TEMPERATURE MEASUREMENTS Methods

9. GEOTHERMAL MEASUREMENTS DURING DEEP SEA DRILLING PROJECT LEG 801

J. P. Foucher, Centre Océanologique de Bretagne, BrestP. Y. Chenet and L. Montadert, Institut Français du Pétrole, Rueil-Malmaison

andJ. M. Roux, Laboratoire de Mécanique Physique, Université de Bordeaux I

ABSTRACT

Geothermal gradients were successfully determined from downhole temperature measurements at Sites 548, 549,and 550. Mean determined temperature gradients are 27.2°C/km between 110.5 and 281.5 m at Site 548, 22.5°C/kmbetween 236.5 and 417 m at Site 549, and 54.9°C/km between 99.5 and 156.0 m at Site 550.

We used over 200 conductivity measurements, including both onboard and shore-based determinations that were ingood agreement, to find heat-flow values corrected for irregularities in seafloor topography and sediment-basementthermal conductivity contrast. The resulting values are 36 ± 15 mW/m2, 37 ± 15 mW/m2, and 78 ± 18 mW/m2 forSites 548, 549, and 550, respectively. The Hercynian continental basement conductivity values average 3.8 ± 0.4 W/m°C. The average for the oceanic basalts at Site 550 is 1.6 ± 0.1 W/m°C.

The heat-flow values at Sites 548 and 549 are comparable to those previously determined farther south on the northBiscay margin, in the Trevelyan-Meriadzek area, in particular the heat-flow determination of 36 mW/m2 at Site 402.

The thermal regime of the Goban Spur margin, as inferred from the heat-flow determinations at Sites 548 and 549,is compatible with a thermal model of continental margin evolution that assumes that the margin formed by lithospher-ic extension about 110 m.y. ago.

The heat contribution from the radioelements in the crust probably does not differ significantly from Site 548 to Site549, and is estimated to be less than 10 mW/m2.

The heat flow of 78 mW/m2 at Site 550 is abnormally high for an ocean crust thought to have formed about100 m.y. ago.

INTRODUCTION

On Leg 80, four holes were drilled on a transect acrossthe Goban Spur area (Fig. 1), providing a unique oppor-tunity to determine the heat flow rates and documentthe thermal regime of the deeper part of a mature conti-nental margin and the adjacent ocean crust.

A particular attraction in carrying out a geothermalinvestigation of the Goban Spur area is that rifting tookplace there in a probably simple geological context. Onthe Goban Spur, rifting affected a Hercynian graniticand metasedimentary basement with a thin sediment cov-er. This is a simpler geological context than that, for ex-ample, in the Meriadzek-Trevelyan area, farther southof the Goban Spur, in the northern Bay of Biscay, wherea thick Mesozoic sequence occurs, evidencing large Me-sozoic vertical movements and a probably more compli-cated geological context. To our sense this makes a com-parison of the present-day thermal structure of the Go-ban Spur area with that of the Hercynian continentalborderland quite desirable, since different geothermalconditions in the two areas are likely to be the result ofthe only thermal effects associated with the rifting event.

Heat flow was successfully determined at Sites 548,549, and 550. Together with a previous heat-flow deter-

Graciansky, P. C. de, Poag, C. W., et al., Init. Repts. DSDP, 80: Washington (US.Govt. Printing Office).

2 Addresses: (Foucher) Centre Océanologique de Bretagne, B.P. 337, 29273 Brest Cedex,France; (Chenet, Montadert) Institut Français du Pétrole, B.P. 311, 92506 Rueil-MalmaisonCedex, France; (Roux) Laboratoire de Mécanique Physique, Université de Bordeaux I, 351,Cours de la Liberation, 33405, Talence Cedex, France.

mination at DSDP Site 402 (Erickson et al., 1979) andearly conventional surface heat-flow measurements (Fou-cher and Sibuet, 1980), the new Leg 80 heat-flow deter-minations contribute toward further outlining the re-gional trends in the thermal regime of the northern Bayof Biscay margin.

TEMPERATURE MEASUREMENTS

MethodsAll temperature data are downhole data acquired us-

ing the DSDP downhole temperature probe described byYokota et al. (1980). The measurement technique con-sists in lowering to the bottom of the hole the tempera-ture probe, rigidly secured to the lower end of the corebarrel, and driving the probe into the undrilled, ther-mally undisturbed sediment ahead of the drill bit. Theprobe is pushed into the sediment by using, as the driv-ing force, the weight of the bottom-hole assembly, intowhich the probe has been previously latched. The probeincludes a self-contained recorder. The sensor tempera-ture is sampled every minute for a total measuring time,after insertion of the probe into the sediment, of about30 min. In normal operating conditions, the sensor hasreached or closely approached the equilibrium sedimenttemperature at the end of the measurement time. In thiscase, the accuracy of the temperature measurement isprobably within 0.05°C.

