30. DOWNHOLE TEMPERATURE MEASUREMENTS AND THERMAL CONDUCTIVITIES OFSAMPLES, SITE 396 DEEP SEA DRILLING PROJECT LEG 46

A. J. Erickson,1 Geology Department, University of Georgia, Athens, Georgiaand

R. D. Hyndman, Pacific Geoscience Centre, Earth Physics Branch, Department of Energy, Mines, and ResourcesVictoria, B.C., Canada

INTRODUCTION

Determination of the rate and distribution of heat flowthrough the Earth's crust provides one of the most directsources of information on the temperature distributionwithin the earth. Many thousands of heat flowmeasurements have been made in the ocean basins bymeasuring the thermal gradient in the uppermost 10 metersof the sea floor using conventional oceanographictechniques (e.g., Langseth, 1965). The Deep Sea DrillingProject has provided the opportunity to measure in situsediment temperatures to depths of hundreds of metersbeneath the sea floor using the equipment and techniquesdescribed by Erickson et al. (1975). Heat flow valuescomputed in the same region using both deep borehole andshallow sediment temperature data are in generally goodagreement, confirming that the conventional measurementshave not been systematically affected by variations inbottom water temperature having durations less than a fewthousand years or by other near surface disturbances in thesediments.

Within the last few years, increasing attention has beenpaid to the question of the nature of the physicalmechanisms by which heat is transmitted through theoceanic crust beneath the sediments. The discrepancybetween heat flow predicted theoretically through youngocean crust using cooling lithosphere models (Parsons andSclater, 1977) and that measured, the very high variabilityof the heat flow measured near the axes of active mid-oceanridges (e.g., Sclater et al., 1974-, Williams et al., 1974) and,more recently, direct observation of active hydrothermalvents on the Galapagos Ridge (Von Herzen, personalcommunication, 1977), has made it clear that hydrothermalcirculation in the ocean crust is an important, if not thedominant factor, in the transmission of heat through the seafloor near spreading mid-ocean ridges.

Lister (1972) proposed a model in which the circulationpattern is controlled by a combination of sea floortopography and variations in the permeability of the seafloor owing to impermeable sediment accumulation andsealing of hydrothermal vents by precipitation of secondaryminerals. Heat flow measurements precisely located withrespect to sea floor topography (Williams et al., 1974)support Lister's idea that elevated areas with little or no

1 Present address: Eureka Resource Associates, Berkeley, California.

sediment cover frequently act as chimneys through whichhot water rises, whereas topographic depressions may beregions of recharge of the hydrothermal system. Ourintention was to measure the temperature distribution bothwithin the sediment and in the basaltic material beneath(Layer 2A) near a spreading ridge crest to determine withmore confidence the nature and extent of heat transmissionby hydrothermal circulation. We here report thermalmeasurements in two holes at DSDP Site 396 into13.5-m.y.-old crust on the Mid-Atlantic Ridge.

TECHNIQUES

Three different types of downhole temperaturemeasurements were made or attempted during DSDP Leg46. These were:

1) Measurements of the temperature of unconsolidatedsediment below the bottom of the hole, at intervals duringdrilling.

2) Measurement of the temperature in the bottom 30meters of the drill hole following re-entry after bitreplacement.

3) Temperature logging of the hole over an extendedvertical interval.

All three types of measurements were made using aself-contained downhole temperature recorder capable ofmeasuring temperatures with an accuracy of about 0.02°Cand a resolution of 0.002°C (see Erickson et al., 1975). Thetemperature data are recorded on magnetic tape for lengthsof time whose duration is variable depending on thesampling time interval chosen. During Leg 46, temperaturemeasurements were made every 8, 16, and 32 seconds forperiods of 1.5, 3, and 6 hours, respectively.

The recorder was inserted into the end of a modified corebarrel as shown in Figure 1. Coil springs above and belowthe pressure case provided support and acted as shockabsorbers. The pressure case was prevented from beingpushed up inside the core barrel by a steel baffle, and fromfalling out through the drill bit by a %-in.-thick steel plateheld in place by the core catcher subassembly.

