Downhole measurements at Site 1254 consisted of one deployment of the DVTP and two deployments of the DVTPP to determine in situ formation temperatures and pressures. In order to remove possible causes for the disturbed measurements collected at Site 1253, we tried several configurations of measurement times in the sediment, at the bottom of the hole, and the mudline, and we also changed the instrument.
The first measurement was performed at 50 mbsf. The DVTPP records still showed severe disturbances during in situ formation measurements. Because we presumed that the latching system might have been responsible for the vertical movement of the tool, the system that connects the probe to the core barrel head was modified. In order to isolate problems which may be associated with the DVTPP design, we ran the DVTP at 150 mbsf, which showed improvement but still not a clear and undisturbed decay. The DVTPP was used again at 200 mbsf, and the improved data quality of the temperature record allowed for extrapolation of undisturbed sediment temperature.
During all runs, the shipboard active heave compensator was activated after the mudline stop.
In contrast to the measurements performed at Site 1253, we stopped 5 min at the mudline, 5 min at the bottom of the hole, and 15 min in the formation. The temperature record is illustrated in Figure F48A and F48C. The temperatures recorded at the mudline (1757-1803 hr UTC) and the bottom of the hole (1805-1810 hr UTC) can hardly be distinguished because of the small depth difference. Both are very close to the bottom water temperature of 1.989°C, determined earlier during this cruise. The penetration into the sediment (1813 hr UTC) is clearly marked, but lacks a decay. Instead, slightly increasing temperatures with significant scatter were recorded; undisturbed sediment temperature cannot be inferred. The strong temperature increase at 1830 hr UTC was caused by pulling the tool out of the sediment.
For a thermal gradient of 0.0083 K/m, as calculated from Site 1040 data, and the bottom water temperature of 1.989°C, determined during Leg 205, the expected temperature at 50 mbsf is 2.404°C. This expected temperature is considerably lower than any temperatures recorded during this penetration. It is possible that the movement of the tool created enough frictional heating to prevent a decay to in situ temperatures.
The observed pressure increased from ~0.1 MPa at sea level to ~43.05 MPa at the bottom of the borehole (Fig. F48B, F48D). During the mudline stop, the high variability of the measured pressure was most likely caused by the vertical heave inducing movement of the tool that acted as a moving piston inside the drill pipe. The same features also occurred in the pressure records at Site 1253 (see "Downhole Measurements" in the "Site 1253" chapter). After the active heave compensator was activated, the record at the bottom of the hole did not show significant distortion; however, while the tool was in the formation, the record was strongly disturbed. The penetration itself is clearly marked, but no pressure decay can be observed and large variations, including excursions to pressure values below hydrostatic, suggest vertical tool movement.
A formation pressure cannot be extrapolated from the penetration record, but the expected hydrostatic pressure at bottom of the hole can be estimated. With a constant seawater salinity, temperature, and density increasing linearly with depth (1030 kg/m3 at the top), the hydrostatic pressure at 50 mbsf is 43.16 MPa, which is close to the observed pressure of 43.05 MPa at the bottom of the hole.
The DVTP was chosen for the second deployment at 150 mbsf. Therefore, we obtained only temperature data here, but it is of slightly better quality (Fig. F49). This second run was performed without a stop at the bottom of the hole. The mudline temperature was recorded for 5 min prior to penetration (0612-0617 hr UTC) and also after penetration. Prior to penetration, it was not as stable as expected and the measured temperature of 1.85°C was considerably lower than the expected bottom water temperature of 1.989°C. After we pulled the tool out of the sediments, the mudline temperature was more stable and had risen to 1.94°C, which was close to the expected temperature. This effect was probably caused by pumping, which was stopped at 0612 hr UTC on 14 October.
The penetration (0624-0637 hr UTC) consisted of two spikes that indicated that the probe had moved down during the equilibration period. Following the second spike, a decay was seen, but data were still too noisy to be extrapolated using CONEFIT (Davis et al., 1997) software. A visual estimate yielded a temperature of 2.48°C for the formation, which is considerably lower than temperature measurements from Leg 170 (Kimura, Silver, Blum, et al., 1997). The expected temperature at 150 mbsf, calculated from the Site 1040 thermal gradient, would yield a value of 3.234°C. Because the recorded data are reasonable and of presumed higher quality than previous data, this difference in sediment temperatures may be caused by formation cracking during penetration with colder bottom water seeping into the formation. Unfortunately, this hypothesis cannot be tested because the temperature at the bottom of the hole was not measured in this run.
After modification of the latching system and recording the less disturbed data with the DVTP, we used the DVTPP again for the third measurement. This time, we performed a 5-min stop at the mudline prior to penetration and the temperature at the bottom of the hole and a second mudline measurement were taken after the penetration. We hoped to see less influence of pumping at the mudline. The record is shown in Figure F50A and F50C.
Starting with 1.9°C, the mudline temperature increases from 2053 to 2100 hr UTC by 0.2°C. After the penetration, the mudline temperature is much more stable at 2.19°C, with a scatter of ~0.02°C. Nevertheless, a slight increase with time is still observed, and the measured temperature is higher than expected.
The penetration is clearly marked at 2107 hr UTC and shows a clear decay. Processing using the software CONEFIT was successful and yielded a sediment temperature of 3.592°C. The most undisturbed part of the decay curve from 210900 to 212520 hr UTC was used for this calculation, as marked in Figure F50C. The expected sediment temperature using the known gradient of Leg 170 data is 3.649°C (Kimura, Silver, Blum, et al., 1997), which is consistent with the extrapolated value.
For the first pressure measurement at the mudline the active heave compensator was already switched on, so we see a very stable pressure record of 42.7 MPa there (Fig. F50B, F50D). In contrast to this, the records at the bottom of the hole and the mudline after the penetration show a highly variable pressure, suggesting vertical motion because active heave compensation was switched off directly after pullout. Pressure at the bottom of the hole is almost equal to the expected hydrostatic value of 44.71 MPa.
Although reasonable pressures are recorded at the mudline and the bottom of the hole, the pressure in the formation is unreasonably high. A maximum overpressure equal to the lithostatic load of ~1 MPa is possible at this depth, but the apparent value of 3 MPa observed here is unreasonably high. One explanation for this observation would be that the pressure port was clogged with mud. A decay is also missing so that no in situ formation pressures can be inferred.
Nevertheless, an improvement in data quality is obvious by the significantly lower variability of the pressure measurement in the formation compared to the first run. This improvement was made possible by the modification of the latching system.