At Site 1253, the DVTPP was deployed twice in order to determine the in situ temperature and pressure of the formation. The first measurement was performed directly beneath the casing of the reentry cone at a depth of 60 mbsf; the second was at a depth of 150 mbsf. Prior to these measurements, the bottom water temperature was determined by attaching a high-resolution miniaturized temperature data logger (MTL) to the video system used for reentering the borehole. A conductivity/temperature/depth (CTD) profile recorded offshore Nicaragua in September 2002 during the German Meteor cruise is presented here as an independent reference. It is used to calibrate the DVTPP tool to this well-constrained bottom water temperature. The MTL was also used during wireline logging to record a high-resolution temperature profile within the pipe and the borehole.
For the determination of the downhole temperature gradient, it is necessary to know the exact bottom water temperature to calibrate the tools to a stable reference temperature. For high-resolution temperature measurements, we used an MTL (Pfender and Villinger, 2002) (see also "Downhole Tools" in the "Explanatory Notes" chapter).
By mounting an MTL on the video system while reentering the borehole, we measured a high-resolution temperature profile of the water column down to 4333 mbrf. Figure F78 shows a part of the record that includes a temperature inversion with the lowest temperature of 1.837°C (1450 hr UTC). The plot indicates a temperature of 1.985°C close to the seafloor at a depth of 4333 mbrf.
In Figure F79, we present data from a CTD performed in September 2002 offshore Nicaragua to compare the temperature profiles. The inversion recorded by the MTL is also seen in the CTD data at a depth of 2817 mbsl and a temperature of 1.849°C. At 4322 mbsl (4333 mbrf), the temperature is 1.983°C, which corresponds very well to the value determined by the logger during the video run at the same depth. At 4376 mbsl, the CTD temperature reading is 1.989°C, which we assume is the bottom water temperature for Site 1253. This bottom water temperature deviates from that determined during Leg 170. At Site 1039, the temperature was 1.81°C, as measured by two different tools (water sampling temperature probe and the Adara temperature tool). The cause of this difference is not clear.
Additional MTL temperature measurements were performed during wireline logging to detect possible hydrologically active horizons. The MTL was mounted in the TAP tool close to the sensor tip of the TAP temperature sensor attached to the bottom of the triple combo tool string (Fig. F80). In a first attempt at logging early on 8 October, a bridge at a depth of 528 mbsf could not be passed and the run was stopped because of tool problems. After replacing the broken tool, the second logging run took place on the afternoon of 8 October and covered the depth interval from 413 to 530 mbsf.
Figure F81A and F81B illustrates the temperature and pressure profiles of both runs, beginning with the mudline stop and ending with the pull-out from the borehole.
In the first run, the 5-min-long stop (0632-0637 hr UTC) at the mudline (4387.1 mbrf) was clearly marked by a constant pressure of 44.3 MPa. Detailed investigation of the pressure shows periodic variations of ±0.1 MPa, corresponding to a vertical movement of ~10-m amplitude, which is interpreted as being caused by the ship's heave. As the tool was lowered into the borehole, the pressure increased up to 48.5 MPa at 4800 mbrf (end of 10
-in casing), where the tool was held for 10 min. The pressure showed smaller variations here, up to ±0.07 MPa, corresponding to ~7 m of vertical movement. At the end of this period, the wireline heave compensator was switched on. The record then shows the tool being lowered through the open hole and reaching a maximum pressure of 50 MPa at 530 mbsf.
The temperature at the mudline varies from 2.05° to 2.52°C for the MTL; the TAP tool temperatures show similar variation but are offset by +0.88°C. This joint change in both records indicates first a fast increase then a slower decrease in temperatures while the tool was held at the mudline, which might be caused by previous pumping and excessive tool movement. Because the MTL has also been used to determine the bottom water temperature and is calibrated to a high-precision thermometer, it is regarded as the more reliable tool. It is also obvious that the scatter of the MTL data (±1 mK) is lower than that of the TAP tool (±30 mK). Nevertheless, both instruments show the same trend and have the same offset over time. As the tool was lowered through the borehole to the end of the casing at 4800 mbrf, the temperature increased to 5.715°C. During the stop there, the temperature slowly increased to 5.735°C at the end of the stop. The trend is not obvious with the TAP tool because its scatter is higher than this increase.