Sediment Temperature MeasurementsTo measure the in situ sediment temperature a thin

thermistor probe extending below the drill bit is pushedthrough the thermally disturbed sediment into theundisturbed sediments below the bottom of the hole. Toaccomplish this the core barrel containing the temperaturerecorder was lowered on the wireline (core barrel recovery

389

A. J. ERICKSON, R. D. HYNDMAN

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Baffle

Spring

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cable) and locked into the bottom hole assembly, then thedrill string was slowly lowered to the bottom of the hole sothat the thermistor probe was forced into thermallyundisturbed sediment ahead of the drill bit. An attempt wasmade to close the first bumper sub (telescoping loadsubassembly) to help isolate the temperature probe fromvertical movements of the drill string caused by ship'smotion. In order for this operation to be successful, thesediment must be soft enough for probe penetration butstrong enough to support the 6500-kg weight of the drillcollars below the bumper sub.

The probe was held as motionless as possible in thesediment for 15 to 20 minutes while the frictional heatgenerated during the penetration of the probe dissipated intothe sediment and the probe approached thermal equilibrium(e.g., Erickson et al., 1975). The core barrel was thenretrieved with the wireline and the recorded data playedback and interpreted on board ship.

Bottom Hole Measurements

Where the sediments or basement rocks are tooconsolidated for the thermistor probe to penetrate, the onlytemperature that can be measured is that in the hole itself,which is seriously disturbed by the drilling process. Withassumptions about the geometry of the hole, the thermalproperties of the rock fluid, the magnitude and duration ofthe thermal disturbance due to drilling and circulation ofdrilling fluid, it is possible to correct for the drillingdisturbance and obtain estimates of the in situ rocktemperatures (e.g., Bullard, 1947; Jaeger, 1956, 1961;Hyndman et al., 1976). Because rock near the bottom of thehole has been subject to drilling circulation for the shortestlength of time, temperatures measured there will be leastdisturbed and will have the most rapid return to equilibrium.However, the disturbance is still significant andtemperatures should be measured as long after terminationof drilling circulation as possible. When the holes are drilledwith re-entry for bit replacement, the best opportunity tomake useful bottom hole temperature measurements isimmediately following each re-entry since considerabletime is required for the re-entry operation.

Bottom hole temperatures were measured using almostthe same method as for measurement of sedimenttemperatures. The pressure case was inserted in the corebarrel which was then lowered on the wireline to the bottomof the drill string. A 30-meter length of pipe was then addedto the drill string. If the drill bit is initially about 30 metersabove the bottom of the hole, the drill string can be loweredslowly in 5-meter increments, allowing the temperatureprobe 2 or 3 minutes to come to equilibrium at each level. Inthis way a set of five or six temperature values can beobtained from which a local bottom hole thermal gradientcan be computed. After completion of the measurement thecore barrel can be retrieved as usual using the wireline.

Temperature Logging

Temperature logging techniques differ from those used tomeasure bottom hole temperatures only in that temperaturesare measured over a substantial portion of the hole depth,rather than only near the bottom of the hole. For thisoperation, the wireline normally used to lower the core

390

DOWNHOLE TEMPERATURE MEASUREMENTS

barrel must either not be used or be removed from the drillstring in order to permit the addition of the drill pipenecessary to enable the bottom hole assembly to be loweredover the interval to be logged. Special equipment is neededto release the core barrel after it is lowered to the bottom ofthe drill string with the wireline; however, it was notavailable during Leg 46. The alternative of allowing thecore barrel containing the temperature probe to freefalldown the drill pipe was used at Hole 396B. The speed ofdescent of the core barrel, with the baffle obstruction abovethe pressure case to impede the flow of water through thecore barrel, was slightly more than 100 m/min. The drillpipe was filled with water to the level of the drill rig floor toeliminate the impact created when the core barrel strikes thewater surface in the drill pipe. The instrument appears tohave suffered no damage; however, this procedure probablyshould be used only when absolutely necessary.

