Bottom-simulating reflectors

In 1967, the Soviets discovered the first major hydrate deposit in the permafrost (Makogon, 1987). The hydrate deposit in the Messoyakha field has been estimated to involve at least one-third of the entire gas reservoir, with depths of hydrates as great as 900 m. During the decade beginning in 1969, more than 5 x 109 m3 of gas were produced from hydrates in the Messoyakha field. The information in the Soviet literature on the production of gas from the Messoyakha field is discussed in Chapter 7. Table 7.4 in Chapter 7 also lists other locations in Russia, including the Black Sea, Caspian Sea, and Lake Baikal, where evidence for hydrates has been provided from sample recovery or BSR (bottom simulating reflectors) data. 

FIGURE 7.12 (a) Bottom simulating reflector for hydrate deposit in Blake-Bahama Ridge,... 

Ocean Gas Hydrate Bottom Simulating Reflector (BSR) Extent ... 

COLOR FIGURE 7.20 Seafloor slump in the Blake-Bahama Ridge shown in both seismic (top) and cartoon (bottom) relief. (From Dillon, W.P., Nealon, J.W., Taylor, M.H., Lee, M.W., Drury, R.M., Anton, C.H., Natural Gas Hydrates Occurrence, Distribution, and Detection, (Pauli, C.K., Dillon, W.P., eds.) American Geophysical Union Monograph, 124, p. 41, Washington DC (2001). With permission.) Note the bottom simulating reflector parallel to the ocean bottom, except in the middle section where it appears a seafloor eruption has occurred. 

J. (1978) Diagenesis of Late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea. Sedimentology 25, 155-181. 

Hyndman, R.D., and Spence, G.D., 1992. A seismic study of methane hydrate marine bottom-simulating reflectors. Journal Geophys. Res, (97) 6683-6698. 

In 1995, Japan s Ministry of Economy, Trade and Industry (METI), formerly Ministry of International Trade and Industry (MITI), launched a project to explore for marine gas hydrate accumulations around Japan. From late 1999 to early 2000, an exploratory hole MITI Nankai Trough was drilled on the landward side of the eastern Nankai Trough, offshore Japan (Fig. 1) by Japan National Oil Corporation (JNOC) along with Japan Petroleum Exploration Co., Ltd (JAPEX) as the well operator. The water depth at the drill site was 945 m and the sub-bottom depth of the hole was 2355 m. The seismic bottom simulating reflector (BSR) is present at around 295 mbsf. There were two exploration objectives one was a gas hydrate survey in shallow Quaternary sediments and the other was conventional oil and gas exploration in deeper Tertiary sediments. In addition to the main hole, seven short holes (two site survey, two pilot and three post-survey holes) were also drilled for the gas hydrate survey around the main hole. In this paper, we will clarify the origins of methane in gas hydrates found in the MITI Nankai Trough Well and discuss gas migration and hydrate formation in the sediments. 

Chi W.-C., Reed D.-L., Liu C.-S. and Lundberg N. (1998) Distribution of the bottom-simulating reflector in the offshore Taiwan collision zone. Terrest. Atmos. Ocean. Sci. 9, 779—794. 

Fig. 1 Methane hydrate, which is stable belou- and to the left of the phase boundary line. Also shovra is the geothermal gradient in permafrost as well as marine environments. Where the curves intersect, natural methane hydrate is stable. Natural methane hydrates are found in the lightly shaded region. BSR labels the "bottom-simulating reflector." an unexpected interface found by sonic exploration techniques and usually associated with the interface between sediments with and without hydrate. View this art in color at...

The importance of adequate calibration is paramount in any ultrasonic inspection and is generally caurried out both to monitor equipment stability and to enable defect echo amplitudes to be referred to those from known standard reflectors. Figure 6 depicts the calibration block specifically designed for this work. One surface was machined concave with the same radius of curvature.as the cone at the position of the outer weld. Two 3 mm diameter flat bottomed holes (FBH) and a 1.5 mm diameter side drilled hole (SDH) were provided at a depth equivalent to the cone plate thickness and spaced sufficiently far apart that reflections could be obtained from each one independently of the others. A second 1.5 mm SDH at 15 mm depth served two purposes, firstly as euti equivalent reflector to the SDH in the standard A2 block and secondly to provide a means, in conjunction with the other SDH, of checking the probe angle. One section of the block, V thick, simulated the cone plate itself and was used for recording backwall echo amplitudes for the focused and normal probes. 

The array reflector simulated a room with overhead water sprinklers and consisted of a 2-ft-thick concrete floor touching the bottom of the array, a 2-ft-thick concrete ceiling offset 10 ft above the floor, two 6-in.-thick full-density water walls touching the sides of the array, and two 12-ft-thlck low-density water-mist walls touching the other two sides of the array. All void tras filled with low-density water mist. 

Figure 4.9 shows the resulting flow characteristics of the CFD-simulation of the 1-hole reflector geometry in the levitator without the process chamber. The sonotrode is located at the top, the concave reflector with the nozzle outlet at the bottom of Fig. 4.9. For orientation purpose, the top three pressure nodes are visualized as black spots. The turbulent free jet expands after leaving the nozzle due to air suctioning at the lower part of the levitator and decelerates continuously before being deflected at the sonotrode to both sides. Since the deflected gas at the top and the jet created by suction at the bottom have opposite directions, ring vortices are generated at both sides.

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