An Effective Method to Determine the Distribution Boundary of Cobalt-Rich Fe-Mn Crusts on a Guyot: Synchronous Application of Sub-bottom Profiling and Deep-Sea Video Recording

Cobalt-rich ferromanganese crust, hereafter called Fe-Mn crust, is one of the important marine mineral resources in the international seabed. Fe-Mn crust occurs on the surface of seamounts, which are enriched in cobalt, nickel, copper, platinum group elements, rare earth elements and other metal elements, and is of considerable interest to many nations. Systematic investigations by the USSR (and later Russia), USA, Germany, France, Japan, UK and Korea have been conducted since the 1980s (Aplin and Cronan, 1985; Hein et al., 1985; Halbach, 1984; Halbach and Puteanus, 1984). In order to eusure ordered activities, the International Seabed Authority (ISA) is providing exploration and prospecting regulations. China's systematic investigations started in the late 1990s (Pan et al., 2002; He et al., 2001), and accelerated for the purpose of finding promising Fe-Mn crust deposits.

According to the summit shape, the seamounts investigated by China currently in the Pacific can be divided into two types: the flat summit seamount (so-called guyot) and the fastigium seamount. The upper boundary of the Fe-Mn crust distribution is easy to determine in the latter type, due to the small summit and wide flank, however the former has a wide and flat summit formed by steady sedimentation, and the crusts formed early on that summit are usually covered by subsequent sedimentation, therefore, it is considered that the upper boundary of the Fe-Mn crust distribution is related to the lower boundary of sediment on the summit or flank.

It is important to estimate exactly the mineral resources that determine the boundary of Fe-Mn crust distribution on the guyot, which is also the key for delineating the mining site. We could not determine the upper limit of the guyot for a long time, which presented a problem for crust resources evaluation because of the limitation of the survey method and facilities. During the period of cruise DY105-16A, conducted by the R/V DaYang Yi Hao in 2004, a long section survey of deep-sea video-recording was adopted specifically on the guyots, and a sub-bottom profiling survey was carried out at the same time, while a stationary sub-bottom drill sampling was done at the shallow water on the guyot. The synchronous application of these methods will help to reveal the boundary of crust distribution for the first time.

DATA ACQUISITION AND PROCESSING

The TOPAS PS018 Parametric Sub-bottom Profiler used on the cruise is the product of Kongsberg Defence & Aerospace AS. This advanced sub-bottom profiler system is comprised of a transceiver unit (including power amplifier, T/R-switch receiver, transmit/receiver transducer array), operator console and output equipment (Kongsberg Defence & Aerospace AS, 2002, TOPAS PS 018 Operator Manual) (Fig. 1). This system is capable of 150 m of maximum penetration and 30 cm of maximum resolution. The work frequencies are: primary 12.5-17.5 kHz and secondary 0.5-5.0 kHz. The parameters of pitch, roll and heave can be corrected in real time. The TOPAS sub-bottom profiler can operate with various pulse forms: single pulse (Ricker wavelet), CW wavelet (Bursts), FM-pulses (Chirps) and other coded wavelets. Choice of wavelet depends on the aim of survey and the water depth: if high resolution is needed or at shallow water, the Ricker wavelet can be selected, however, if the aim is to gain high penetration or at deep water, Chirps can be selected.

Figure  1.  Schematic block diagram of the TOPAS PS 018 system.

DownLoad: Full-Size Img PowerPoint

The deep sea video-recording system, DYDEEPSEAVIDEO2001-2, developed by the Institute of Mineral Resources, Chinese Academy of Geological Sciences, consists of an underwater unit and deck unit. The communication between the two units is through a 10 000 m armored coaxial cable (Fig. 2). The system can work for a long time in the water, because of the power supply from the ship's deck.

Figure  2.  Schematic block diagram of the deep-sea video-recording system.

DownLoad: Full-Size Img PowerPoint

The video-recording system was drawn by the armored coaxial cable, from the outer edge of the summit to the flank, along the designated survey line, 3-5 m above the sea floor, thus ensuring a clear image. At the same time, the sub-bottom profiler worked synchronously; a multibeam sounding system recorded the water depth data. According to the water depth, Ricker, Chirps and Bursts wavelets were selected, and ping interval and delay time were adjusted in real time, in order to obtain the perfect profile. The ship speed kept 1-2 knots, course steady, during the survey.

Real time filtering matching, bandpass filtering, time variable filtering and time variable gain were applied to process the sub-bottom profiling data. Digitized processing and location correction preliminary to the video-recording data were implemented.

RESULTS AND DISCUSSION

The results from the synchronous application of sub-bottom profiling and video-recording on several guyots in the west Pacific indicate that an obvious relationship exists between the seamount sub-bottom structure and the Fe-Mn crust distribution.

