool was able to continue into open hole and thus the bottom ~30 m owere in open hole followed by the remainder of the hole through PQ pipe. For M0028A no processing was carried out prior to the first break picking of the seismic arrival through the sediment, which was a trough.
For M0028A the seafloor geophone was used in
conjunction with airgun firing time data from M0029A measurements to correct
for variations in airgun firing times, and the stacked downhole
data is shifted to reflect the airgun firing at time zero. At M0028A, the seafloor
geophone first arrival did not have a clear peak. Here the first breaks for the seafloor
geophone were picked when the signal came up from zero amplitude. The first breaks (peaks) of the
hydrophone suspended 20 m above the seafloor geophone are assessed in
conjunction with the seafloor arrivals to confirm a relatively vertical ray
path for the seismic wave between the source and the seafloor receiver. The static shifts were applied and saved
within the stacked data file, as above.
The layout of the VSP survey can be seen athttp://brg.ldeo.columbia.edu/data/iodp-eso/exp313/exp_documents/iodp-eso-313-vsp-layout.pdf and http://brg.ldeo.columbia.edu/data/iodp-eso/exp313/M0027A/documents/313-M0087A_vsp-survey-layout.pdf. Each VSP shot was recorded and
stored digitally. The signal
received by the geophone was digitized in a Geometrics geode and recorded using
the Geometrics Seismodule Controller software. Each VSP shot was recorded on 7 channels
(Table 1) to measure the vertical component on the downhole geophone, the
vertical and two horizontal components on a seafloor geophone, a seafloor
hydrophone (within the housing of the seafloor geophone), a suspended
hydrophone, and the electrical signal caused by the movement of the airgun
shuttle. Of the two airguns (20 and
40 cubic inches), only one was fired during each shot. The record lengths were 2.5 s long
covering the time period from -0.5 to 2 seconds. The sampling rate was 250 microseconds.
Data flow was monitored for quality in real
time and data quality was recorded on paper to be entered into an Excel
spreadsheet post survey, along with shot and receiver coordinates (datum
WGS84). Where available, borehole
tilt and azimuth measured by borehole logging is used to calculate horizontal
location of the downhole receiver. Where this information is not available, the borehole is assumed to be
vertical. The Excel quality and
coordinate measurements were entered into the VSP data files using MATLAB and
Processing involved removing poor quality
shots and stacking the remaining shots at a given depth to improve the signal
to noise. 3-D offset information
was calculated and entered. Where
possible, first break arrival times were picked for the downhole data, the
vertical seafloor geophone, the suspended hydrophone, and the airgun shuttle
signal. Based on a combination of
data from the vertical seafloor geophone, suspended hydrophone and airgun
shuttle signal, static correction were calculated and applied to account for
any time shift caused by the airgun firing past time zero. The wave form and arrival times of these
data were assessed for each survey to determine the best technique of
calculating static corrections.
The unstacked raw seismic data were organized into a SEG-Y revision 1 format file that included the necessary shot and receiver positions. A smaller corresponding file that contained the stacked data at each depth is 313-M0028A_vsp-stacked.segy. This file is available online and should be accessible by any program that is able to read the SEG-Y format data.
Additional
information about the drilling and logging operations can be found in the
Operations section of the Site Chapter in IODP Proceedings of Expedition 313.
For further questions about the data, please contact:
Jennifer
Inwood
University
of Leicester
Phone:
011-44-116-252-3327
Fax:
011-44--116-252-3918
E-mail: iodp@le.ac.uk
Johanna Lofi
University
of Montpellier 2
Phone: 033-
467-149- 309
Fax: 033- 467-
143- 244
E-mail: iodp@le.ac.uk
For any web
site-related problem please contact:
E-mail: logdb@ldeo.columbia.edu