Minerals Report - International Seabed Authority
Minerals Report - International Seabed Authority Minerals Report - International Seabed Authority
identification of gas hydrate in marine sediments has been the presence of anomalous signature on marine seismic records known as Bottom Simulating Reflections (BSR). The Carolina Rise along the eastern United States margin, particularly over the Blake Ridge, was the area where marine gas hydrate was first identified 32 on the basis of a BSR. It has been suggested 33 that reflection polarity reversal, a large reflection coefficient and increasing sub-bottom depth with increasing water depth are the different criteria that characterize a BSR. However, at the same time it was felt 33 that none of these criteria are unique to hydrates. Subsequent studies 33-35 have highlighted three manifestations of hydrates in sea-floor sediments, which have been used to recognize gas hydrates in seismic profiles on the U.S. Atlantic continental margin. These are: i) Bottom Simulating Reflections (BSR), ii) amplitude blanking, and iii) velocity inversion. 4.1.1. Bottom Simulating Reflections (BSR) Sloan 1 observed that hydrate has a very strong effect on acoustic reflections because it has a high acoustic velocity (approximately 3.3 km/sec, about twice that of sea floor sediments), and thus cementation of grains by hydrates produces a high velocity section. Sediments below the hydrate-cemented zone, if water saturated, have lower velocities (water velocity is about 1.5 km/sec), and if gas is trapped in the sediments below the hydrate, the velocity is much lower (even with just a few percent of gas). Because reflection strength at an interface is proportional to the change of acoustic impedance, which is the product of velocity and density, the base of the hydrate-cemented zone produces a very strong reflection. As the phase boundary is a distinct limit to hydrate occurrence, this reflection is sharply defined. The base of the gas hydrate stability occurs at approximately uniform sub-bottom depths throughout a small area, therefore, the reflection from its base roughly parallels the sea floor. This reflection has therefore come to known as 'Bottom Simulating Reflection' or BSR. In literature the term BSR has been used to represent either 'Bottom Simulating Reflector' or 'Bottom Simulating Reflection', causing a degree of ambiguity. According to Huene and Pecher 36 the bottom simulating reflection is the anomalous reflection seen on marine seismic records indicating the position in the sediment column of the base of the INTERNATIONAL SEABED AUTHORITY 528
zone of gas hydrate stability. On the other hand, the bottom-simulating reflector is the region of impedance contrast in the sediment column at the base of the zone of gas hydrate stability that gives rise to the anomalous reflection on seismic records. Several different criteria have been used to characterize the anomalous reflections inferred to be a BSR and to help differentiate these reflections from other unusual ones. These criteria 37 are i) Reversed polarity of the seismic wavelet relative to the reflection from the sea floor (equivalent to a negative reflection coefficient) at the BSR. ii) Frequent crosscutting of the seismic reflections from bedding planes by the BSR, indicating that it is not a bedding plane reflection. iii) Close mimicking of the sea-floor topography by the BSR. A BSR-like reflection may not necessarily arise from gas hydrate formations. In some areas, these acoustic features may also result from temperature-controlled diagenetic effects. During the Deep Sea Drilling Project (DSDP) Leg 19, seismic reflections akin to BSR in some Bering Sea sediments draped on the Umnak Plateau were observed (Kvenvolden 5 and references therein). The drilling at one of these sites (site 185) however indicated the presence of only methane in the sediment with no evidence of the existence of gas hydrates. In view of this, some researchers (cf. Kvenvolden 5 ) attributed those BSR-like seismic reflections to a lithologic transition from hemipelagic diatom ooze to indurated claystone. Thus, there are two types of BSRs, one indicating the base of the gas hydrate zone and the other signalling a diagenetic boundary (cf. Kvenvolden 5 ). Since the BSRs represent an acoustic velocity contrast in the sediment, it is not necessary that all BSRs reflect the existence of hydrates; similarly a BSR may not always be discernible when a hydrate is present 38 . Nevertheless the coincidence in depth of the BSR with the theoretical, extrapolated pressure and temperature conditions that define the hydrate phase boundary and sampling of hydrate above BSRs give confidence that this seismic indication of hydrates is dependable 4 . Therefore, gas hydrate areas are generally inferred when the so-called INTERNATIONAL SEABED AUTHORITY 529
- Page 486 and 487: workshop, in relation to deposit ev
- Page 488 and 489: an economic perspective, including
- Page 490 and 491: sulphides and cobalt-rich ferromang
- Page 492 and 493: CHAPTER 13 PETROLEUM POTENTIAL AND
- Page 494 and 495: structure and filled with sediments