Figures 3 and 4 show two sections (local) of the sub-bottom profile and their interpretative results, which were acquired during the video-recording survey. These data were compared with the seismic profile and ocean drilling project (ODP) data (Yang et al., 2004; Bergersen, 1995; Haggerty and Premoli Silva, 1995; Abrams et al., 1992). The revealed sub-bottom structure can be divided into two layers, layer Ⅰ and layer Ⅱ from upper to lower (Fig. 3). Layer Ⅰ is sediment composed of foraminifer ooze. Layer Ⅱ is substrate with bio-clasts limestone located above and a volcanic edifice (including volcaniclastite and basalt) located below. The interface between two faces can be further identified according to the seismic data.

Figure  3.  (a) Section A of sub-bottom profile from a video recording site (water depth 1 425-1 620 m). The upper figure is the original profile, the interpreted profile is beneath. SD denotes site of deep-sea sub-bottom drilling. (b) Section B of sub-bottom profile from x video-recording site (water depth 2 505-2 610 m). a-h denote the corresponding picture numbers in Fig. 5.

DownLoad: Full-Size Img PowerPoint

Figure  4.  Images of special location from x video recording site. (a). sediment; (b). gravel crust; (c). plank crust; (d). gravel and plank crust; (e) plank crust; (f). sediment; (g). gravel and plank crust; (h). gravel crust.

DownLoad: Full-Size Img PowerPoint

The picture of the core sample that was obtained by sub-bottom drilling in cruise DY105-16A is shown in Fig. 3. The core sample is about 40 cm in length, the uppermost 10 cm of that is Fe-Mn crust layer, the other is bio-clasts limestone, both of them contact closely. Thus, it can be seen that the substrate should be mainly limestone on the upper flank of the seamount. However, the volcaniclastite or basalt were usually caught at the upper slope by dredge sampling before, perhaps because the volcaniclastite or basalt is easily weathered, consequently, it is easy to be caught by dredge. According to the drilling result mentioned above, the original locations of the dredged samples should be deeper in water depth.

From the video images (Fig. 4), it can be seen that there is little occurrence of Fe-Mn crust in the area with abundant sediment distribution on the summit of the seamount (Fig. 4a). The Fe-Mn crust begins to appear (Fig. 4b), starting from the nip-out of the sediment. The flat position on the flank of the guyot is the ideal place where the sediment deposited, the Fe-Mn crusts generally do not appear and the gravel crust can occasionally be seen at the surface of this kind of place (Fig. 4f), but the Fe-Mn crusts are usually well developed on the surface of the substrate next to the place covered by sediment.

Therefore, according to the sub-bottom structure of the seamount revealed by the sub-bottom profile, the sediment distribution area can be identified, and then the undeveloped or blank area Fe-Mn crusts can be determined. The ability to determine this kind of area in resources evaluation will improve assessment results. Based on the analysis of the sub-bottom profile (Fig. 3), the water depth of the upper boundary of the Fe-Mn crust distribution of guyot x in the west Pacific is about 1 560 m. According to the analysis of the sub-bottom profile at different positions on the seamount, we can circle the limit line of the upper boundary distribution of Fe-Mn crusts. The more profiles can be obtained, the more exact the circled limit line will be.

Presently, there are some shortcomings in the acquisition of these two kinds of data. Firstly, the exact position of the video-recording system in the water can not be obtained. Improvements could include a mounted underwater positioning system or the installation of a pressure sensor. Secondly, the signal of the sub-bottom profiler is difficult to follow efficiently, when the topography greatly changes, or in deep water.

CONCLUSIONS

(1) Based on the sub-bottom profiling data, the lower boundary of sediment distribution on the summit of a guyot can be determined, and consequently the upper boundary of Fe-Mn crust distribution can be inferred. (2) By comparing the sub-bottom profiling data and topography of the seamount, the relationship between topographic form and the distribution of sediment can be judged. This can help in determining the boundary of the sediment distribution area. Further, the distribution area of Fe-Mn crusts can be determined. (3) The basis of Fe-Mn crust resources evaluation of a guyot is important for determining the distribution boundary and undeveloped or blank area of Fe-Mn crusts. The synchronous application of sub-bottom profiling and deep-sea video-recording is an efficient method for obtaining an answer to this question. By analyzing the sub-bottom profiling data and the lower boundary of sediment on the guyot summit, and comparing with deep-sea video-recording images, the water depth range of Fe-Mn crust occurrence can be inferred. By combining this data with the sub-bottom drilling sampling results, the thickness of Fe-Mn crusts in a certain area can be known. Therefore, the evaluation result of the available resources will be improved based on this information.