The quality of the measurements depends on severalcritical factors, some of which are difficult to control.Clearly, two main factors are the depth of insertion of

423

J. P. FOUCHER, P. Y. CHENET, L. MONTADERT, J. M. ROUX

the probe into the sediment and the degree of stabiliza-tion of the probe in the sediment. In extreme cases—ei-ther of too hard rocks, which prevent normal penetra-tion of the probe into the sediment, or of vertical move-ments of the probe after its insertion into the sediment,which produce large frictional heating—measurementsare unsuccessful.

ResultsTable 1 summarizes the downhole temperature mea-

surements attempted during Leg 80.

Hole 548ATemperatures were measured at four levels in Hole

548A (Fig. 2). Two good quality measurements, at 110.5and 281.5 m sub-bottom, were used to calculate a meantemperature gradient of 27.2°C/km. By extrapolationof this gradient to the sea bottom, a bottom seawatertemperature of 9.9°C was calculated; this is significantlyhigher than the measured value between 7.1 and 7.6°C.The approximately 2°C difference between the observedand calculated temperatures can be ascribed to the knownseasonal fluctuations of bottom-water temperature atthe location of Hole 548A. Hole 548A is the hole drilledat the shallowest water depth during Leg 80, only 1251 m.

Hole 549Seven temperature measurements were attempted in

Hole 549 at six different levels (of which four are repre-sented in Fig. 3). The three measurements at sub-bottomdepths of 198.5, 236.5, and 417.0 m gave good quality

Table 1. Summary of downhole temperaturemeasurements, Leg 80.

Sub-bottomdepth(m)

Hole 548A

053.5

110.5167.5281.5

Hole 549

095.0

151.0198.5236.5274.5274.5417.0

Hole 550

099.5

156.5213.5

Hole 55OB

0323.0

Temperature(°C)

7.1-7.6

12.9

17.5

3.4

10.09.9

10.4?13.3-13.6

2.6-2.76.69.7

2.816.0?

Quality/observations

GoodNo stabilizationExcellentNo stabilizationGood

ExcellentNo recordingNo recordingGoodGoodNo penetrationNo stabilizationGood

GoodExcellentExcellentUncertain penetration

GoodNo stabilization

temperature determinations. There is, however, some un-certainty as to the value of 10°C obtained at 198.5 m,since the bottom-water temperature of 7.2°C derivedfrom the temperature record for this measurement isclearly not valid. It will be also noted that the tempera-ture at 417.0 m had not fully stabilized at the end of themeasuring time. The minimum temperature recorded atthis depth was 13.6°C. A possibly better estimate, ob-tained from a simple plot of the temperature versus theinverse of time, is 13.3°C. The mean geothermal gradi-ent calculated between the 236.5 and 417.0 m depths is20.5°C/km, taking a temperature of 13.6°C at 417.0 m,or 24.4°C/km, taking the temperature at that depth tobe 13.3°C. We consider this range of values, 20.5 to24.4°C/km, as the best estimate of the geothermal gra-dient in Hole 549.

Hole 550Three temperature measurements were attempted in

Hole 550 (Figs. 4A-4C). Two excellent temperature de-terminations, at 99.5 and 156.5 m sub-bottom depth,give a mean temperature gradient value of 54.9°C/km.

Hole 550BA temperature measurement was attempted at a sub-

bottom depth of 323 m (Fig. 4D). The measurement isof very poor quality, owing to apparent frictional move-ments of the probe in the sediment, causing temperaturedisturbances. A rough estimate of the sediment temper-ature, about 16°C, can be obtained by looking at theshape of the temperature record. In spite of its large un-certainty, a major interest of this estimate is that it tendsto confirm the high temperature gradient calculated forHole 550. One will note that the 16°C temperature valuenearly aligns with other temperature data for Site 550 ona temperature-versus-depth plot (Fig. 5).

THERMAL CONDUCTIVITY MEASUREMENTSIn total, 142 conductivity measurements were made

in the shipboard laboratory on recovered samples fromSites 549 and 550, using the needle-probe technique (VonHerzen and Maxwell, 1959).

