海底から探る地球-地震・地殻変動観測 (地震研究所海域観測研究グループ)
金沢 敏彦、塩原 肇、篠原 雅尚、望月 公広、山田 知朗 (地震研究所)

Seismic structure survey for the uppermost mantle and crust in the segment B4, the AAD using OBS and MCS


Masanao Shinohara, Kimihiro Mochizuki,Tomoaki Yamada, Kazuo Nakahigashi, David M. Christie and Chiaki Igarashi

1. Introduction
The Australian-Antarctic Discordance (AAD) is an anomalously deep and rugged zone of the Southeast Indian Ridge (SEIR) between 120°E and 128°E (Fig. 1). The AAD is also defined by the intersection of the SEIR and Australian-Antarctic Depression (Veevers, 1982) which is associated with a V-shaped trend of residual depth anomalies (e.g. Marks et al., 1990) and a saddle-shaped low in the geiod (Weissel and Hayes, 1974). The SEIR south of Australia is devided into three distinct zones (Weissel and Hayes, 1974). Zone B indicates the AAD itself. The anomalous large depths within the AAD are interpreted to result from an unusually cold underlying mantle that yields a low melt supply to spreading plate boundary (e.g. West et al., 1994). The AAD is also associated with distinct geochemical boundaries. The eastern AAD area is a distinctive boundary between Indian-type and Pacific-type upper mantle provinces (Klein et al., 1988).

The AAD has unusual geometry with altering north- and south-offset spreading sections of the spreading center (Vogt et al., 1984). There are five sections denoted as B1 - B5. At the spreading centers within the AAD, relatively thin oceanic crust may be formed by a low melt supply. The crust with a total thickness of 4.5 km is found in the B5 (Tolstoy et al., 1995). Their results indicates that the Oceanic Layer 2 is -3 km thick, whereas the Oceanic Layer 3 is unusually thin (- 1.2 km). From the multibeam bathymetric mapping, westernmost Zone A has normal seafloor characterized by axis-parallel abyssal hills. In B5 area, normal seafloor terrain occupies a west-pointing V-shaped region that is symmetrical about the spreading axis (Christie et al., 1998). In B4 area and outside of the V-shaped region in B5, the sea floor terrain is unusually choatic. In the Zone A and within the V-shaped region of the B5, Pacific-type mantle source rocks were dredged. In the other hand, all samples from the B4 area are Indian-type mantle source. (Pyle et al, 1992). The previous seismic experiment was carried out within the V-shaped region in the B5. The crustal and uppermost mantle structure of choatic seafloor topography area is not known yet.

Fig. 1. Batjymetric map of Southeast Indian Ridge showing the location of the AAD. A gray dash line shows the plate boundary. Contour interval is 1000m.

During the Hakuho-maru KH01-03 cruise Leg 3, we carried out the seismic experiment using large capacity airguns, Ocean Bottom Seismometer (OBS) and Multi Channel Streamer (MCS) in the segment B4 where the sea floor topography is choatic and Indian-type mantle source rocks were dredged.

2. Instruments
2.1 OBS

We used a pop-up type OBS (Fig. 2) equipped with flashing light, radio beacon and transponder. This OBS has one vertical component and two horizontal components velocity sensor with a natural frequency of 4.5 Hz (Mark Products L25B or L28B) on a gimbals mechanism. The seismic signal is sent to pre-amp and 16 bits AD converter, and sent to a Digital Audio Tape (DAT) for continuous and compressed data recording. We set 100 Hz data sampling each channel for this experiment.

Fig. 2. Photograph of the OBS used in this study.

2.2 Airgun
We used two Bolt 1500C type airguns;the airgun with a 17-liter chamber on the starboard side, and the airgun with a 20-liter chamber on the port side (Fig. 3). The air pressure was set at 1500 lb/IN2G, and airguns were shot every 20 or 60 seconds.

Fig. 3. Assembly of an airgun on the deck of the R/V Hakuho-maru before the cruise. Capacity of the chamber is 1000 cubic inchs (approximately 17 liters).

2.3 MCS
We used an Innovative Transducers Inc. solid-cable 24 channel analogue hydrophone streamer as receivers. The group interval was 25 meters, and each group consists of 5 hydrophone sensors. An approximately 300 m long nylon rope was attached at the tail in order to stabilize the streamer cable.

The Syntron MultiTRAK system was used to handle streamer positioning. Two MTUs (MultiTRAK Units) were mounted at the head and the tail of the streamer cable. These units control the cable depth by moving a set of motor driven wings. The MTUs were also equipped with a two-axis fluxgate magnetometer, and measured the horizontal components of the Earth’s magnetic filed. Given their mounted locations on the streamer cable, the host computer calculated its shape.

Geoscope StrataView was used as a data logger. It converted analogue signals from the hydrophones to digital data, and recorded the data on 4 mm DAT tapes.

3. Operations
3.1 OBS

We deployed five OBSs on the seafloor at spacing 20km to observe seismic waves from airgun source as shown in Fig. 4. Positions and time-adjustment data of all OBS are listed in Table 1 and 2. The OBS-airgun survey line crosses the ridge axis in the B4 segment. Although we planned to use ten OBSs, we could deploy five OBSs due to a bad sea condition (Fig. 5). After the shooting, we retrieved all OBS.

