MARINE

A number of 10 SEDIS-V OBS by GEOPRO were deployed at sea from 2006-11-29 to 2006-12-02 (Fig.11). The instruments (Table 11, Figures 14 and 15) employed INPUT/OUTPUT SM-6, B-Coil broad band geophones and hydrophones, and high performance, low-drift clocks. The system recorded the vertical and horizontal components on channels 1,2 and 3, respectively. Due to lack of compass, the true orientation for the horizontal channels will possibly be obtained by analysis. The hydrophone data were recorded and channel 6. Table 12 shows the instrumental settings and characteristics. Appendix 7.4 reports the coordinates of deployment of the OBS stations (Tables 15).

From tables 11, 12 and 13 the conversion factor is

• Velocity = N Vref / Digitizer_span / Gain / sensor_sensitivity (V/m/s), that is
• Velocity = N 4.5 / 5242878 / 10.6 / 28.8 = N 2.81154092$^{-9}$ (m/s), where N is signal amplitude in counts.

True ground motion can be obtained by deconvolving with transfer function of geophone, that can be found from 2 Zeroes (0,0) ; (0,0) and 2 Poles (-15.83362697,23.42507659) (-15.83362697,-23.42507659).

Table 11: GEOPRO's SEDIS-V OBS characteristics.

Analogue inputs	6 differential channels
Input signal range (Vref)	+/- 4.5 V
Over voltage protection	+/- 40 V
A/D converter	High performance Delta-Sigma CS 5321 on each channel
Digital filters	CS 5322, cut-off frequencies: 500 Hz, 250 Hz,125 Hz, 62.5 Hz
Dynamic range	120 dB @ 250 Hz
Clock oscillator	TCXO, 0.01 PPM, 0-50 C
Main processor,Onboard memory	Intel XScall PXA255, 64 Mbytes DRAM
Data storage interface	PCMCIA, CF memory card Radio, Modem telemetry
Power supply, Consumption	voltage:9-36VDC; standby:1.2 W,recording:2.1 W 6 channels

Table 12: Data Acquisition parameters, geophone characteristics and settings.

Sampling rate	4 msec
Number of channels per OBS	4
Type of reconding	continuosly recording
Start recording	08:00:00 29/11/2006 UTC
End recording	12:00:00 05/12/2006 UTC
channel 1 (Vertical)	SM6 B-Coil (4.5Hz, 28.8 V/m/s +/-5%)
channel 2 (Horizontal 1)	SM6 B-Coil (4.5Hz, 28.8 V/m/s +/- 5%)
channel 3 (Horizontal 2)	SM6 B-Coil (4.5Hz, 28.8 V/m/s +/-5%)
channel 6 (Hydrophone)	HTI-1
Sedis V Gain	10.6 (1,2,3) 50.4 (6)
Digitizer Resolution	24 bits (1,2,3,6)
Cutoff Frequencies	93.6 - 102.9 (-3dB) - 125 (-130dB) Hz

Table 13: GEOPRO's SEDIS-V Digitizer span.

Voltage	Sedis reading (Hex)	Sedis reading (Dec)
+VREF	4FFFFF(H)	+5242879
0V	000000(H)	0
-VREF	500000(H)	-5242880

Upon recovery, the OBS data were downloaded and a QC check was performed.

figure

Figure 14: OBS frequency and phase lag response.

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Figure 15: R/V Urania : (left) Assembling of an OBS inside a 17" glass Benthosphere; (right) Release procedure of an OBS off Urania.