MethodsThe needle-probe technique was originally developed

to measure the thermal conductivity of a soft sedimentsample, in which case the needle-probe is inserted intothe sediment sample. Thermal conductivities of hard rocksamples can also be measured, using a half-space adap-tation of the method (Carvalho et al, 1980), with theneedle sensor applied to the polished flat surface of arock sample. During Leg 80, thermal conductivities ofhard rock samples were measured using the half-spaceversion of the method. The probe was calibrated usinga fused-silica standard (1.38 Wm-•K"1). The accuracyof the needle-probe technique has been estimated to bewithin 5% (Von Herzen and Maxwell, 1959).

In addition to the shipboard needle-probe measure-ments, 61 steady-state measurements were completedashore, after the cruise, on rock samples from Sites 549and 550, in a thorough experimental study of a newly

424

GEOTHERMAL MEASUREMENTS

developed hotplate-type thermal-conductivity measure-ment device (Roux, 1982; and see Appendix).

Results

Tables 2 and 3 summarize all the conductivity dataacquired using the needle-probe technique and the hot-plate steady-state method. The data show the normal in-crease of thermal conductivity with depth, from meanvalues around 1.5 W/m°C just below the seafloor tomore than 3.5 W/m°C near the bottom of Hole 549(Table 2). Conductivities measured on the Paleozoic mi-caceous sandstones of the basement sampled at this holeaverage 3.8 ± 0.4 W/m°C. In Hole 55OB, the conduc-tivity decreases in basalts near the bottom of the hole(Table 3). The mean thermal conductivity measured onthe basalt samples is 1.60 ± 0.1 W/m°C in this hole.This value is in close agreement with previously report-ed conductivity values for basalts: for example, 1.66 ±0.02 W/m°C for DSDP Leg 37 basalts (Hyndman andDrury, 1976).

Comparison of Thermal Conductivity Data AcquiredUsing the Needle-Probe Technique and the

Hotplate Apparatus

Steady-state measurements could not be made on thesame rock samples used on board the ship for needle-probe measurements. This prevents direct comparisonof the conductivity data acquired using the two meth-ods. Nevertheless, comparison of the mean conductivityvalues over the same depth intervals suggests good agree-ment between results obtained using the needle-probetechnique and those obtained with the hotplate appa-ratus (Table 4).

Dependence of Conductivity on Temperature

Preliminary experiments on four sedimentary sam-ples from Hole 549 indicate a decrease of the conductiv-ity by an amount between 0.007 and 0.011 W/m°C perdegree over the temperature range zero to 80°C (Fig. 6).

Table 2. Thermal conductivity measurements at Site 549.

Core-Section

Hole 549A

2-14-14-16-18-110-112-114-116-118-124-126-128-132-134-134-1

Hole 549

22-324-328-128-128-228-228-226-229-140-142-242-244-446-152-156-556-457-157-357-474-374-276-281-183-1

Sub-bottomdepth(m)

Thermalconductivity

(W/m°C)

Needle-probe apparatus

92826476685

104112120125133138141149157157

1.5091.3951.6241.4421.4101.509.395.395.458.426.546.509.442.565.527.565


392412449449449449449449455550570570593608664701705711713715815813827861879

1.8331.7981.7462.0051.8252.1451.6301.7461.9721.8251.9411.8531.9111.7983.9551.9151.9742.6192.5162.5663.3772.4212.4212.3332.251

Core-Section

Sub-bottomdepth(m)

Thermalconductivity

(W/m°C)

Hole 549 Needle-probe apparatus (Cont.)

84-184-184-185-285-287-289-290-192-1

Hole 549

14-334-142.CC43-144-446-347-452-157-458-659-160-661-172-172-273-173-377.CC78-180-281-182-283-286-387-288-390-291-395-196-197-198-1

885885885888888906921929947

2.5673.4682.3762.0371.7823.3772.4682.9842.790

Steady-state apparatus

313502570580590608620664714726730740750800806809834845865875875875880900910915930940973980985990

2.4311.6821.5682.0511.7241.4371.6813.2632.2802.1302.1532.2222.1881.9162.1612.3012.8022.4852.7022.6982.6982.6973.1042.2023.0012.9253.1422.5533.6804.3423.0624.062

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Table 3. Thermal conductivity measure-ments at Site 550.

