Most of the Arctic Ocean is covered with ice with the thickness of 1 ~ 6m, so it is very challenging to carry out geophysical survey. Although the ice on the continental shelf will melt in summer, the ice sheet in the center of the Arctic Ocean will not melt all year round. Due to the special natural conditions of the Arctic Ocean, the geophysical survey in the study area is extremely difficult. Geophysical surveys in the deep waters of the Arctic Ocean began in the early 1960s. Only a few countries such as the United States, Canada, Russia, Germany and Sweden use floating Iceland (ARLIS II and Lorex), submarines (SCICEX) and icebreakers as carriers or scientific platforms to collect various geophysical data in Lomonosov Ridge and its vicinity. Figure 10- 1 shows the routes of previous scientific research and the positions of collected seismic lines. The following will briefly introduce the scientific research activities of previous geophysical surveys in Lomonosov Ridge of the Arctic Ocean in chronological order.

I. ARLIS Ⅱ1961~1965 Scientific Research

196 1 year, the US Navy's second Arctic research experimental ice station (ARLIS II) was located on an ice floe island about 350km away from Barrow Point, Alaska (Ostenso et al., 1977). During 1963 and 65438+February, floating Iceland drifted to the Faroe Strait, and the northernmost part reached Lomonosov Ridge, facing the makarov Basin (Figure 10- 1). This scientific research project includes water depth, gravity measurement and seismic exploration. In the ARLIS II scientific research, seismic data were collected along the path from the edge of makarov basin to Lomonosov Ridge. Through the re-processing of seismic reflection data (Weber et al., 1985), the continuous sedimentary cover 60 meters above Lomonosov Ridge was revealed. In addition, in the drift path through Lomonosov Ridge along the inclined angle, the seismic data also reveal this kind of continuous sedimentary cover with good stratification, and its thickness increases to > 850 m at the flat top of the Ridge (Weber et al., 1985).

Second, LOREX 1979 scientific research

1979, Canada organized and implemented the lomonosov ridge experimental project (LOREX), and used floating Iceland for seismic exploration again (Weber, 1979). This seismic exploration is equipped with 0. 164 L( 10 in3) air gun as the seismic source, which is excited through a hole on the ice floe island. In addition, a deep-penetrating multi-channel seismic excitation system (Weber, 1979) is adopted. In order to obtain high-precision seismic data, the LOREX project also assembled a 3 kHz basement detection profiler (Weber, 1979). This seismic data reflects the geological structure characteristics of the narrowest part of Lomonosov Ridge near the pole (Figure 10- 1). The analysis results of these seismic data show that Lomonosov Ridge is composed of a series of inclined fault blocks arranged in echelon, and the top of it is covered with a layer of loose sediment with a thickness less than 75 m (Weber et al., 1985). In addition, in the area where the experimental ice station of LOREX project passes through Lomonosov Ridge, the phenomenon of consolidated sediment erosion was also found (blasco et al., 1979).

Iii. Arctic Ocean 199 1/ARK-VIII/3 Scientific Research

After the completion of the LOREX project, the Arctic expedition plan was implemented again in 12 and 199 1 year, and the entire Lomonosov Ridge (Fü tter,1992; Qiao Kate et al., 1992).Polarstern (German) and Oden (Sweden) icebreakers * * * both escorted the expedition. The seismic equipment towed by Polarstern has a device that can run on the Arctic sea ice. This method draws lessons from some successful experiences in the past Arctic seismic exploration activities, and has been proved to be very effective in the extreme environment of the Arctic (Grantz et al., 1986).

In this investigation, two 3L (about 183 in3) air guns were used as seismic sources, and the air guns were suspended under a hydraulic device with a weight of 1t, so that the air guns were as close as possible to the stern deck of the icebreaker (Jokat et al., 1992). In addition, a 300-meter-long 12 seismic streamer was used. The seismic reflection data (Jokat et al., 1992) of Lomonosov Ridge under the ice cover were successfully obtained in this expedition. In addition, sonar buoys are also used to detect the velocity structure and coverage characteristics of Lomonosov Ridge (Jokat et al., 1992). Among them, two seismic reflection profiles (AWI-9 1090 and AWI-9 109 1) located at 87 55' and 87 40' north latitude respectively completely pass through Lomonosov Ridge for the first time (Figure10-/KLOC) This set of strata has good bedding and undisturbed sedimentary coating characteristics (Jokat et al., 1992). These two survey lines provide a basis for selecting drilling locations in paleoceanography.

Figure 10- 1 Seismic Acquisition Roadmap Covering Lomonosov Ridge

(According to jakobsson et al., 2000)

In this expedition, high-precision shallow seabed profile data (Fütterer et al., 1992) were obtained at the frequency of 2.5~5.5 kHz by using the infrasound system installed on the Polarstern icebreaker. Shallow seabed profile data collected along AWI-9 1090 and AWI-9 109 1 line clearly show undisturbed sediments with a thickness of 30 ~ 40m on Lomonosov Ridge. Polarstern icebreaker is also equipped with Atlas Hydrosweep multi-beam bathymetric sonar to collect water depth data of Lomonosov Ridge (Fü tter et al., 1992). All the data collected above have been imported into the Arctic Ocean International Water Depth Database (IBCAO) (jakobsson et al., 2000), and later this database also provided the water depth data of Lomonosov Ridge for 302 scientific research.