RESULTS

Heat Flow Measurement 1 — Hole 396A, 58 MetersSub-bottom

No downhole temperature data were obtained in theborehole because the instrument did not reach the sea flooruntil after the end of the measurement cycle.

Heat Flow Measurement 2 — Hole 396A, 75 MetersSub-bottom

Although the instrument worked perfectly, the large seaswells and resultant vertical movement of the drill pipealong with too soft sediment produced a highly disturbedtemperature record. Examination of Figure 2 suggests thatthe thermistor probe was nearly constantly in motion, withsudden temperature rises associated with frictional heatingduring downward movement, followed almost immediatelyby rapid cooling as the probe was pulled up into the coolerfluid within the borehole. The sediment at the bottom of thehole was not consolidated enough to support the lowermostbumper sub even with successive lowerings of the drill pipeby a few meters. Temperatures recorded while the probewas moving in the sediments vary between 5.61° and7.82°C. In a few instances, it appears that the probe wasstationary for periods long enough that segments of anapparent decay curve are discernible. Figure 3 shows somevery speculative attempts to fit decay curves to these datapoints. The estimated equilibrium temperatures are between4.8° and 5.4°C, with the lower temperature being moreprobable. It is possible that the gradual temperature increaseof about 0.5°C observed during the measurement period wascaused by conduction to the sensor of the frictional heatgenerated by the motion of the bottom hole assembly. Abottom water temperature of 3.43 ±0.02°C was estimatedfrom temperatures recorded in the drill pipe just before thetemperature probe within the drill string passed below thesea floor.

Heat Flow Measurement 3—Hole 396A, 95 MetersSub-bottom

Heat flow measurement 3 also suffered from motion ofthe thermistor probe; however, the sediment was somewhatmore consolidated so that longer intervals of temperature

data undisturbed by movement are apparent in thetemperature-time plot (Figure 4). Each of these apparentlyundisturbed intervals has been used to estimate anequilibrium sediment temperature (Figure 5) on theassumption that the well-recorded temperature-time decaycurve of the next deeper heat flow measurement, 4, can beused to extrapolate the short intervals recorded withoutdisturbance during measurement 3. Although excellentagreement exists between the form of the best decay curvesrecorded during measurements 3 and 4, the estimated in situsediment temperature of between 5.1° and 5.7°C is slightlyhigher than the interpolated temperature predicted on theassumption of a linear thermal gradient between the seafloor and the reliable downhole temperature, 4, measured at150.5 meters sub-bottom. We suspect that the observedtemperatures are again higher than the in situ sedimenttemperatures due to the conversion of mechanical to thermalenergy at the bottom of the borehole. A bottom watertemperature of 3.43 ±0.02°C also was recorded on thislowering.

Heat Flow Measurement 4—Hole 396B, 150.5 MetersSub-bottom

As a result of firmer sediment and greatly reduced shipmotion in calmer sea conditions, a reliable in situ sedimenttemperature of 6.11 ±0.02°C was recorded within aboutone meter of the sediment-basalt contact at Hole 396B(Figure 6). The thermal decay curve obtained during thismeasurement has been extrapolated to equilibrium using thesimple 1/t method discussed by Hyndman et al. (1974)(Figure 7). As previously discussed this curve has been usedto aid in the interpretation of heat flow data recorded duringmeasurements 2 and 3. A bottom water temperature of 3.42±0.02°C was recorded on this lowering, almost identical tothe previous two lowerings.

Bottom Hole Measurement 5—Hole 396B

Heat flow measurement 5 was made followingtermination of drilling because of unstable hole conditionsand after the hole had been re-entered with a drill bitmodified for use with the Schlumberger well loggingequipment and a downhole seismic experiment. Afterfreefalling the core barrel containing the heat flow probeinto the bottom hole assembly, temperatures were measuredfor 3 minutes each at approximately 4.75-meter increments(one-half pipe length intervals) from near the bottom of thecased portion of the drill hole in the sediments down to340.5 meters sub-bottom where drilling problems hadpreviously become evident. Unfortunately, the thermistorprobe was bent and flooded when it encountered anobstruction after logging only the uppermost 19 meters ofthe uncased portion of the hole. Sufficient force was exertedon the heat flow probe to push the top coil spring throughthe baffle.