Core-Section

Hole 550

4-16-18-410-112-414-215-116-317-218-319-221-223-124-125-426-127-528-129-130-131-132-134-133-134-136-437-139-238-440-142-143-1

Hole 550B

3-24-15-17-28-49-410-411-112-416-117-218-1

Hole 550B

4-110-313-720-422-425-4

Sub-bottomdepth(m)

Thermalconductivity

(W/m°C)


120146161180200218224233244255261282301311323327343347357365376385404394404427432452445460480489

1.4501.5101.4021.3511.5651.5551.7821.4921.5271.4581.6141.5551.7111.4561.5271.458.645

1.7581.8331.8601.6451.7581.7111.7001.6671.7111.851.7581.6242.1391.6662.070


477485494515527537546551565598610618

2.071.9151.9442.1212.2712.1392.072.1752.2712.0373.4182.121

Steady-state apparatus

485545580640655685

.975

.8011.5341.527.742.425

Correlation between Porosity andThermal Conductivity

Variations in thermal conductivity with depth are cor-related with variations in porosity (Figs. 7 and 8). Thedependence of thermal conductivity on porosity can bedescribed, to a first approximation, by a linear law. Thefollowing laws were calculated by a least-squares fittingtechnique. For Site 549,

K = 3.26 - 0.031e (Correlation factor = 0.76)

and for Site 550,

K = 2.25 - 0.012e (Correlation factor = 0.54)

with K, thermal conductivity, in W/m°C and e, porosi-ty, in percent.

HEAT-FLOW ESTIMATES AND CORRECTIONS

Heat flow is calculated as the product of the geother-mal gradient and the thermal conductivity. The selectedconductivity value is the inverse of the mean thermal re-sistivity over the depth interval used for determining thegeothermal gradient. At Site 548, where no conductivitydata are available, conductivity was estimated using da-ta from Sites 549 and 550, as it is noted that conductivi-ty values do not significantly differ from one site to theother at similar depths.

Table 5 summarizes the heat-flow results for Sites 548,549, and 550. Calculated heat-flow values were subse-quently corrected to account for lateral heat-conductioneffects caused by changes in topography and contrastsin thermal conductivity (Fig. 9). Corrections amount toonly a few percent.

DISCUSSIONLow heat-flow values were obtained at Sites 548 and

549 on the transitional crust of the Goban Spur area: 36and 37 mW m2, respectively. These values do not signifi-cantly differ from previously reported heat-flow deter-minations farther south on the north Bay margin, in theMeriadzek-Trevelyan area. In that area, heat flow wasestimated to be 36 mW/m2 (Erickson et al., 1979) atDSDP Site 402, and surface measurements (Fig. 10) werereported in the range 36 to 43 mW/m2 (Foucher andSibuet, 1980). It can be suggested, therefore, that thewhole deeper part of the north Biscay margin is charac-terized, on a regional scale, by relatively low heat-flowvalues, around 40 mW/m2.

Heat-flow values at Site 548 are similar to those atSite 549. This is a notable result, since the crust at Site549, in 3485 m of water, is expected to be considerablythinner than that at Site 548, in only 1691 m of water.On the assumption that local isostasy conditions prevail

Table 4. Comparison of the conductivity data ob-tained using the needle-probe technique andusing the hotplate apparatus.

Sub-bottomdepth

interval(m)

Thermal conductivity (W/m°C)

Transient method(needle-probe

technique)

Steady-state method(hotplate

apparatus)

Hole 549

400-600600-750750-950

Hole 55OB

450-685

1.86 ± 0.13 (13)2.48 ± 0.75 (7)2.62 ± 0.52 (14)

1.89 ± 0.35 (5)2.17 ± 0.53 (8)2.63 ± 0.36 (16)

2.21 ± 0.39 (12) 1.67 ± 0.21 (6)

Note: Number of measurements in parentheses.

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Table 5. Summary of Leg 80 heat-flow results.

Sub-bottomdepth

intervalSite (m)

Temperaturegradient

(10-3 oC/m)

Mean thermalconductivity

(W/m°C)

Uncorrectedheat flow(mW/m2)

Estimated heat flowat a depth of6 km below

the sea surface(mW/m2)

548549550

110.0-281.5236.0-417.099.5-156.0

27.220.554.9

1.50a

1.821.48

413781

±±±

151020

36 ±37 ±78 ±

151418

a Estimated from Sites 549 and 550.