Iv. Arctic Ocean 1996/ARK-XII/ 1 Scientific Research

1996, Auden and Polarstern cooperated again in the Arctic Ocean expedition. Geophysical data collected include seismic reflection data, seismic reflection test data (Kristoffersen et al., 1997) and high-precision shallow seabed survey data collected by FM sonar (Backman et al., 1997). This is also the first time to use FM sonar to collect data in the central area of the Arctic Ocean. Compared with 199 1, this geophysical survey is mainly focused on the lomonosov ridge near the edge of Siberia (figure 10- 1). The seismic reflection profile obtained also successfully transects the Lomonosov Ridge (Figure 10- 1). A total of more than 700 kilometers of seismic data were collected. In this investigation, an air gun group (5.5 liters) consisting of four telescopic air guns provided the seismic source. The air gun is assembled in an iron cage and dropped by a hydraulic device with a weight of1t. Unfortunately, due to the influence of ice floes, this device was lost (Kristoffersen et al., 1997). After that, two 3L Prakla earthquake air guns were installed and a light sinker was assembled for them, and the problem was solved. In addition, the expedition team is equipped with 16 seismic streamers, which are 200m long and offset by150m.

Figure10-2awi-91091seismic profile

(According to jakobsson et al., 2000)

See figure 10- 1 for the profile position.

Another achievement of this expedition is to supplement the water depth data of Lomonosov Ridge in the range of 85 20 ′ n,135 e and 87 40 ′ n,155 e (Jalobsson, 1999). These data also provide important help for the construction of the Arctic Ocean International Deep Database (jakobsson et al., 2000).

V. ark -XIV/ 1a scientific research

The main purpose of ARK-XIV/ 1a scientific expedition is to collect samples and obtain geophysical data at the Alpha Ridge (Jokat, 1998) in the American Arctic Ocean. Due to the extremely harsh natural conditions in the Alpha Ridge area, two icebreakers were used: Polarstern as a scientific research platform, and Russian nuclear-powered icebreaker Arktika for icebreaking. Although the ice layer in the study area is 6 m thick and very hard, 320km multi-channel seismic data were collected along three survey lines, revealing 500 ~ 1200 m thick sedimentary layers.

On the way back to Laptev Sea, we chose a route with good ice conditions along Lomonosov Ridge, and successfully obtained several seismic profiles (Jokat, 1998) (Figure 10- 1 and 10-3). These profiles reveal the geomorphological features of Lomonosov Ridge to the south of 85° N, while the two scientific expeditions of 199 1 and 1996 were both carried out in the north of 85° N. In addition, the results of this scientific expedition also reflect the characteristics that the top sediments of Lomonosov Ridge gradually thicken towards the edge of Laptev Sea. After that, the 1 main well and 3 spare wells of 302 scientific research are located in the above survey line. The location of the standby well is mainly determined by considering the unexpected situation caused by bad ice conditions.

Fig.10-31ARK-XIV/ 1a AWI-98590 seismic profile obtained from scientific research in 1998.

(According to jakobsson et al., 2000)

VI. SCIEX1999 Scientific Research

Sciex1999 used USS Hawkbill, an American nuclear-powered submarine, to avoid the interference of polar ice floes, so a large-scale investigation was conducted on Lomonosov Ridge (Edward et al., 2003) (Figure 10- 1). Along the designed routine survey route, the SCAMP system installed on the USS Eagle beak is used for continuous side-scan sonar and multi-beam sounding survey. Its main purpose is to explore the erosion characteristics of Lomonosov Ridge, and make supplementary investigations near two original seismic lines (AWI-9 1090 and AWI-9 109 1). The analysis of SCAMP FM sonar data shows that the erosion of Hailing Ridge is mainly confined to the area with water depth less than 1000 m (Polyak et al., 200 1), and in AWI-9 1090 and AWI-91earthquake.

7. Arctic Ocean 200 1

200 1 in order to drill for voyage IODP302, it is necessary to supplement the collection of seismic contact line of lomonosov ridge. Therefore, the Swedish expedition team made a supplementary investigation on Arctic Ocean 200 1 (Kristoffersen et al., 200 1). Two 8.5-liter GI air guns (about 5 19 in3) and eight 200-meter-long seismic streamers (Kristoffersen et al., 200 1) were used in this investigation. Under severe ice conditions, seismic data of1091(Figure101) were collected. However, due to the complex ice conditions, the investigation days were shortened from 5 days to 3 days, and the 400-meter geophone streamer was damaged.

Eight. IODP 302 voyage

In 2004, four wells were drilled in the voyage of IODP302 (along the AWI-9 1090 survey line), and single-channel seismic and 15 kHz echo detection data were collected near the well site, aiming at four wells of M00 1 ~ M0004 respectively. In this data acquisition, a 0.65-liter (40 cubic inches) PAR 1600 air gun with corrugated casing is used as the seismic source. The signal receiving system is a single seismic streamer with an effective length of 16m. The seismic streamer consists of 100 AQ- 1 geophone and preamplifier. The shot spacing is set to 2.7s, the delay time is 1.0s, the frequency of sampling received signal is 4 kHz, and the sampling point spacing is1.3s.. ..

On the seismic profile of 302 voyage scientific research, Cenozoic stratigraphic sequence can be divided into two sets of seismic stratigraphic units, which are equivalent to LR5/LR6 and LR3/LR4. The seismic reflection characteristics are basically not deformed in the whole section. The seismic reflection in the upper part of LR5~LR6 strata is weak and discontinuous, indicating that the wave impedance difference is small. The wave-like seismic reflection characteristics of several layers show that the seabed topography is uneven. There is a set of flat layered seismic reflection at the bottom of LR5~LR6 strata (Figure 10-4). The relationship between seismic reflection and actual formation response needs to be further studied in combination with well and earthquake.

Figure 10-4 M000 1 ~ M0003 3D image of seismic profile near well point.

(According to jakobsson et al., 2000)