Correlation of the temperature-time record (Figures 8 and9) and the log of the events occurring on the drill rig floorduring the measurement suggests that valid temperature datawere only obtained from the first four 4.75-meter intervals(to about 115 minutes on Figures 8 and 9), after which timethe probe bent and was subject to frictional heating as itrubbed against the side of the borehole. Final failure of the

391

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probe occurred at 133 minutes when the tip of the thermistorprobe broke off and the probe flooded. The obstructionresponsible for bending the thermistor probe occurredbetween 172 and 177 meters sub-bottom, only 20 to 25meters into the basement. When the probe broke it filledwith sediment, suggesting that the hole was full of sedimentat that depth. For future measurements where obstructionsmay be encountered, temperature measurement within thedrill pipe is suggested.

A temperature of 3.05 ±0.02°C was measured bothwithin the casing and in the uncased borehole in the basalticbasement. This value has been increased by 0.47°C to bringthe observed bottom water temperature of 2.95°C intoagreement with the consistently recorded values of 3.42°Cmeasured with the same instrument, but a differentthermistor, during the preceding three heat flowmeasurements (2, 3, and 4). The change in calibrationprobably arose from the severe shock to which thisthermistor was subjected. Thus borehole temperaturesbetween 153 and 172 meters were nearly constant and onlyslightly (0.10°C) warmer than the bottom watertemperature.

THERMAL CONDUCTIVITY MEASUREMENTS

Sediments

Thermal conductivity measurements were made aboardGlomar Challenger on the sediment cores recovered from

392

DOWNHOLE TEMPERATURE MEASUREMENTS

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the holes using the transient needle-probe method describedby Von Herzen and Maxwell (1959). This method uses therate of temperature increase as a function of time in athin-walled hypodermic needle containing a heating elementand a thermistor. The temperature increase for a constantheat input is recorded in analog form, then digitized, and acurve of the form T = A + Bt + C bi(t) is fitted to thetemperature (T) versus time (t) data using a nonlinearregression program. The value of C depends on the thermalconductivity. Reduction of the data in this manner permitsthe removal of nearly linear temperature drift (given by thevalue of B) arising from the difference in ambienttemperature between the sediment core and the laboratory.The needle is inserted into the sediment through small holesdrilled in the plastic core liner.

The results of the sediment thermal conductivitymeasurements are listed in Table 1. Data from both Holes396A and 396B have been combined in Figure 10. Themean of the five conductivity values measured in a singlesection of severely disturbed, totally unconsolidatednanno-foram ooze recovered from just below the sedimentsurface at Hole 396A is 2.84 ±0.07 mcal/cm sec °C. The 18conductivity values measured in sediments recovered fromgreater depth, 122.50 to 142.27 meters sub-bottom, in Hole396B are highly variable, with a mean and standarddeviation of 3.04 ±0.24 mcal/cm sec °C. This value is

similar but slightly higher than the mean for previouslymeasured carbonate sediments. Mean thermal conductivityvalues for each of the four sections from which data wereavailable (Table 2), although quite variable, show nosystematic variation with depth. Cores 1 and 2 at Hole 396Bconsist of nannofossil ooze; Core 3, with a lowerconductivity, is described as marly nanofossil ooze. Allcores were severely to moderately disturbed by drilling.

Basalt

The thermal conductivity and wet density of 10 basaltsamples from Hole 396B were measured using a divided barapparatus with constant temperature ends (Jessop, 1970).Values for the quartz and fused silica standards used in theapparatus were from Ratcliffe (1959). The basalt sampleswere in the form of discs 2.5 cm in diameter by 1.0 cm thickwhich were cut from small 2.5-cm-diameter cores(mini-cores) drilled from chunks of basalt. The minicoreswere stored in water as soon as possible after their recoveryaboard D/V Glomar Challenger and kept saturated untiltheir conductivity was measured.