(which is a simple assumption, made in the absence ofdirect seismic information on the crustal structure at orin the vicinity of the drilling sites), the crust at Site 549would be more than 10 km thinner than at Site 548. Theobserved lack of any significant difference between theheat-flow values at Sites 548 and 549, despite expectedsubstantially different crustal thicknesses, tends to con-firm the earlier observation by Foucher and Sibuet (1980)that there is no significant heat-flow variation across theMeriadzek-Trevelyan area, although the crustal thick-ness decreases seaward from 20 km to 6 km (Avedik etal., 1981).

In interpreting heat-flow data from the top of a con-tinental margin, it is usual to consider the heat-flow val-ues as the sums of three components: the heat from themantle, the heat produced by the radioelements in thecrust, and the transient heat flux associated with thethermal rifting event. In the case of the Goban Spurmargin, the rifting event ended about 110 m.y. ago (Mas-son et al., this vol.), which implies that the transientheat flux associated with the rifting event is unlikely toexceed 10 mW/m2 (McKenzie, 1978).

This therefore leaves, for the sum of the heat flowfrom the mantle and the heat originating from the ra-dioelements in the crust, a value of 26 to 36 mW/m2.This is a low value, and suggests, when compared withthe mantle heat-flow estimate for the U.K. by Oxburghet al. (1980) of 27 mW/m2, that the radioelements in thecrust at Sites 548 and 549 contribute to the surface heatflow at these sites by less than 10 mW/m2.

Whether this low estimate of 10 mW/m2 for the crust-al heat production could result from a large decrease as-sociated with rifting of an initially high crustal contri-bution in the pre-rifting stage, or from a low or van-ishingly small decrease of an initially low crustal con-tribution, is difficult to establish, because of the largepossible range of values that can be assumed for the ini-tial crustal contribution: from less than 20 to more than80 mW/m2, as one may estimate from a recent analysisof the heat-flow determinations on the U.K. and Frenchborderland (Oxburgh et al., 1980; Vasseur et al., 1980).

Nevertheless, the fact that similar heat-flow valueswere obtained at Sites 548 and 549, and farther southacross the Meriadzek-Trevelyan area, even though vari-ations in crustal thickness are large, tends to indicatethat similar quantities of heat are produced by the crustat the different sites sampled or drilled across the mar-gin, despite considerably different amounts of crustalthinning. This conclusion rests on the assumption that

the heat flowing from the mantle remains constant ornearly constant at all these sites.

Does this result provide clues on the nature of thecrustal thinning processes across the margin (Le Pichonand Sibuet, 1981; Chenet et al., 1982; Brun and Chou-kroune, 1983)? Comparable crustal heat productions sug-gest comparable amounts of stretching of the uppercrustal layer, where radioelements are thought to con-centrate. This means that the amount of stretching ofthe upper crust may not increase as rapidly as the amountof crustal thinning across the margin, which implies non-uniform amounts of crustal thinning with depth duringrifting.

Finally, an interesting result is the surprisingly highheat-flow value of 78 mW/m2 obtained at Site 550. Thissite is on ocean crust formed about 100 m.y. ago (Massonet al., this vol.). Typical heat-flow values for ocean crust ofthis age are 45 to 55 raW/m2. The high heat-flow valuecould result from additional heat brought close to the sur-face by convective processes in the faulted basement ob-served near this site. Further measurements would be re-quired to map the geothermal anomaly and clarify thethermal structure near Site 550.

SUMMARY AND CONCLUSIONS1. Geothermal gradients were successfully determined

at Sites 548, 549, and 550. The geothermal gradientswere converted to heat-flow values, using over 200 ther-mal conductivity measurements on samples from Sites549 and 550. Determined heat-flow values, after correc-tions for topography and sedimentary effects, are 36 ±13 mW/m2 at Site 548, 37 ± 14 mW/m2 at Site 549,and 78 ± 18 mW/m2 at Site 550.

2. Low heat-flow values at Sites 548 and 549, on thetransitional crust of the Goban Spur margin, tend toconfirm that the northern Bay of Biscay margin is ther-mally characterized by heat-flow values somewhat lowerthan on the Hercynian borderland. The crustal contri-bution to the surface heat flow at Sites 548 and 549 is es-timated to be less than 10 mW/m2.

3. The thermal regime of the Goban Spur margin iscompatible with a thermal model of continental marginevolution that assumes that the margin formed by litho-spheric extension about 110 m.y. ago. That no decreasein the heat flow occurs from Site 548 to Site 549, thoughthe crust becomes thinner by probably more than 10km, may indicate similar amounts of extension of theupper crust at Sites 548 and 549, despite considerablydifferent amounts of crustal thinning.

