IODP-MSP drilling and logging contractor: ESO

Hole: M0027A

Expedition: 313

Location: New Jersey (NW Atlantic)

Latitude: 39° 38.0406' N

Longitude: 73° 37.3015' W

Logging date: May 19 - 21, 2009

Sea floor depth (driller's): 50.15 m DRF

Sea floor depth (logger's): 49.50 m WRF

Total penetration: 631.01 m DSF

Total core recovered:  471.59 m (86.21 % of cored section)

Oldest sediment recovered: Eocene

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)
21-May-09	328.72	0.45	0.91	204-329	0-204

The VSP data was acquired on the May 21, 2009 from 328.72 to 0.45 m WSF with a spacing of 0.91 m. Part of this survey (from 328.72 to 204.00 m WSF) was carried out in open hole. The lower part of the borehole was not surveyed due to a technical issue with the VSP tool. No processing was carried out prior to the first break picking of the seismic arrival through the sediment, which was a trough.

The seafloor geophone and the airgun shuttle signal were used to determine the traveltime from the source to the seafloor. The seafloor geophone peak and the time the airgun shuttle signal amplitude began to be clipped were pikced. Variations in airgun elevation and firing time differences between the two airguns were accounted for using this time. Within the stacked data, the data is time-shifted to reflect the source firing at time zero and at a constant elevation. The time shift reflects the source firing at time zero. The stacked data files are all time-shifted, with the static shift used saved within the stacked data files. Due to the nature of SEG-Y formatting, the time used for the static shift has been truncated to be an integer; the static shifts, however, reflected the 250 microsecond sampling rate and were calculated and applied with 250 microsecond accuracy.

The general layout of the VSP the layout of the survey conducted in this borehole are available at http://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-M0027A_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 VISTA software. Data was then processed using the Mathworks MATLAB and Seismic Imaging Software VISTA programs.

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-M0027A_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.

VSP data was recorded and processed by the University of Alberta who were contracted by EPC.

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