Hole 395A was drilled 21 years ago during DSDP (Deep Sea Drilling Project) Leg 45 to a total depth of 664 mbsf (meter below sea floor) into the oceanic crust in the middle of the Atlantic Ocean. It is located at latitude 22°45'N and longitude 46°5'W, on the edge of a small sediment pond ( Fig. 1). Hole 395A penetrates 93 m of sediments and the uppermost 112m of the hole were cased. Hole 395A was logged during DSDP Leg 78B and ODP Leg 109 prior to the present occupation of the hole during this leg, Leg 174B. Since Hole 395A is among a few of the deep holes into the oceanic crust, the downhole logs at this site have been continuously providing vital information for documenting the in-situ physical, chemical, and structural properties of oceanic subsurface formations and for our understanding of geophysical and geological dynamic processes occurring in the young oceanic crust. The reoccupation of the Hole 395A during this Leg is to further strength such knowledge with more advanced downhole logging technologies.
Fig. 1. Location of DSDP Hole 395A. (After Langseth et al., 1992)
Investigation of heat transfer mechanisms in the oceanic crust as a function of age and distance from the ridge axis is important for understanding of the mantle-crust-ocean interactions and their evolution. The role of cooling processes associated with ocean water circulation into the upper basement should not be underestimated in these mechanisms and in forming different crustal geological, hydrological, and biological systems. The effects of the presence of overlying sediment layers on the hydrogeologic processes are also important in affecting the hydrothermal circulation in the ocean crust (Langseth et al., 1984, 1992; Davis et al., 1996). The success of hydrothermal studies depends largely upon the accurate knowledge on crustal and sediment structures and on the spatial and temporal distributions of in-situ permeability, thermal conductivity, pressure and temperature gradients. The logging and corking operations during Leg 174B were planned to collect first-hand in-situ measurements on some of these variables for the active circulation system at Hole 395A. Above all, in-situ structural and physical properties of the fractured and/or porous structures of both crust and sediment layers from geophysical logging and later corking results provide essential ''ground-truth'' information to constrain crustal geological and hydrological models.
The main objective of Leg 174B was to reenter Hole 395A for downhole logs and CORK experiment (Circulation Obviation Retrofit Kit) to address the above-mentioned important questions. The primary scientific objectives of this leg are (1) to document the in-situ physical properties and hydrogeology at this young crustal reference site; and (2) to test a hydrological model developed from observations obtained during 3 earlier reentries since the hole wad drilled in 1975-1976.
Three tool strings were used for downhole logging prior to CORK operations. The first tool string was 22 m long and included the Azimuthal Resistivity Imager (ARI), spectral gamma ray (HNGS), and Lamont high-resolution Temperature (TLT) sondes. The second tool string included the spectral gamma ray (NGS), Dipole Sonic Imager (DSI), and Formation MicroScanner (FMS) tools, having a total length of 33 m. The third 'Triple-combo' tool string was 31.5 m long and included the spectral gamma ray (HNGS), Advanced Porosity Sonde (APS), Lithodensity Sonde (LDS), and Dual Induct ion Tool (DIT). An SP log was also acquired over the open hole interval.
There was the debut deployment of the deep-penetrating ARI in an ODP hole. The pipe was set at 32 mbsf and the wireline heave compensator was used. The overall quality of the log data acquired during Leg 174B is excellent, with the exception of the two washout intervals noted above. Logging operations were completed at 0645 hr, 30 July 1997, using a total of 28.75 hrs of rig time.
The logged depth interval in Hole 395A during Leg 174B is summarized in Table 1.
|Run (pass) 1||Run (pass) 2||Run (pass) 3|
|Cored Interval||no core||no core||no core|
|Triple Combo||603 - 0 mbsf||136 - 106 mbsf||
|FMS||603 - 113 mbsf||603 - 0 mbsf||603 - 113 mbsf|
|DSI||603 - 113 mbsf||603 - 0 mbsf||
|ARI||603 - 113 mbsf||236 - 181 mbsf|
|SP||603 - 113 mbsf|
|TLT||0 - 603 mbsf||603 - 0 mbsf|
Integration of core measurements, high-quality in-situ logs, and previous work will improve our knowledge about the fine-scale structural and physical models of the upper oceanic crust at Hole 395A and directly provide the essential information needed to define its fine-scale permeability structure. In addition, integration of core and log results with the available seismic data may make it possible to laterally extend our knowledge about the in-situ physical properties, heterogeneity, and permeability at the drill site, and refine the regional hydrogeological model proposed in previous studies (e.g., Langseth et al., 1984, 1992).