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4. The heat-flow determination of 78 mW/m2 at Site550 is abnormally high for an ocean crust thought tohave formed about 100 m.y. ago. Further measurementsin the area would be required to clarify the geothermalfield. The high heat-flow determination has not been con-firmed by recent surface heat-flow measurements, which,instead, indicate a regional heat flow of 50 to 60 mW/m2 (Sibuet, personal communication).

ACKNOWLEDGMENTS

We are grateful to Claude Jaupart for a helpful review.

REFERENCES

Avedik, E, Camus, A. L., Ginzburg, A., Montadert, L., Roberts, D.G., et al., 1982. A seismic refraction and reflexion study of thecontinent-ocean transition beneath the north Biscay margin. Phil.Trans. R. Soc. London Ser. A, 305:5-25.

Brun, J. P., and Choukroune, P., 1983. Normal faulting, block tilting,and décollement in a stretched crust. Tectonophysics, 2:345-356.

Carvalho, H. da S., Purwoko, S., Thamrin, M., and Vacquier, V.,1980. Terrestrial heat flow in the Tertiary basin of central Sumatra.Tectonophysics, 69:163-188.

Chenet, P. Y., Montadert, L., Gairaud, H., and Roberts, D. E, 1982.Extension ratio measurements on the Galicia, Portugal and north-ern Biscay continental margins. Implications for some evolutionmodels of passive continental margins. Mem. Am. Assoc. Petrol.Geol, 34:703-715.

de Graciansky, P. C , Poag, C. W, et al., in press. The Goban Spurtransect—geological evolution of a sediment-starved passive conti-nental margin. Geol. Soc. Am. Bull.

Erickson, A. J., Avera, WE., and Byrne, R., 1979. Heat flow results,DSDP Leg 48. In Montadert, L., Roberts, D. G., et al., Init. Repts.DSDP, 48: Washington (U.S. Govt. Printing Office), 277-288.

Foucher, J. P., and Sibuet, J. C , 1980. Thermal regime of the north-ern Bay of Biscay continental margin in the vicinity of the DSDPSite 400-402. Phil. Trans. R. Soc. London Ser. A, 294:157-167.

Hyndman, R. D., and Drury, M. J., 1976. The physical properties ofoceanic basement rocks from deep drilling on the Mid-AtlanticRidge. J. Geophys. Res., 81:4042-4052.

Le Pichon, X., and Sibuet, J. C , 1981. Passive margins, a model offormation. J. Geophys. Res., 86:3708-3720.

McKenzie, D., 1978. Some remarks on the development of sedimenta-ry basins. Earth Planet Sci. Lett., 40:25-32.

Montadert, L., Roberts D. G., de Charpal, O., and Guennoc, P., 1979.Rifting and subsidence of the northern continental margin of theBay of Biscay. In Montadert, L., Roberts, D. G., et al., Init. Repts.DSDP, 48: Washington (U.S. Govt. Printing Office), 1025-1060.

Oxburgh, E. R., Richardson, S. W., Wright, S. M., Jones, M. Q. W,Penny, S. R., et al., 1980. Heat flow pattern of the United King-dom. In Strub, A. S., and Ungemach, P. (Eds.), Proc. SecondInternal. Seminar on Results of EC Geothermal Energy Res.: D.Reidel, pp. 447-455.

Roux, J. M., 1982. Etude de conductivimètres pour les hautes et bassestemperatures. Application à la mesure du flux géothermique dansles sediments marins [These]. Université de Bordeaux I.

Vasseur, G., and Groupe FLUXCHAF, 1980. A statistical study ofheat-flow data in France. In Strub, A. S., and Ungemach, P. (Eds.),Proc. Second Internat. Seminar on Results of EC Geothermal En-ergy Res.: D. Reidel, pp. 474-484.

Von Herzen, R. P., and Maxwell, A. E., 1959. The measurement ofthermal conductivity of deep-sea sediments by a needle-probe meth-od. J. Geophys. Res., 64:1557-1563.

Watremez, P., 1980. Flux de chaleur sur le massif armoricain et sur lamarge continentale: essai de modélisation de 1'évolution thermiquede la marge armoricaine [These]. Université de Brest.

Yokota, T., Kinoshita, H., and Uyeda, S., 1980. New DSDP (Deep SeaDrilling Project) downhole temperature probe utilizing ICRAM(memory) elements. Bull. Earthquake Res. Inst., Univ. Tokyo, 54:441-462.

