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10. DOWNHOLE LOGGING AT DEEP SEA DRILLING PROJECT SITES 501, 504, AND 505, NEARTHE COSTA RICA RIFT1
J. R. Cann, Department of Geology, The University, Newcastle upon Tyne, NE1 7RU, United Kingdomand
R. P. Von Herzen, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
ABSTRACT
Successful logs were taken in four holes in the area of the Costa Rica Rift during DSDP Legs 69 and 70. The logsclearly show the chert layers in limestone at the base of the sedimentary section of Holes 501 and 504A and the alterna-tion of the massive flow units and pillow basalts in the basement section of Holes 501 and 504B. Nearly continuousneutron and resistivity logs of the 562 meters of basement section of Hole 504B show that the porosity of the basaltdecreases gradually from about 13% near the sediment/basalt interface to about 6% 250 meters into the basement.Below that depth porosity is approximately constant. A comparison of the resistivity and neutron logs suggests thatpores in regions of high porosity near the top of the basement are more connected than in similar regions deep in thehole. The caliper log for Hole 504B shows that the size of the hole is generally close to the size of the bit but has varia-tions that are not clearly related to either porosity or permeability. The compensated density log, although it gives agood indication of the variation of porosity, is not a reliable indicator of the values of formation porosity in highporosity zones. The sonic log is noisy on a fine scale but gives an idea of large-scale structure that correlates well withthe values of porosity derived from resistivity.
INTRODUCTION
During Legs 68 (501), 69, and 70 of the Deep Sea Drill-ing Project, logs were obtained in four holes drilled intwo contrasting areas of the Costa Rica Rift to investi-gate both open and sealed circulation of heated oceanwater within the ocean crust (Fig. 1).
In the area where circulation is believed to be open tothe ocean, where heat flow is low, drilling the brittle,fractured, little altered basalt proved to be very difficult.It was only with great effort that in one hole (Hole 5O5B)we penetrated 40 meters into basement. We managed tolog most of that basement section. We also logged a sec-tion of the overlying sediment, but the cold, unalteredsediment had washed out beyond caliper range duringdrilling, and the logs were not very satisfactory.
Drilling was much better in the high-heat-flow area,where hydrothermal circulation was believed to be sealedbeneath a continuous blanket of sediments. Hole 501,the pilot hole for re-entry, penetrated 70 meters intobasement beneath 250 meters of sediment that had beendiagenetically altered to chert and limestone over itslowest 30 meters. Here the lower 120 meters of sedimentand most of the basement was logged. We made a firstattempt at re-entry only a few hundred meters from thissite, at Hole 504A. The attempt failed when the casingbit broke up only a few meters into basement. We madeone logging run, primarily to record temperature butalso to acquire a gamma density record of the chert lay-ers. The eventually successful re-entry hole, Hole 504B,was cased to basement and then drilled, first to 212 me-ters of basement penetration on Leg 69 and eventually
Cann, J. R., Langseth, M. G., Honnorez, J., Von Herzen, R. P., White, S. M., et al.,Init. Repts. DSDP, 69: Washington (U.S. Govt. Printing Office).
to 562 meters of basement penetration on Leg 70. Suitesof logs were run in the basement section at the end ofthe drilling on both legs, with different rates of successon each leg. These logs provide the principal interest ofthe Costa Rica Rift drilling, if only because of theirlength, which allows a clear view of the extent of thefluctuations of the physical properties of the basement.
The logging tools used for the standard logging runswere part of a logging package that was provided un-der contract by Gearhart-Owen, Incorporated. Gearhart-Owen logging engineers operated the tools: Larry Ax-line sailed on Legs 68 and 69 and David Collart on Leg70. The technical specifications for the logging tools aregiven in Table 1. The logging results were interpreted byusing the appropriate charts in the Gearhart-Owen man-ual, supplemented where necessary by charts in theSchlumberger logging manual. The results of all logs ex-cept the temperature log are discussed here. The temper-ature logs are discussed by Becker et al. (this volume) inconjunction with the other temperature measurements.Two special tools that were not part of the Gearhart-Owen package were also run during the logging pro-gram. They are both discussed in other chapters of thisvolume, the downhole magnetometer by Ponomarev andNekhoroshkov and the borehole televiewer by Andersonand Zoback.