The mean sample temperature was about 25°C, and theaccuracy of individual values is better than 5 per cent (Table3). The harmonic mean conductivity is 4.08 ±0.05 mcal/cmsec °C, very similar to that measured previously on sea floorbasalts (e.g., Hyndman and Drury, 1976). The thermal

393

A. J. ERICKSON, R. D. HYNDMAN

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conductivities measured on basalt samples have beenplotted as a function of depth in Figure 10. Although clearlyhigher than conductivity values measured in the sedimentsabove, there is no obvious relationship of the measuredconductivities with depth. The in situ conductivity of theupper crust may be significantly lower than this meanbecause of the presence of extensive voids and fractures notrepresented in the small laboratory specimens, perhaps aslow as 3.5 mcal/cm sec °C. The effect of the differencebetween laboratory and in situ temperature and pressure onthe measured thermal conductivity should be negligible (seeHyndman and Drury, 1976).

INTERPRETATIONS

Heat Flow Calculations

From the borehole temperatures measured in thesedimentary portion of Holes 3 96A and 396B (Table 4) theaverage thermal gradient is 17.8 ±1.0°C/km between thesea floor and the sediment-basalt interface (Figure 11). Thisvalue is based upon the three bottom water temperaturemeasurements in the drill pipe at the sea floor, and upon thereliable temperature measured at 150.5 meters sub-bottom.No correction of the observed mean conductivity (3.04

±0.24 mcal/cm sec°C) to in situ temperature and pressureconditions at sub-bottom was made as the two effectsapproximately cancel and the variability in the conductivitydata is much larger than either the pressure or temperaturecorrection. This thermal gradient and mean thermalconductivity give a heat flow estimate of 0.54 ±0.07µcal/cm2sec through the sediment at Site 396. A somewhathigher value is obtained using the gradient between theseafloor and the less reliable temperature measured at 95meters sub-bottom.

During the site survey for Site 396 two heat flowmeasurements were made in the same sediment pond usinga standard Ewing-type heat flow instrument with outriggerprobes mounted on a piston corer (J. Crowe, personalcommunication, 1977, Purdy et al., 1978). Heat flowvalues of 0.27 and of less than 0.7 µcal/cm2sec weremeasured, in agreement with the low value of 0.54µ,cal/cm2sec measured in the borehole.

Basement Temperatures

The measurement of a nearly constant temperature ofonly 3.05°C ( corrected to 3.52°C as previously discussed)just below the sediment-basalt interface was surprising forseveral reasons. Firstly, downward extrapolation of the

394

DOWNHOLE TEMPERATURE MEASUREMENTS

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HOLE 396B150.5 m Sub-bottom

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Figure 6. Plot of temperature versus time recorded during downhole temperaturemeasurement #4, 150.5 m sub-bottom, at Hole 396B.

temperatures in the sediment using the mean basalt thermalconductivity gives a temperature increase of 0.25°Cexpected in the borehole between 153 and 172 meterssub-bottom. The observed increase was less than 0.02°C.Secondly, a reliable sediment temperature of 6.11°C wasdetermined within about a meter of the top of the sectionthrough which the temperature was logged. The onlyreasonable explanations for the result are either that thedrilling circulation thermal disturbance had not significantlydecayed or that seawater is flowing down the borehole intothe permeable basaltic clastic layers encountered deeper inthe borehole (see Site Chapter), this bathymetric lowperhaps being a hydrothermal recharge area.

The following argument shows that the anomalously lowtemperature measured in the upper 19 meters of thebasement cannot be attributed to cooling of the rocks due tocirculation of drilling fluid. The temperature in the boreholeat any depth will be a function of (1) the initial temperature

of the wall rock at that depth, (2) the temperature of thecirculating fluid (close to the bottom water temperature), (3)the history of circulation, (4) the length of time that theborehole has been undisturbed since last circulation, (5) thethermal properties of the wall rock and borehole contents,and (6) the size of the borehole.