Date of Initial Receipt: August 23, 1983Date of Acceptance: February 29, 1984

APPENDIXSteady-State Measurements of Thermal Conductivity

The principle of the steady-state technique is to compare the ther-mal conductivity of a given sample to the conductivity of a standard.For this purpose, a heating element is placed between the sample andthe standard (Fig. 11 A). Both extremities of the cylinder of section sare maintained at a constant temperature Tu and a constant quantityof electrical power W is supplied in the central heater. When steady-state conditions are reached, the central heater is at a constant temper-ature T, + r(Fig. 11B). Let us designate by

λj the thermal conductivity of the sample 1, byλ2 the thermal conductivity of the sample 2, and byE the thickness of the samples.

If lateral thermal flow is neglected, Fourier's law can be expressedin monodimensional form in sample 1 and in sample 2:

For sample 1: ΔTE

Φ = heat flux through sample 1.

For sample 2: Φ2 = λ2 — ;E

Φ2 = heat flux through sample 2.

The total heat loss, W, across the external sides of the apparatus,is equal to the electrical power:

W = Φ\S + Φis

where 5 is the surface of each external side. The mean conductivity isthen expressed by

λ, + λ,= W/ls • —

AT(1)

If the thermal conductivity λ of one of the samples is known withaccuracy, the measurement concerns λ2, and

λ, = W/s W) - λ, (2)

In the expression of the thermal conductivity (1), the lateral heat flowis neglected. To predict this lateral thermal loss, we use a finite-ele-ment method to compute a correction factor, a, defined by

It is a cylindrical geometry problem. Figure 11C shows the networkused with the boundary conditions. The finite-element program in-volves (1) determination of the heat flux through the boundary, and(2) calculation of the nodal temperature. The computation was per-formed for different values of the A parameter, where A is the ratio be-tween the average thermal conductivity λm and the insulating thermalconductivity λ;. The correction factor is also a function of the thick-ness of the sample (see Fig. 12).

428


52° N P14° W 12 08

44

200 km - : 3

Figure 1. Location map of DSDP geothermal measurements on the northern Bay of Biscay margin. Sites areshown by filled circles. Open circles represent DSDP sites at which no geothermal measurement was ob-tained. Main physiographic features of Western Europe and of the Bay of Biscay after Montadert et al.(1979). Magnetic Arfomaly 34 after Foucher and Sibuet (1980). Key to symbols (bottom of figure): 1 =Hercynian ranges and Paleozoic basins; 2 = continent-ocean boundary (Montadert et al., 1979); 3 =boundaries of inshore basins; blank areas inland represent the Mesozoic and Cenozoic basins; 4 = mainfractured zones and faults; 5 = main Hercynian fold trends.

429


53.5 m sub-bottomBottom seawater: T = 7.6°C

10 20 30 40 50 60 70 80 90 100 110 120Time (min.)

110.5 m sub-bottomBottom seawater: T = 7.6°CSediment: T = 12.9°C

10 20 30 40 50 60 70 80 90 100 110 120Time (min.)

10 20 30 40 50 60 70 80 90 100 110 120Time (min.)

Figure 2. Temperature-versus-time records, Site 548 (Hole 548A).

X 281.5 m sub-bottomBottom seawater: 7~=7.1°CSediment: T= 17.5°C

10 20 30 40 50 60 70 80

Time (min.)

90 100 110 120

Figure 2. (Continued).

198.5 m sub-bottomSediment: T= 10°CWater: 7"=7.2°C?

100 110

236.5 m sub-bottomSediment: 7"= 9.9°CWater: 7"=3.4eC

10 20 30 40 50 60Time (min.)

70 80 90 100

Figure 3. Temperature-versus-time records, Site 549 (Hole 549).

430


20

19

18

17

16

15

14

13

— 12

EnK 1 0

9

8

7

6

5

4

3

2

274.5 m sub-bottomSediment: 7" = <10.4°C?Water: 7~=3.4°C

10 80 90 100


i. 1

20

19

18

1716

15

14

13

— 12O— 11

**" 10

9

8

7

6

5

4

3

2

1

A

\

I I 1 1

-

-

99.5 m sub-bottomSediment: r=6.6°C

Water: r=2.6°C

/

i i i i j i i i_10 20 30 40 50 60 70

Time (min.80 90 100 110 120

18

17

16

15

14

13

12

9

8

7

6

5

4

3

2

-Λ•

7•

B

• V\\\\\

-

156.5 m sub-bottomSediment: 7"=9.7°CWater: 7"=2.6°C

I 1

10 20 30 40 50 60 70 80 90 100 110 120Time (min.)

