IODP-MSP drilling and logging contractor: ESO

Hole: M0077A

Expedition: 364

Location: Chicxulub Impact Crater (Gulf of Mexico)

Latitude: 21° 27.009' N

Longitude: 89° 56.962' W

Logging date: April 14-15 (upper section), May 1-4 (middle section), May 26-28 (lower section)

Sea floor depth (driller's): 34.2 m DRF

Sea floor depth (logger's): 33.94 m WSF

Total penetration: 1368.89 m DRF (1334.69 m DSF)

Total core recovered:  839.51 m (100 % of cored section)

Oldest sediment recovered: Paleogene

Lithologies: Carbonates, granite, melt

Date	Top depth (m WSF)	Bottom depth (m WSF)	Spacing (m)	Open hole survey (m WSF)	Through pipe survey (m WSF)
April 14-15, 2016	200	500	1.25	200-500	None
April 14-15, 2016	47.5	200	2.5	47.5-200	None
May 2, 2016	462.5	695	1.25	506-695	462.5-506
May 28, 2016	650	1325	5	700-1325	650-700

The VSP was recorded in three phases between April 14^th and May 28^th, 2016 (see deployment table). Four triaxial sondes and a telemetry unit were deployed with a spacing of 15 meters between each, giving a total tool string length of 60 meters (see VSP survey layout). The travel time was recorded from source to downhole receiver with no correction made for sea depth. All data was recorded digitally with the Sercel Slimwave Electronics package. UTIG provided the airgun, which was a dual solenoid design. Recording was triggered by firing of the airgun at 2 m below sea surface with 3.0 seconds of recording time for each shot. Sampling rate is 250 ms.

Prior to commencement of VSP recording, the telemetry unit was zeroed at 34.1 m below the driller's table, giving all recorded values in m WSF.

UPPER INTERVAL: April 14-15, 2016. The borehole was rotary drilled to 503 m DSF with 7 7/8 inch diameter. VSP data was acquired from 500 to 200 m WSF at sonde spacing of 1.25 m and from 200 to 47.5 m WSF at a spacing of 2.5 m. Data acquisition commenced at 17:20 (local time) on April 14 and ceased at 23:26 April 14.

MIDDLE INTERVAL: May 2, 2016. Coring ceased at 701.70 m DSF on May 1, 2016 and it was decided to change the drill string size. During this VSP deployment the drill rods remained in the borehole with a bottom depth of 506 m WSF. The VSP system was prepared for deployment by 00:00 May 2 with the first shots recorded at 13:20 and the final shots at 17:28 with recording from 695 to 462.5 m WSF at a 1.25 m spacing.

LOWER INTERVAL: May 28, 2016. Coring ceased about 00:00 May 26 with the total borehole depth of 1334 m DSF and logging operations began after the pipe has been removed. Coring operations were completed by 07:00 May 28 and rig up for the VSP commenced. The first shot was recorded at 10:07 am with the bottom sonde at a depth of 1325 m WSF with the last shot at 13:17 with the topmost sonde at 650 m WSF with a spacing of 5 m. This overlap with the middle interval allowed for correction of gaps.

Processing

The VSP data was recorded and processed by the University of Alberta who were contracted by EPC. Processing has been performed with the assumption of a vertical borehole.

The raw data acquired directly from the survey in the upper, middle and lower intervals are provided as as one merged SEGY file. The initial processing described here has been applied to only the vertical component of the data (azimuthal and radial signals were recorded as well for anisotropy investigations).

The processing steps are as follows:

1) There was a short lag between the first break of the lower interval as compared to the middle and upper intervals (presumably from the airgun being in a slightly different location). This may also be an effect of tides as the airgun was placed relative to the ocean surface. The data has been static shifted 1.46 ms to account for this and provide a continuous first break trough.

2) Particularly bad waveforms have been selected and removed (known as "kills").

3) The upper, middle and lower interval surveys have been combined in to one data set.

4) All data collected at the same depth has been added together in to one waveform per depth (stacking). This improves the signal to noise ratio.

5) First breaks have been manually picked by the center of first trough.

6) Some problems with the header values have been corrected (depths were being improperly recorded at some points).

7) Interval velocities and standard deviations have been calculated. A moving window differential method was used, with a window of 21 data points. Due to the 10 data point radius of this window, the first and last 10 depths cannot be calculated.

Data Quality

Data in the upper and middle sections was crisp and high frequency. The lower section still has crisp first breaks, but is missing the high frequency component. This is suspected to be caused by the manual triggering of the airgun (technical difficulties with software drivers appeared during VSP acquisition in the lower interval).

Quality control was performed in real time with additional shots performed when necessary. Afterwards, Coherent Comparison of interval velocities with preliminary analysis from the wireline sonic tool P-wave signal was performed. Damping of the wireline was done with available softeners due to oscillations caused by ocean surface waves. Noise in the seismic data is unavoidable, but has been reduced to a practical minimum.

Additional information about the VSP operations can be found in the IODP Proceedings of Expedition 364.

For further questions about the data, please contact:

Johanna Lofi

Geosciences Montpellier - UMR 5243 - CC 060 - Bat. 22 
Universite de Montpellier 2
Place E. Bataillon
34095 MONTPELLIER Cedex 05

France

Phone: 011-33-467-149-309

E-mail: johanna.lofi@gm.univ-montp2.fr

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