Chirp systems are sub-bottom profilers so named because of their wideband FM sound sources (e.g., 2-16 kHz over 20 ms). These systems belong to the linear driven source group driving an amplifier with a known voltage waveform. At frequencies at the low end of the linear source range, the acoustic return received at the hydrophone is matched filtered with the outgoing FM pulse to generate a high-resolution image of the stratigraphy. These systems are most successful at higher frequencies (3.5kHz and up), can produce sediment classification based on sediment reflection coefficients, but have restricted depth penetration (about 60m).
Another alternative technology involves the flextensional transducer, a hollow elliptical cylinder (Class IV). This is driven by a stack of ceramic elements mounted internally across the major diameter of the cylinder causing a large volume change. They have been most widely used in the frequency range 1-3 kHz, but designs exist for 300-500 Hz systems (Oswin and Dunn, 1988) although they have a limited operational depth range to about 300 m.
4.3. Special seismic systems
Special seismic systems are those with a source and receiver array geometry different to the conventional position near the sea surface. Several systems have been used to investigate marine sediments containing hydrate. They produce multi-channel seismic datasets that provide very detailed velocity analyses. These analyses can be processed to generate estimates of hydrate and methane occurrence.
A high frequency instrument for seafloor characterization of fine-scale velocity structure in near-seafloor sediments is the Deep-Tow Acoustic
Geophysical System (DTAGS). It consists of a Helmholtz resonator with a broad frequency output between 250 and 650 Hz, and a 24 channel (42 m spacing) towed array, both are towed 500 to 700 m above the seafloor (Rowe and Gettrust, 1993). Penetration depths have proven to be on the order of 700 m. The higher frequency and low attenuation from volume allows 2 to 3 m scale resolution of small geologic features in the HEZ while the larger angles between transmitted and received energy in this deep-tow configuration allows for fine scale 2-D velocity analyses. DTAGS has been used successfully to map hydrate BSRs.
Other specialised systems with either shallow-deep configurations (Pasisar) or narrow beam higher frequency sub bottom profilers (Parasound) can be developed for hydrate applications. They may make a significant contribution to detecting and quantifying the near-surface occurrence of hydrate and be particularly applicable where geotechnical information is required.
Apart from identifying hydrate we need to be able to measure its concentration and physical properties. Collecting seismic data in both single and multichannel modes permits a wide variety of processing and visualization techniques to be applied. These can reveal the geological and acoustic impedance structures in sufficient detail to identify drilling targets, determine depths, and provide input to economic assessment. Because the economic targets of the oceanic hydrate system occur in the upper 0.25-1.5 km the precise nature of the mechanical strengthening conferred by the formation of hydrate is very important. In addition to the increase in strength, the deformational character of the hydrate is important for determining the level of stress, and the resultant adaptation to strain, that can be tolerated during methane extraction operations (Stern et al., 1996). These can create abnormal thermal and pressure gradients that are important in developing safe extraction strategies.
Although simple pore filling increases the bulk modulus of the sediment, the shear modulus and sediment strength is much less affected than where cementation takes place. It is likely that analyses of the relationship of pressure to Vs and S-wave attenuation in the HEZ may be required to accurately classify the geotechnical properties of hydrate and associated methane deposits. S-wave velocities have been determined from incident P-wave source acoustic experiments (Caiti et al., 1991) and conventional seismic surveys, but more fine-scale characterization is probably necessary. The ideal survey would consist of concurrent P-wave and S-wave measurement using a hydrophone array close to or at the seafloor. This would allow measurement and calibration of incident energy levels at a variety of frequencies and the associated attenuation necessary for estimating methane concentrations and percentages.
4.5. Choosing seismic systems for hydrate assessment
Seismic sources and hydrophone arrays have been developed for the deeper targets in the hydrocarbon industry. The receiver system is generally an array of point receivers up to several km in length controlled at operational depth by 'birds' (Figure 5). This surface-surface system becomes less effective in deep-water because the geometry reduces the CDP advantages, but it is very practicable for a deep-tow system, if slow to operate. However the surface-surface system will remain a primary survey tool because of its availability and practicality.
Deep-tow operations are slow. Therefore in order to better monitor the acoustic energy entering the sediment from surface-surface systems it is desirable to measure the incident wavelet. This can be achieved by locating a hydrophone near the seafloor and will enable attenuation to be used for the analysis of both hydrate and methane charged sediments. This monitoring can also be achieved with a single hydrophone suspended at manageable depths or by using stationary ocean bottom seismometers (OBS), although these are an expensive addition to the operations.
An advantage of OBSs is that additional seismic information such as shear-wave arrivals and surface waves can be observed. Both provide valuable information on Vs at different depths beneath the seabed. For the top 50-100 m Scholte waves, a solid-liquid interface wave similar to a Raleigh wave, shows frequency dispersion from seabed impulse sources. In the presence of a positive S-wave velocity gradient the lower frequency energy travels faster. The seismogram can be inverted to give the velocity structure. Additional S-wave data can be obtained by OBS operated in the conventional refraction mode but owing to the expense of operation should only be considered when specific problems need to be addressed.
Both compressional-wave (Vp) and shear-wave (Vs) velocities should be used to characterize (pore filling) bulk modulus and (cementation) shear modulus properties. Normal seismic reflection profiling (single or multichannel) operates principally with P-wave arrivals. However S-waves are often generated by the incident wavelet at the seabed or some depth below it. The problem in recording these S-wave paths using a streamer array is that they have to be re-converted back to P-waves prior to re-entering the water layer which generates some unknowns in the interpretation. Hence the desirability of seabed monitoring.
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