50 60 70Time (min.)

90 100 110 120


Figure 4. Temperature-versus-time records, Site 550. A-C. Hole 550.D. Hole 55OB.

431


400 u

Temperature (°C)

10

Site 549

Figure 5. Temperature profiles at Sites 548, 549, and 550.

Oo

ε

20 40 60 80

Figure 6. Plot of measured thermal conductivities versus temperature for four sedimentary samples from Hole549. Lithologies: 549-44-4 (590 m sub-bottom), calcareous silty mudstone; 549-59-1 (730 m), calcareoussandy mudstone; 549-82-1 (875 m), sandy limestone; 549-90-2 (930 m), sandy siltstone.

432


100

200

300

400

ε 500

600

700

800

900

1000

10 20 30

Porosity (%)

40 50 60 70 80 90 100

O

o

#

s 5$ 5 x × * x

oo

^ o

X > xx ^ X

o

x * ×* ×

:VQj o

cP

xx

* *

×X X X × y O

Q O X = Porosity

OO O = Thermal conductivity

O

oo o

XX< ° nX OO

o

1 i

5 4 3 2 1

Thermal conductivity (W/m°C)

Figure 7. Plot of porosity and thermal conductivity data versus sub-bottom depth at Site 549.

433


10

100

200

~E 300

400

500

600

700 -

20 30

Porosity (%)

40 50 60 70

O

O

oo

D°o

8

X × oo°

oooo

o

X ×

80 90 100

X X

X = Porosity

O = Thermal conductivity

O

O

X

5 4 3 2 1Thermal conductivity (W/m°C)

Figure 8. Plot of porosity and thermal conductivity data versus sub-bottom depth at Site 550.

434


I Post-rift | Syn-rift {'.'•'•'• j Continental basement E:: : : : 3 Oceanic basement

Schematic cross-section of Goban Spur margin(after de Graciansky et al. [in press])

Predicted heat flow

After profile CM 11

I 0=78mW/m^??200 m approx.

I 1000 m approx.

Plio. Quat.—

EocenePaleocene

__ Maestrichtian

^ 5 0E40

30

. Predicted heat flow

: / ~

Profile CM15

1 km

L

K= 1.8W/m°C-v

K = 2.5 w/m°cxfT:::::

1km tΦ =

\1 —

5 4 g K= 1.8 W/m°C

r*zdCl K = 2.5\

:::K- 3.75 w/m°c:::::

t37.5 mW/m2

, '_ — •

III 5

1—'

"g 40

I 30

Profile OC 202 548 \K= 1.8 W/m°C

NE

1 km

= 2.5W/m"Ci

m°c:: : : : : : : q

-1 kmF T

»= 36 mW/m'

Heat-flow variations at depth across the Goban Spur margin

Figure 9. Two-dimensional finite-element numerical modeling of the lateral conduction effects of topography changes and thermal conductivitycontrasts in the vicinity of Sites 548, 549, and 550. A constant heat flow, defined as the corrected heat flow, Φ, is assumed at the base of eachmodel, taken at a depth of 6 km below the sea surface.

50°N

47

Figure 10. Compilation of available heat-flow data, including DSDP results, for the northern Bay of Biscay margin. From Erickson et al. (1979),Foucher and Sibuet (1980), and Watremez (1980). DSDP results are represented by black squares. Heat-flow values in mW/m2. Bathymetry inm × 10"3 .

435


Thermal insulation

Heat-flow evacuation

B

T. + AT

r,+ AT

11111111111tt 1111tt 11Φi

1111111111 n 111111111

t u t t i t t t n i m t u i u iΦ 2 λ 2

Sample

Heatingelement

Sample

ilδr

= 0 >T+ AT

m t u t t i H t u t i t n t i

Figure 11. A. Sketch of the thermal conductivity measurement apparatus. B. Thermal state of the appa-ratus in steady-state conditions. C. Finite-element modeling of the apparatus with boundary condi-tions (T is time).

4 6 8 101214161820 Sample thickness (mm)

Figure 12. Plot of the correction factor, a, versus the relative thermalconductivity, A, for different thicknesses of the sample, as given bythe finite-element modeling.

436

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