Although the small interval of sediments (26 cm) recovered in Core 203-1243B-1R was disturbed and not suitable for physical properties measurements, moisture, density, and sonic velocities of 20 basalt samples from Cores 203-1243B-2R through 18R were measured in the laboratory.
The samples were minicores cut from the working half. The minicores are right circular cylinders 2.5 cm in diameter and 2.0-2.2 cm long. The same samples were used to make paleomagnetic measurements. Sonic velocities were measured using the modified Hamilton frame velocimeter (PWS3) then the wet mass was measured as described in "Physical Properties" in the "Explanatory Notes" chapter. The samples were then dried for 48 hr at 100°C before the dry mass and dry volume were measured.
Wet bulk densities, grain densities, porosities, and sonic velocities are summarized in Table T6. The sonic velocities range from 4.3 to 5.7 km/s, with a mean of 5.26 ± 0.08 km/s. Porosities range from 4% to 17%, with a mean of 7.7 ± 0.7%. Wet bulk densities range from 2.52 to 2.82, with a mean of 2.69 ± 0.02 g/cm3, whereas the grain densities range from 2.64 to 2.98 g/cm3, with a mean of 2.85 ± 0.02 g/cm3.
The relationships among these properties are summarized in Figure F34. Except for two samples that have particularly high porosities (Samples 203-1243B-7R-1, 106-108 cm, and 14R-1, 36-38 cm) (see Table T6), wet bulk densities decrease markedly with increasing porosity. We also observe a marked decrease of grain density with increasing porosity and a strong increase of wet bulk density with increasing grain density. Lower grain densities are likely to reflect the abundance of low-temperature, low-density alteration products such as clay minerals in the samples. Hence, taken together, these relationships suggest that higher porosities are associated with higher permeabilities, which in turn lead to higher degrees of hydrous alteration.
A relationship between sonic velocity and porosity is expected because cracks in the rock have a profound effect on the elastic properties of the material. Probably because the low-density phyllosilicate minerals produced by alteration also have very low elastic moduli, Figure F34 shows a positive correlation between the sonic velocity and the grain density of the samples. Velocity decreases with increasing porosity and with decreasing grain density. Thus, if the grain density is an index of alteration, the seismic velocities in these samples reflect the combined effects of porosity (cracks) and alteration on the properties of the rocks.
In addition to the usual suite of downhole logs, including gamma density, porosity, and sonic logs, the Schlumberger WST was used to conduct a seismic survey through the basement section in Hole 1243B. Details of the survey are given in the "Downhole Measurements." The data are summarized in Table T7. Data were recorded at eight stations; except for the interval between stations 7 and 8 (at the top of the basement section), the stations were located 10 m apart, and 5-16 shots were stacked at each station to improve the signal-to-noise ratio. Traveltimes were determined automatically by a threshold method. Table T7 lists the average traveltimes and their standard errors (estimated from the standard deviations of the measured traveltimes).
The traveltimes recorded in Table T7 were used to compute interval velocities between the stations; the results are given in Table T8 and shown in Figure F35. The precision of the interval velocities, estimated from the uncertainties in the traveltimes, is 5%-7.5%. Also shown in Figure F35 are the downhole sonic (DTCO) log and the sonic velocities and porosities of the laboratory samples (see Table T6). There is a discrepancy between the WST and downhole log depths and the core depths. The log depths have been adjusted to conform to the core values, which place the top of basement near 110 mbsf.
In general, Figure F35 shows good agreement between the velocities in the laboratory samples, the sonic log, and the well seismic data. The laboratory velocities are, on average, higher (5.26 km/s) than the velocities recorded by the sonic log, which average 4.72 km/s. Cracks cause markedly lower seismic velocities in rocks (e.g., O'Connell and Budiansky, 1974; Kuster and Toksöz, 1974; Gangi, 1978) and are known to have a profound effect on the seismic properties of the uppermost oceanic basement (e.g., Hyndman and Drury, 1976; Wilkens et al., 1991). Thus, the difference between the laboratory velocities and the sonic log probably reflects the presence of large-scale cracks in the formation that are not present in the laboratory samples.
The uppermost interval velocity from the WST survey is anomalously low (1.59 ± 0.12 km/s), which is more characteristic of the overlying sediments than of the basaltic basement. On average, the variation of velocity with depth in deep-sea sediments is given by (Carlson et al., 1986)
Accordingly, the velocity of the sediments overlying basement in Hole 1243B should be near 1.63 km/s. Similarly, the average velocity in the 117-m sediment column in Hole 1243A, where the one-way traveltime is near 0.075 s, is 1.56 km/s. Elsewhere, the WST interval velocities are slightly lower than the sonic velocities (Fig. F35); over the logged interval, excluding the uppermost interval of low velocities, the average of the WST velocities is 4.60 km/s. The difference may be interpreted in two ways. One possibility is that the difference between the WST interval velocities and the sonic log velocities reflects a systematic error in the WST traveltime picks. If so, a more rigorous analysis of the data using correlation techniques will resolve the difference. The second possibility is that the observed difference reflects the different scales to which the sonic log and seismic experiments are sensitive. The sonic tool samples the formation over intervals of 2 ft (0.6 m). Large-scale cracks that occur at intervals of >0.6 m are recorded as dropouts or abrupt minima, such as the one near 133 mbsf in Figure F35. The sonic log is, thus, likely to record the properties of the formation between these features. Seismic waves, on the other hand, sample the formation on scales of tens to hundreds of meters, and the WST interval velocities may, therefore, reflect the presence of cracks that occur on this larger scale.