Introduction

The geographic extent of hydrate deposits and their distribution within the sediment column are still relatively undefined. Both single and multi-channel seismic reflection surveys remain the principle methods of identifying the presence of hydrate but the effects of methane flux through the seabed is best visualized with several other acoustic sensors, namely side-scan. The strong acoustic impedance contrast reflector at the base of the layer - the bottom simulating reflector or BSR - is normally seen where free methane is present beneath the hydrate. Hydrate formation may also cause blanking of the sediment acoustic stratigraphy through cementation of the sediment structure.

Figure 1. Schematic of geophysical tool targets for characterising hydrate, (a) pockmark; (b) mud volcanoe; HSZ = hydrate stability zone; HEZ = hydrate economic zone.

Deep ocean surveys using relatively high resolution side-scan sonar acoustic imagery of the seafloor can reveal pockmarks, mud diapirs and mud volcanoes that are often related to methane flux. These surveys have also mapped large-scale slope disturbances and sediment mass wasting attributed to hydrate instability (Kenyon, 1987;Masson et al., 1997). Larger scale failures appear to correlate with regions of higher hydrate occurrence, at least on the European margins (Miles, 1995). However this relationship can be tenuous in some localities, such as in the Mediterranean Sea, where hydrates are not known to occur widely but are still associated with large scale sediment movement (Rothwell et al., 1998). These factors illustrate the need for quantitative measurement of hydrate distribution in both area and depth. The only tools that can achieve this on the large scale at acceptable cost are geophysical.

To date the acoustic systems and processing were usually designed to reveal hydrocarbon exploration targets or crustal features of scientific interest. However hydrates are confined to the uppermost 0.5 to 2 km of marine sediments and these systems need to be re-tuned when mapping hydrate resource issues at between 200 and 800 m depth in the seafloor. The impact of this hydrate on seafloor stability, safety and the environment is probably confined to the top few hundred meters of sediment (Figure 1).

A commercially successful strategy for methane production from hydrate does not yet exist, though Japan has started to assess their resources as reserves. Therefore we consider effective geophysical techniques for identifying and characterizing the various hydrate issues. Technical specifications are listed in Table 1.

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