- Page 496 and 497: Basin groups are subdivided into ty
- Page 498 and 499: during the late Mesozoic-Cenozoic s
- Page 500 and 501: Figure 4: Volume density of initial
- Page 502 and 503: ITIPRHC = Vnr φHC γ 103 (Mmtoe),
- Page 504 and 505: Table 1: Offshore (deepwater) initi
- Page 506 and 507: Speaking about particular geographi
- Page 508 and 509: World Ocean, including areas off no
- Page 510 and 511: Table 4: Largest oil and gas discov
- Page 512 and 513: The extremely harsh environment bro
- Page 514 and 515: In other regions of the world, enco
- Page 516 and 517: REFERENCES 1. L.G. Weeks (1971), Ma
- Page 518 and 519: SUMMARY OF PRESENTATION AND DISCUSS
- Page 520 and 521: Dr. Vysotsky said that estimates of
- Page 522 and 523: In Southeast Asia, Dr. Vysotsky spo
- Page 524 and 525: Philippines, and possibly Brazil. W
- Page 526 and 527: and we shall encounter severe deple
- Page 528 and 529: In the 1960's scientists discovered
- Page 530 and 531: Figure 2. Worldwide locations of kn
- Page 532 and 533: methane by bacteria in an anoxic en
- Page 534 and 535: It has been assumed that the struct
- Page 538 and 539: ottom simulating reflections (BSR)
- Page 540 and 541: Figure 6: Interpretative plot of hy
- Page 542 and 543: floor have indicated the presence o
- Page 544 and 545: of free gas below the BSR is usuall
- Page 546 and 547: sediment and to provide an indicati
- Page 548 and 549: 6. Harvesting methane hydrates -Som
- Page 550 and 551: deposits can be commercial, even in
- Page 552 and 553: at a catastrophic scale. Some of th
- Page 554 and 555: Figure 8. Development of hydrate re
- Page 556 and 557: This is perhaps due to the percepti
- Page 558 and 559: would provide Japan with methane fo
- Page 560 and 561: NOTES AND REFERENCES 1. E.D. Sloan
- Page 562 and 563: 20. R.F. Meyer (1981), Speculation
- Page 564 and 565: Master Workshop on Gas Hydrates: Re
- Page 566 and 567: 60. M.D. Max and M.J. Cruickshank (
- Page 568 and 569: is that their exploitation might re
- Page 570 and 571: further pointed out that even if on
- Page 572 and 573: Dr. Desa also informed participants
- Page 574 and 575: thickness of the sediments, the thi
- Page 576 and 577: pressure at sea level leads to hydr
- Page 578 and 579: aware of any studies of natural ear
- Page 580 and 581: CHAPTER 15 A CASE STUDY IN THE DEVE
- Page 582 and 583: NAMIBIA L W OR CT BOTSWANA SOUTH AF
- Page 584 and 585: The Orange River, one of Africa’s
identification of gas hydrate in marine sediments has been the presence of<br />
anomalous signature on marine seismic records known as Bottom<br />
Simulating Reflections (BSR). The Carolina Rise along the eastern United<br />
States margin, particularly over the Blake Ridge, was the area where<br />
marine gas hydrate was first identified 32 on the basis of a BSR. It has been<br />
suggested 33 that reflection polarity reversal, a large reflection coefficient<br />
and increasing sub-bottom depth with increasing water depth are the<br />
different criteria that characterize a BSR. However, at the same time it was<br />
felt 33 that none of these criteria are unique to hydrates. Subsequent<br />
studies 33-35 have highlighted three manifestations of hydrates in sea-floor<br />
sediments, which have been used to recognize gas hydrates in seismic<br />
profiles on the U.S. Atlantic continental margin. These are: i) Bottom<br />
Simulating Reflections (BSR), ii) amplitude blanking, and iii) velocity<br />
inversion.<br />
4.1.1. Bottom Simulating Reflections (BSR)<br />
Sloan 1 observed that hydrate has a very strong effect on acoustic<br />
reflections because it has a high acoustic velocity (approximately 3.3<br />
km/sec, about twice that of sea floor sediments), and thus cementation of<br />
grains by hydrates produces a high velocity section. Sediments below the<br />
hydrate-cemented zone, if water saturated, have lower velocities (water<br />
velocity is about 1.5 km/sec), and if gas is trapped in the sediments below<br />
the hydrate, the velocity is much lower (even with just a few percent of<br />
gas). Because reflection strength at an interface is proportional to the<br />
change of acoustic impedance, which is the product of velocity and<br />
density, the base of the hydrate-cemented zone produces a very strong<br />
reflection. As the phase boundary is a distinct limit to hydrate occurrence,<br />
this reflection is sharply defined. The base of the gas hydrate stability<br />
occurs at approximately uniform sub-bottom depths throughout a small<br />
area, therefore, the reflection from its base roughly parallels the sea floor.<br />
This reflection has therefore come to known as 'Bottom Simulating<br />
Reflection' or BSR.<br />
In literature the term BSR has been used to represent either<br />
'Bottom Simulating Reflector' or 'Bottom Simulating Reflection', causing<br />
a degree of ambiguity. According to Huene and Pecher 36 the bottom<br />
simulating reflection is the anomalous reflection seen on marine seismic<br />
records indicating the position in the sediment column of the base of the<br />
INTERNATIONAL SEABED AUTHORITY 528
Change language