Logs play a crucial role in linking core data with regional geophysical surveys and in providing data where core sections could not be obtained. To study hydrate, researchers typically address the problem with a multidisciplinary strategy, integrating measurements made on core samples with those made using logs and placing both in the context of regional geophysical and seismic studies. The multiple scales of investigation used with seismic, downhole, and core data acquired in the same geological environment complement each other extremely well. Seismic sections are the basis for a regional description and enable a cross-section to be inferred; logs typically have an intermediate resolution of approximately 0.5 m giving continuous information in the region surrounding the borehole; and core samples provide detailed information on physical properties and age. Core, log, and seismic data used jointly also contribute to the confidence in each data set individually. Unlike measurements on core samples, however which are often disturbed during the process of recovery, downhole logs provide a set of continuous information and sample a larger volume of rock than core measurements. In addition, for most hydrate drilling environments, continuous coring does not result in continuous core recovery. Techniques other than piston coring typically results in less than 50% recovery and this proportion is often disturbed by the drilling process. As a result, the true depth of the core becomes ambiguous and the accuracy of measurements made on it may be severely degraded. In situ measurements reduce uncertainty in the sampling depth and eliminate the disturbance of properties that occurs as a sample is taken. Furthermore, because logs have much greater vertical resolution than seismic data, but little lateral resolution, the combination of the two defines subsurface geological structures far better than either data type alone.
The difference in the scale of the physical phenomena affecting each type of measurement may be extreme. The scale ratio from core to log may be greater than 2 x 1(P; the ratio from log to seismic may be 10^ to 10^ times larger. In most integrated scientific applications, therefore, downhole logs provide three complementary advantages: 1) data is acquired under in situ conditions, 2) data is acquired in continuous profiles measured throughout the interval with no missing sections, and 3) data is sampled at a larger scale, intermediate between core and seismic measurements. With the expectation that recent advances in technology will continue, research using interdisciplinary strategies that integrate core, log, and seismic data will undoubtedly expand, particularly in studying gas hydrates. Applying complementary techniques to a particular hydrate environment, either in the deep sea or in permafrost areas, is the now the expected scientific approach.
An example of a comparison between surface geophysical data and downhole data is given in Figure 2. Velocity profiles obtained from logs, vertical seismic profiles (VSPs)—another downhole tool measuring the formation velocity—and multichannel seismic experiments are shown as a function of depth in a marine sedimentary environment [data from Yuan et al., 1996]. The increase in velocity above the BSR to about 1800 m/s is attributed to the presence of high-velocity hydrate in all three types of data.
The detailed diagram on the right highlights the similar response of the three experiments, though each at a different scale, to the presence of hydrate above a reference no-hydrate, no-gas trend; however, significant differences in the resolution of fine-scale variation puts obvious emphasis on the importance of using multiple methods. For example, low velocities below the BSR detected only in the VSP data are interpreted to represent a thin layer of low concentration (<1%) free gas (also see section 3.3). This gas layer is not evident in the logs because they have a shallower effective depth of penetration into the formation and do not sense gas that may have been released near the borehole during drilling. The multichannel seismic data also do not have adequate resolution in this area to detect the thin gas layer.
A comparison of downhole resistivity to seafloor electrical sounding surveys in this area similarly agrees over the broad measurement scale of these two experiments. Log-measured resistivity is about 2.1 ohm-m in the hydrate zone, compared to a reference of about 1 ohm-m for no hydrate, and electrical sounding profiles yield similar values of about 1.8-2.0 ohm-m over the same depth interval [Hyndman et al., 1999; Yuan and Edwards, 2000].
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