The response of the borehole to a particular drillinghistory can be approximated by assuming (1) that thecirculation of cool seawater through the hole is equivalent tokeeping the walls of the borehole at the temperature of thecirculating fluid whenever circulation is in progress (e.g.,Jaeger, 1956, 1961); or less accurately by assuming (2) thatheat is removed from the rocks surrounding the borehole ata constant rate during circulation (e.g., Bullard, 1947). Thesecond set of boundary conditions will overestimate thethermal disturbance caused by circulation since ascirculation continues for longer and longer times, the wallrocks will gradually cool and will lose heat at a lower rate

395

A. J. ERICKSON, R. D. HYNDMAN

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600.03 004 0.05 0.06

1/ Data Word Number0.07 008 009

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than they did initially when the temperature differencebetween the circulating water and wall rocks was at amaximum.

The total drilling and circulation time from the first corein basalt was 16.5 days, interspersed with long periods ofnon-circulation during re-entry operations, and otherdelays. By assuming that circulation was continuous themaximum thermal disturbance can be calculated. The holewas permitted to equilibrate for about 37.5 hours from thetime the last core was obtained until the temperaturelogging. This time is much longer than the thermal responsetime of the borehole fluid so that equilibration of thematerial in the borehole to wall rock temperature can beassumed.

Using the simple approximate theory of Bullard (1947), aminimum of 40 per cent of the disturbance should havedecayed so that the observed temperature in the boreholeshould be at least 1.1°C warmer than the temperature of thedrilling fluid, rather than the 0.10°C observed.

We thus conclude that water is flowing down the holefrom the sea floor, with only slight warming due to heatflow from the walls of the hole, and is then entering the

highly permeable layers of basaltic sand and gravelencountered between about 310 and 405 meters sub-bottom.If this conclusion is correct, at least after having penetratedby drilling, the porous layers became part of ahydro-thermal circulatory system, possibly involvingbasement outcrops which act as chimneys for rising hotwater which is replaced through or near the edges ofsediment-covered basement lows by the slow downwarddiffusion of cool water. An illustrative schematiccross-section is shown in Figure 12.

CONCLUSIONS

Temperature measurements in the sedimentary portion ofthe borehole at Site 396 demonstrate a low approximatelylinear thermal gradient. A heat flow of 0.54 µcal/cm2sec iscalculated. Although this value is much lower than the heatflow theoretically expected (3.1 µ,cal/cm2sec) throughocean crust of this age (13.5 m.y.) on the basis of coolinglithosphere models (Parsons and Sclater, 1977), it is inagreement with the other low values observed in the samesediment pond, and is consistent with the generally low, butvariable heat flow observed through sediment ponds inregions of rough topography and young crust. The low heatflow through the sediment is attributed to the effects ofhydrothermal circulation within the fractured, permeablebasement beneath the sediment, rather than to fluidmovement within the sediment itself.

The observation of low heat flow in the sediment andnearly constant temperatures only slightly greater than thatof the bottom water in the basement portion of the boreholeis nearly identical with the results obtained at Site 335during Leg 37 (Hyndman et al., 1976). Our interpretation ofthe data at Site 396 is similar to that proposed for Site 335,that when the drill bit penetrated the relatively impermeablesediment cover it initiated the downward flow of seawaterthrough the hole into the highly permeable, basement rocks,particularly into the glass-rich basaltic sand and gravellayers encountered below about 310 meters sub-bottom, 160meters below the sediment.

It is perhaps significant that at both Sites 335 and 396,located in oceanic crust of approximately the same age(16.5 and 13.5 m.y., respectively), strong evidence existsfor flow of seawater down the borehole, whereas for Sites332 and 334, drilled 3.5- and 8.9-m.y.-old crust, no suchdownward flow is indicated. One possible interpretation ofthis observation is that the permeability of the basalticbasement increases with age near the ridge crest, and thathighly permeable layers of basaltic sand and gravel are lesscommon in basement younger than 9 or 10 million years.This possibility is in agreement with suggestions by Dick etal. (this volume) that clastic material is formed by tectonicbrecciation, erosion, transport, and deposition on the ridgeflanks away from the median valley.