The preliminary results of the shipboard analyses of the Leg 174B logs and comparisons to Leg 45 core description and logs from Legs 78B and 109 clearly reveal that Hole 395A consists of definable layers of pillow basalts, massive lava flows, and fluid aquifers that can be mapped to changes in the resistivity, wave velocity, and bulk density logs. Distinct changes in the high-resolution temperature gradient derived from TLT log (Fig. 2) and anomalies in the SP log (Fig. 3) show that at least two major aquifers are active in the hole at about 310 and 420 mbsf respectively. The borehole temperatures logged with the Davis-Villinger Temperature Probe (DVTP) agree well with the high-resolution temperature log with the Lamont Temperature Tool (TLT) at coincident depths (Fig. 2).
Fig. 2. Temperature logs recorded in Hole 395A during Leg 174B. The circles show the station measurements recorded with DVTP, the blank line is the high-resolution downgoing TLT log, and the light gray the upgoing TLT log. The temperature gradient from the downgoing TLT log is shown as dark gray line on the right side.
Fig. 3. Composite-log of hole parameters, electrical logs, sonic logs, density and porosity logs recorded in Hole 395A during Leg 174B. Track 1: calipers C1 and C2 from the FMS tool; Track 2: spontaneous potential; Track 3: improved shallow and deep Laterolog with ARI; Track 4: Vp, Vs logs; Track 5: density log; and Track 6: neutron porosity log.
The sonic logs (in Fig. 3) recorded using the DSI tool represent the first use of this tool in the ocean crust. These data were recorded during the second logging run with three separate passes of the tool through the open hole interval. In total, five different modes of the DSI were enabled and allowed for acquisition of both compressional and shear waveforms using different acoustic sources. Both high-frequency compressional and shear and dipole shear modes produced excellent quality sonic waveforms. The data quality was directly affected by the waveform coherence between receivers (not shown), which is high overall and generally greater than 50% for the dipole shear data. The dipole shear waveforms also have systematically higher coherence than the high-frequency compressional and high-frequency shear waveforms, which is in part the result of less scattering from small fractures and pillow basalt morphologies affecting the shorter wavelengths. The average value of compressional and shear travel times generally agree with the Multi-Channel Sonic log results from Leg 109 (Moos, 1990) with Vp/Vs ratios averaging approximately 1.7 in massive units and ranging between 1.8 and 2.2 in pillow basalts.
Analyses of FMS (Fig. 4) and ARI (Fig. 5) images acquired during this leg enable quantification of the resistivity structure of the oceanic crust at 5mm - 1m scale, respectively. Combined with the resistivity measurements from the Azimuthal Resistivity Imager (ARI), the temperature gradient profile shown in Fig. 2 may be used to classify three temperature regimes: (A) isothermal regime from the sea floor down to 250 mbsf (meters below sea floor), (B) shallow-gradient regime from 250 mbsf down to 420 mbsf, and (C) steep-gradient regime from 420 mbsf downward. The isothermal regime is believed to contain the largest number of active aquifers of all the three regimes, while the steep-gradient regime contains the fewest. The steep-gradient regime coincides with a large aquifer at about 420 mbsf and continues into massive lavas below.
Fig. 4. Enlarged portion of ARI image in Figure 3 in comparison with FMS images, shallow and deep Laterolog, and sonic logs.
Fig. 5. ARI image shows massive lava flows, aquifers, and 1m-scale vertical heterogeneity.
Y.F. Sun & D. Goldberg Borehole Research Group, Lamont-Doherty Earth Observatory
Davis, E. E., D. S. Chapman, and C. B. Forster, Observations concerning the vigor of hydrothermal circulation in young oceanic crust, J. Geophys. Res., 101, 2927-2942, 1996.
Langseth, M. G., R. D. Hyndman, K. Becker, S. H. Hickman, and M. H. Salisbury, The hydrogeological regime of isolated sediment ponds in mid-oceanic ridges, in Hyndman, R. D. et al., DSDP Init. Repts., 78B, 825-837, 1984.
Langseth, M. G., K. Becker, R. P. Von Herzen, and P. Schultheiss, Heat and fluid flux through sediment on the western flank of the mid-Atlantic Ridge: a hydrogeological study of North Pond, Geophys. Res. Lett., 19, 517-520, 1992.
Moos, D., 1990. Petrophysical results from logging in DSDP Hole 395A, ODP Leg 109. In Detrick, R., Honnorez, J., Bryan, W.B., Juteau, T. et al., Proc. ODP, Sci. Results,106/109: College Station, TX (Ocean Drilling Program), 237-253.