No abstract

A bottom-simulating reflection (BSR) is a seismic reflectivity phenomenon that is widely accepted as indicating the base of the gas-hydrate stability zone. The acoustic impedance difference between sediments invaded with gas hydrate above the BSR and sediments without gas hydrate, but commonly with free gas below, are accepted as the conditions that create this reflection. The relationship between BSRs and marine gas hydrate has become so well known since the 1970s that investigators, when asked to define the most important seismic attribute of marine gas-hydrate systems, usually reply, "a BSR event." Research conducted over the last decade has focused on calibrating seafloor seismic reflectivity across the geology of the northern Gulf of Mexico (GoM) continental slope surface to the seafloor. This research indicates that the presence and character of seafloor bright spots (SBS) can be indicators of gas hydrates in surface and near-surface sediments (Figure 1). It has become apparent that SBSs on the continental slope generally are responses to fluid and gas expulsion processes. Gas-hydrate formation is, in turn, related to these processes. As gas-hydrate research expands around the world, it will be interesting to find if SBS behavior in other deepwater settings is as useful for identifying gas-hydrate sites as in the GoM. Research background. Joint research with the Minerals Management Service (MMS), through a cooperative agreement with the Coastal Marine Institute at Louisiana State University, has resulted in a study using the university's GoMwide 3D seismic data coverage. Seafloor reflectivity across the northern GoM's continental slope has been mapped, and numerous SBS areas like those illustrated in Figure 1 have been identified. Through research projects funded primarily by MMS and NOAA, manned submersible dives on many SBS sites have provided direct observations and samplings to calibrate the actual geologic character of the seafloor to seismic reflectivity. These investigations have led to the model illustrated in Figure 2 which defines three qualitative ranges in the rates of fluid and gas expulsion coupled to geologic-biologic response at the seafloor. These reflector-anomaly sites also indicate three gas-hydrate domains that can be associated with these seafloor reflectivity behaviors. The block diagram shown in Figure 2 illustrates that fluids and gases have various migration pathways to the modern seafloor and that delivery rates, as well as fluid-gas composition, impact geologic response on the continental slope surface. Deep-cutting faults that intersect the overpressured zone frequently form the migration routes for hydrocarbons, formation fluids, and sometimes fluidized sediments. In addition, considerable heat can accompany these products, which eliminates the gas-hydrate stability

v ertical seismic profiling is one of the rapidly developing areas of geophysical technology for exploring in mature basins. The measurement basically involves recording the total upgoing and downgoing seismic wave fields propagating through a stratigraphic section by means of geophones clamped to the wall of a drilled well.in most seismic measurements, both the energy source and receivers are positioned on the earth' s surface. What happens to the seismic wavelet as it propagates from the source to a subsurface reflector and back to the receivers is mostly a matter of inference based on the characteristics of the source and on the properties of the wave field measured at the surface.Vertical seismic profiling replaces much of this inference with several closely spaced direct physical measurements of the seismic wave field in the real earth conditions that exist between the earth' s surface and the subsurface reflector. These measurements are proving to be invaluable in structural, stratigraphic, and lithological interpretations of the subsurface and are particularly valuable when combined with surface-recorded seismic data covering a prospective area in a mature, producing basin.

No abstract

The seismic body wave radiation pattern of a working roller-cone drill bit can be characterized by theoretical modeling and field data examples. Our model of drill-bit signal generation is a pseudo-random series of bit-tooth impacts that create both axial forces and tangential torques about the borehole axis. Each drill tooth impact creates an extensional wave that travels up the drill string and body waves that radiate into the earth. The model predicts that P-waves radiate primarily along the axis of the borehole, and shear waves radiate primarily perpendicular to the borehole axis. In a vertical hole, the largest P-waves will be recorded directly above and below the drill bit; whereas, the largest shear waves will be recorded in a horizontal plane containing the drill bit. In a deviated borehole, the radiation patterns should be rotated by the inclination angle of the drill bit. This proposed seismic body wave radiation pattern is investigated with field data examples.