ACKNOWLEDGMENTSWe wish to express our appreciation for the assistance provided

by the Deep Sea Drilling Project technical staff in making thetemperature measurements. We also thank Dr. A. M. Jessop forarranging the thermal conductivity measurements on the basaltsamples. Contribution of the Earth Physics Branch.

396

DOWNHOLE TEMPERATURE MEASUREMENTS

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Figure 8. Plot of temperature versus time recorded during downhole temperaturemeasurement #6 in Hole 396B. Temperatures were measured every 4.75 m fromthe base of the cased portion of the borehole until the thermistor probe bent(117 min.) Erratic temperatures between 117 and 128 minutes are believed tobe due to frictional heating of the probe as it scraped up and down the wall ofthe borehole prior to finally flooding at 128 min.

REFERENCES

Bullard, E. C , 1947. The time necessary for a borehole to attaintemperature equilibrium, Mon. Node. Roy. Astron. Soc,Geophys. Suppl.,\. 5, p. 127-130.

Erickson, A. J., Von Herzen, R. P., Sclater, J. C , Girdler,R. W., Marshall, B. V., and Hyndman, R. D., 1975.Geothermal measurements in deep-sea drill holes, J. Geophys.Res., v. 80, p. 2515-2528.

Hyndman, R. D. and Drury, M. J., 1976. The physical propertiesof oceanic basement rocks from deep drilling on themid-Atlantic ridge, J. Geophys. Res., v. 81, p. 4042-4052.

Hyndman, R. D., Erickson, A. J., and Von Herzen, R. P., 1974.Geothermal measurements on DSDP Leg 26: In Davis, T. A.,Luyendyk, B. P., et al., Initial Reports of the Deep SeaDrilling Project, v. 26: Washington (U.S. GovernmentPrinting Office), p. 451-463.

Hyndman, R. D., Von Herzen, R. P., Erickson, A. J., andJolivet, J., 1976. Heat flow measurements in deep crustal holes

on the mid-Atlantic ridge, J. Geophys. Res., v. 81,p. 4053-4060.

Jaeger, J. C., 1956. Numerical values for the temperature in radialheat flow, J. Math. Phys., v. 34, p. 316-321.

, 1961. The effect of the drilling fluid on temperaturesmeasured in boreholes, /. Geophys. Res., v. 66, p. 563-569.

Jessop, A. M., 1970. The effect of environment on divided barmeasurements, Tectonophysics, v. 10, p. 39.

Langseth, M. G., 1965. Techniques of measuring heat flowthrough the ocean floor: In Lee, W. H. K (Ed.), Terrestrialheat flow, Washington (American Geophysical Union),p.58-77.

Lister, C. R. B., 1972. On the thermal balance of a mid-oceanridge, Geophys. J., v. 26, p. 515-535.

Parsons, B. and J. G. Sclater, 1977. An analysis of the variationof ocean floor bathymetry and heat flow with age, J. Geophys.Res., v. 82, p. 803-827.

Purdy, G. M., Schouten, H., Crowe, J., Barrett, D. L., Falconer,R. K. H., Litvin, V. M., Udintsev, G. B., Marova, N. A.,

397

A. J. ERICKSON, R. D. HYNDMAN

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TABLE 1Thermal Conductivity Values Measured in

Sediments Recovered From Holes 396A and 396B

Sample(Interval in cm)

Hole 3 96A

1-1, 1061-1, 1151-1, 1221-1, 1311-1,140

Hole 396B

1-1,501-1, 661-1, 801-1, 1121-1, 1351-2, 101-2,181-2, 252-1,532-1,612-1, 792-1, 1002-1, 1192-1,1323-1, 763-1,903-1, 1063-1, 127

Depth(m)

1.041.151.221.311.40

122.50122.66122.80123.12123.35123.60123.68123.75132.03132.11132.29132.50132.69132.82141.76141.90142.06142.27

Conductivity(mcal/cm sec°C)

2.952.782.802.762.89

3.273.053.073.053.012.702.752.953.513.383.303.032.683.152.862.672.963.29

64 80 96Time (min)

112 128

Figure 9. An explanded scale plot of data in Figure 8.