Fig.4. Bathmetry obtained during KH01-03 Leg 3 by the Sea Beam 2100 on the Hakuho-maru and the positions of deployed OBSs. A black solid line indicates the OBS profile where the airguns were shot every 60s. Gray lines are the MCS profiles.
OBS# Deployment Retrieval Depth(m)
Latitude(S) Longitude(E) Latitude(S) Longitude(E)  
1 49°24.70’ 125°42.66’ 49°24.83’ 125°42.71’ 3916
2 49°34.93’ 125°36.89’ 49°34.98’ 125°37.17’ 3571
3 49°44.99’ 125°31.21’ 49°44.95’ 125°31.19’ 4136
4 49°55.11’ 125°25.38’ 49°54.99’ 125°25.54’ 3314
5 50°05.27’ 125°19.64’ 50°05.18’ 125°19.79’ 3456
Table 1. OBS position information
OBS# time offset at deployment (UTC) time offset at retrieval (UTC)
1 2002 01 31 23 05 50 +0.183 s 2002 02 02 20 51 30 +0.606 s
2 2002 01 31 23 09 00 +0.247 s 2002 02 02 23 27 10 +0.597 s
3 2002 02 01 03 09 50 +0.255 s 2002 02 03 01 54 30 +0.567 s
4 2002 02 01 04 31 40 +0.277 s 2002 02 03 04 07 00 +0.650 s
5 2002 02 01 05 55 40 ?0.031 s 2002 02 03 05 27 30 +0.148 s
Table 2 OBS time-adjustment data
Fig. 5. Deployment of an OBS during a rough sea condition.

3.2 MCS
Once the streamer was deployed, and all the configurations on the MTUs and StrataView were set up, nothing particular was required to be done. The watch standers were asked to visually confirm current MTU locations and increments of the recorded File Number every 30 minutes.

The MTUs were programmed to maintain the streamer depth at 10 m below the sea surface. The sampling interval of the seismic reflection data was 4 msec. Their record lengths were either 20 seconds or 16 seconds; 20 seconds during shooting the first OBS-inline profile at a 60-second interval, and 16 seconds otherwise. The data were stored in SEGD 8058 rev.1 format.

4. Post cruise research plan
We will make analyses of the OBS data by conventional and other methods including ray tracing technique, seismic tomography, seismic imaging of reflectors and/or scatters and so on. The OBS data may give us information of microseismicity in spite of short observation period. The MCS data will be also processed using the conventional method to image beneath the profiles. These analyses will bring us knowledge about a structure of uppermost mantle and crust in the segment B4 of the AAD. We try to reveal the complicated tectonics of the AAD through comparison between the structure in the AAD and those of other ridge systems.

5. Summary
We successfully carried out the seismic experiment to obtain detailed crustal and uppermost mantle structure in the segment B4 of the AAD. The main profile is perpendicular to the spreading center and has a length of 100km. Five OBS were deployed at an interval of 25 km on the main profile and all OBS were recovered. In addition, MCS survey was performed on the main profile and additional five profiles.

References
Christie, D. M., B. P. West, D. G. Pyle and B. B. Hanan, Chaotic topography, mantle flow and mantle migration in the Australian-Antarctic Discordance, Nature, 394, 637-644, 1998.

Klein, E. M., C. H. Langmuir, A. Zindler, H. Staudigel and B. Hamelin, Isotope evidence of a mantle convection boundary at the Australian-Antarctic Discordance, Nature, 333, 623-629, 1988.

Marks, K. M., P. R. Vogt and S. A. Hall, Residual depth anomalies and the origin of the Australian-Antarctic Discordance Zone, J. Geophys. Res., 95, 17325-17337, 1990.

Pyle, D. G., D. M. Chistie and J. J. Mahoney, Resolving an isotope boundary within the Australian-Antarctic Discordance, Earth Plnet Sci. Lett., 112, 161-178, 1992.

Tolstoy, M., A. J. Harding, J. A. Orcutt and J. P. Morgan, Crustal Thickness at the Australian-Antarctic Discordance and neighboring south east Indian ridge, AGU meeting, 1995

Veevers, J. J., Australian-Antarctic Depression from midocean ridge to adjacent continents, Nature, 295, 315-317, 1982.

Vogt, P. R., N. Z. Cherkis and G. A. Morgan, Project Investigator - I: Evolution of the Australian-Antarctic Discordance from a detailed aeromagnetic study, Antarctic Earth Science: proceedings 4th International Symposium on Antarctic Earth Science, Australian Academy of Science, 1984.

Weissel, J. K. and D. Hayes, The Australian- Antarctic Discordance: New results and implications, J. Geophys. Res., 79, 2579-2587, 1972.

West, B. P. J.-C. Sempere, D. G. Pyle, P. J. Morgan and D. M. Christie, Evidence for variable upper mantle temperature and crustal thickness in and near the Australian-Antarctic Discordance, Earth Planet Sci. Lett., 128, 135-153, 1994.

Appendix. Geophysics research team of KH01-03 Leg 3

Back row: left to right, K. Matsuda, T. Yamada, K. Mochizuki, Y. Nogi, C. Igarashi, D. M. Chirstie.
Front row: left to right, K. Koizumi, K. Nakahigashi, M. Shinohara, K. Okino.