Further lowering the tool resulted in a further increase in measured temperatures followed by a decrease when the tool actually reached the open hole. The temperature recorded with time in the hole was mainly determined by raising and lowering the tool during the attempts to pass the bridge at 530 mbsf. Unable to pass the bridge, the tool was pulled out of the hole.
Pressure and temperatures recorded during run 2 in general show the same features as run 1. At the mudline stop, a constant pressure corresponding to the seafloor depth of 4387 mbrf was seen. The observed temperatures were the same as in the first attempt while the tool was lowered to the seafloor, but the increase during the mudline stop was not as prominent. As the tool was lowered down in the borehole, pressure and temperatures both increased in the same manner as in the previous run. During the stop close to the casing shoe, the pressure at 4700 mbrf remained constant at 47.5 MPa and temperature increased from 4.482° to 4.492°C. As the tool was lowered, temperature increased and suddenly dropped as the tool entered the open hole. In the open hole, the attempts to pass the bridge were marked by the two peaks in the pressure at ~1457 and 1505 hr UTC. The peaks marked the vertical position of the tool pulled up; the low parts in between marked the deepest position of the tool at ~4917 mbrf. Temperatures mirrored this vertical movement and increased with increasing depth. Around 1510 hr UTC, the logging run started from 4917 mbrf (530 mbsf) upward. Again, the zone of the temperature disturbance in the borehole was crossed at the end of the record.
The temperature vs. borehole depth plot can be seen in Figure F82A and F82B. Only temperatures that were collected while the tool was lowered down through the borehole are used. In contrast to lowering the tools where the sensors measure undisturbed temperature fields, the measurements taken while the tool was pulled up take place in a turbulent temperature field and might slightly be shifted against the undisturbed values and are for that reason neglected.
A significant feature in the profiles is the strong increase of temperatures around the end of the casing at 413 mbsf. This structure is almost symmetrical around the end of the 10-in casing and extends ~40 m above and below the casing shoe. The amount of incoming heat to produce the observed 0.8°C temperature increase is too large to be caused by incoming warm fluids. Instead, it is probably due to cement curing, which is an exothermic reaction. Cementing the borehole was done ~36 hr prior to logging.
The temperature gradient in the casing (from seafloor to 50 m above the casing shoe) is ~0.006 K/m and still lower than the expected gradient of 0.0098 K/m. The temperature profile in the open hole is shown in Figure F82B. The large negative gradient up to 450-460 mbsf probably characterizes the influence of the cement curing described above. Below 460 mbsf, the gradient is much smaller and becomes positive at 525 mbsf. Increased borehole temperatures between runs 1 and 2 indicate that the borehole had not yet completely equilibrated to the formation temperatures.
The in situ measurements of temperature and pressure took place on 23 September between 60 and 150 mbsf. During the deployment, the tool was held for 10 min at the mudline and another 10 min within 1 or 2 m of the bottom of the hole. After penetrating the sediments, the tool was left for 30 min in the formation to record the decay of in situ temperature and pressure to allow extrapolation to undisturbed values.
The ship's heave was compensated by the shipboard active heave compensator, which was activated each time after the mudline stop. Figure F83 shows a general overview of temperature and pressure data collected during both DVTPP deployments.
The temperature channel of the DVTPP showed a noise level that was considerably higher than expected from the manufacturer's specifications. After the thermistor was replaced with a fixed resistor in the laboratory for test purposes, it became obvious that the noise was created by the probe electronics. After we had discussions with RBR Ltd., the manufacturer of the probe's logger, we came to the conclusion that oscillations in the amplifier circuit probably caused the noise. Unfortunately, repair at sea was not possible.