Valyashko, G. M., Markushevich, V. M., and Zdoroven,V. V., 1978. IPOD Site 6:A Site Survey. In Melson, W. G.,Rabinowitz, P. D., et al., Initial Reports of the Deep Sea

Drilling Project, Volume 45: Washington (U.S. GovernmentPrinting Office).

Ratcliffe, E. H., 1959. Thermal conductivities of fused andcrystalline quartz, British J. Appl. Phys., v. 10, p. 22.

Sclater, J. G., Von Herzen, R. P., Williams, D. L., Anderson,R. N. and Klitgord, K. D., 1974. The Galapagos spreadingcenter: Heat flow low on the north flank, Geophys. J., v. 38,p. 609-625.

Von Herzen, R. P. and Maxwell, A. E., 1959. The measurementof thermal conductivity of deep-sea sediments by a needleprobe method, J. Geophys. Res., v. 64, p. 1557-1563.

Williams, D. L., Von Herzen, R. P., Sclater, J. G., andAnderson, R. N., 1974. The Galapagos spreading center:Lithospheric cooling and hydrothermal circulation, Geophys.J., v. 38, p. 587-608.

398

DOWNHOLE TEMPERATURE MEASUREMENTS

2.6 2.80

Thermal Conductivity (mcal/cm sec° C)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

Figure 10. Plot of thermal conductivity versus sub-bottom at Site 396. Data obtainedfrom sediments at Hole 396A are shown as squares; the balance of the data (shownas circles) are from Hole 396B.

TABLE 2Mean and Standard Deviation of the ThermalConductivity Values Measured on Sectionsof Sediment Cores Recovered From Holes

396A and 396B

TABLE 3Thermal Conductivity and Wet Density of Pieces of Basalt

Recovered From Hole 396B

Core

Hole 396A1

Hole 396B123

Section

1

211

Conductivity(mcal/cm sec°C)

Mean S. D. N

3.09

2.803.152.95

0.09

0.110.270.22

5

364

Sample(Interval in cm)

5-1,61-637-3,9-119-2, 29-41

10-1,57-5912-1, 103-10514-2, 73-7516-2,71-7317-1,69-7120-3, 137-13922-1,45-47

Depth(m)

157.62177.10194.90203.08215.54227.74246.72268.70290.87305.96

Conductivity(mcal/cm sec°C)

4.324.323.863.984.124.084.204.184.133.86

Wet Density(g/cc)

2.612.802.662.832.822.822.84—

2.752.75

TABLE 4Downhole Temperature Data Used in Calculating the Steady State Geothermal

Gradient in the Sediment at Site 396

Measurement Hole Depth (m) Temperature (°C) Remarks

396A and B396A396A396B

0.0 ±5.075.0 ±1.095.0 ±1.0

150.0 ±1.0

3.43 ±0.024.8 to 5.45.1 to 5.56.11 ±0.02

Water temperatureUnreliableUnreliableExcellent

399

A. J. ERICKSON, R. D. HYNDMAN

3.0 3.5

50 -

100 -

150

Temperature (°C)4.0 4.5 5.0 5.5 6.0 6.5

-

HHH

\

Sediment

w \ v •XN x V \ \ ;M

Basalt

H

1v 1 measure

\

measure

t 2

nent3

\

2 00

Figure 11. Plot of downhole temperatures measured inHoles 396A (#2, #3) and 396B (#4) versus sub-bottom depth. The error bars shown are subjectiveestimates of the uncertainty in in situ sediment temper-ature. Temperature values in the basalt were logged inthe water and mud filled, uncased portion of the bore-hole following re-entry.

3.5

4.0

4

Sediment

//\

\\

Λ\\\\ Site396

| /

v^7/

Uy

^ . iv sk >

VERTICAL

EXAGGERATIONI2 l I0KM

3.5

4.0

4.5

43°40' 43°20'

Figure 12. Profile across Site 396 showing sediment and illustrative patterns of watermovement in the basaltic crust.

400