The stops at the mudline (1545-1555 hr UTC), the bottom of the hole (1555-1605 hr UTC), and the penetration into the sediment (1605 hr UTC) can be clearly seen in Figures F83A and F84A. Large negative excursions were observed while the tool was lowered and raised in the drill pipe. Following the first deployment, we detected an intermittent contact between a cable and the tool's housing, which caused the spikes in temperature during movement of the tool. After the repair, temperature spikes were not observed in subsequent deployments.
Because of the electrical problem, the bottom water temperature is offset by 9°C and is not of any use. The active heave compensator was switched on shortly before reaching the bottom of the hole. Its effect can be clearly seen when comparing the relative temperature changes at the mudline and at the bottom of the hole, where the scatter is strongly reduced. With tool movement minimized by active heave, the intermittent electrical problem was eliminated and valid temperatures were measured. Temperatures at the bottom of the hole are 2.04° ± 0.02°C. The frictional heating caused by the penetration of the tool tip in the sediment is clearly marked but lacks a clear decay (Fig. F83A). Only the first 5 min can be seen as a decay; the rest of the record is strongly spiked. The spikes in the temperature record while the probe is resting in the formation indicate movement of the probe, which produces further spikes of frictional heating with a following decay. Pressure data also show vertical movement, although the heave compensation is active. One reason for the different behavior of temperatures and pressures at the bottom of the hole and in the formation is that the effect of any movement is enhanced in the formation, causing frictional heating and pressure pulses.
It is possible to estimate the expected temperature at 60 mbsf using the temperature gradient of 0.0173 K/m, as calculated from Site 1039 data, and the bottom water temperature of 1.989°C, determined during Leg 205 with independent methods. This calculation yields 3.026°C, which is in good agreement with a visual extrapolation of the early and mostly undisturbed temperature decay.
The observed pressure increases from ~0.1 MPa at sea level to ~45 MPa at the bottom of the borehole (Figs. F83B, F84A). The record at the mudline can be clearly distinguished from the stop at the bottom of the borehole and the penetration. During the mudline stop, the heave compensator was not yet activated and high variability of the measured pressure is observed. Large pressure changes during the stop at the mudline are most probably caused by vertical heave-induced movement of the tool, which acts as a moving piston inside the drill pipe. After the heave compensation is activated, the bottom-hole record does not show significant heave-induced distortion. The penetration record does, however, show disturbance. The penetration itself is clearly marked, but the pressure decay trend is quite noisy and indicates vertical movement of the tool, which becomes more obvious in the formation in contrast to the open hole. The decay of measured pressure at the bottom of the hole points to a formation pressure of ~45.2 MPa. Assuming a constant seawater salinity, temperature, and, therefore, density of 1030 kg/m3, calculations yield a hydrostatic pressure of 44.8 MPa at 60 mbsf. Accounting for the increasing density of water with pressure, the result is 45.1 MPa, which is very close to the estimated formation pressure.
After thoroughly checking the electrical contacts of the DVTPP after the first deployment and correcting a possible intermittent contact, the second deployment retrieved data of only slightly better quality (Figs. F83C, F83D, F84B). The numerous negative spikes observed during the first run disappeared, and a reasonable bottom water temperature of 1.99° ± 0.02°C was recorded at the mudline (2305-2315 hr UTC). The bottom-hole temperature at 150 mbsf was ~2.58°C. The frictional decay following the penetration into the sediments at 2325 hr UTC cannot be processed because it is highly disturbed and does not show any sign of a interpretable decay. The expected temperature at 150 mbsf, calculated in the same way as that for 60 mbsf, would give a value of 4.102°C, which could be possible but is not obvious from the measured temperature data.
For the pressure measurement, we see the same characteristic features as were observed during the first DVTPP run (Figs. F83D, F84B). High pressure variations at the mudline, which suggest vertical motion, were observed when heave compensation was not switched on (~2300 hr UTC). However, the suggested vertical motion of up to 80 m is unreasonable. After the heave compensation was switched on before reaching the bottom of the hole, a stable pressure of 45.9 MPa was recorded. The calculated pressure at this depth, taking into account the increasing density of seawater with pressure, is 46.1 MPa and corresponds well to the observation. The disturbed pressure decay after penetration at 2325 hr UTC cannot be processed because of tool movement.