Different suites of logs were run during five loggingperiods with varying degrees of success (Table 2). Thesonic tool did not work at all effectively on Legs 68 and69; it worked better, though still noisily, on Leg 70.Conversely, the gamma density tool worked well on theearlier two legs but failed during the logging of the deepHole 504B on Leg 70. Despite occasionally good opera-tion during Legs 68 and 69, the natural gamma tool wasineffective because of the scarcity of radioactive ele-ments in the section, and it gave particularly noisy read-
281
J. R. CANN, R. P. VON HERZEN
86"W 84°W 82"W 80°W 78"W
8°N
2°S
4°S
6°S
86"W 84°W 82°W 80°W 78°W
Figure 1. Location of Sites 501, 504, and 505 near the Costa Rica Rift. Sites 501 and 504 are in the high-heat-flow area, and Site 505 is in the low-heat-flow area (see text). After Lonsdale and Klitgord (1978). Contour interval 250 m.
ings during Leg 70, when it may not have been transmit-ting any meaningful signal. It was equally ineffective incombination with the neutron tool because of the acti-vated radioactivity of the formation resulting from neu-tron bombardment. Both the neutron and the resistivitylogs duplicated themselves nearly exactly from run to
successfully in Holes 501, 505 B, and both basement sec-tions of Hole 504B.
RESULTSThe results from the different logs run in each hole
are shown in Figures 2 to 6. Where the same log was runrun, however, and they could be interpreted successfully more than once, the record shown is the best obtained,in relation to one another. Both types of logs were run Some logging runs have been omitted, such as the sonic
282
DOWNHOLE LOGGING
Table 1. Characteristics of the logging tools used.
Epithermal neutron tool—Diameter, in 3 ViRadioactive element:
Source 241AmBeRadioactivity, curies 3
Tool spacing, in 173He counter Proportional
Sonic velocity tool—Diameter, in VAIn-hole position during operation CentralizedSpacing: transmitter-receiver-receiver-transmitter, ft 3-2-3
Focussed resistivity tool—Make Laterolog 3In-hole position during operation Free
Compensated gamma density tool—Radioactive element:
Source CesiumRadioactivity, curies 2
In-hole position during operation EccentralizedNatural gamma tool—
Diameter of scintillometer, in 3'/2Length, in 6Time constant, sec 2
Caliper tool measurement capacity when fully open, in 22Bit diameter, in 9%
log for Hole 501, because the resulting data are clearlyunusable. In the case of that log, for example, valueswere frequently reached that either fell below the veloc-ity of sound in water or exceeded the velocity of soundin peridotite. Other records have been included despitetheir poor quality in case something can be extractedfrom them.
Most of the logs showed great reproducibility fromrun to run in the same hole. This was particularly true ofthe neutron, resistivity, and caliper logs. The excellenceof their agreement can be seen by comparing Figures 4and 5A, which cover the same section of Hole 504B butwere run on different legs. The compensated gammadensity results were broadly comparable from run torun but did not agree so closely, probably because thetool is run eccentralized and is incapable of penetratingvery far into the formation. The natural gamma log re-sults showed only fair reproducibility, partly as a conse-quence of the noisy signal on Leg 70 but also because ofvariable time constant and scale problems in other holes.The details of the best sonic log failed to match those ofa second run of the same section, but when the valueswere averaged over intervals of 10 meters or more,meaningful data began to appear.
Qualitatively, the logs show some features particular-ly well. Near the base of the sedimentary section inHoles 501 and 504A (Figs. 2 and 3), the appearance oflayers of chert is clearly visible. The borehole televiewerimages show these layers to be sharply defined, horizon-tal, and highly reflective and to be separated by layers oflower reflectivity that are probably the softer limestones(Anderson and Zoback, this volume). From the com-pensated gamma density, neutron, and resistivity logs inFigure 2, the upper boundary of the chert-bearing unitat 3699 meters is in sharp contrast to the softer, uniformsiliceous chalk above. The complexity of the chert-bear-ing unit shows up clearly in the spiky records that resultfrom the 10-cm-scale alternation of chert and limestone.
Table 2. Logging operations in the Costa Rica Rift.
Leg
68
69
69
70
69
Hole
501
504A
5O4Ba
504B
5O5B
Date
12 to 13 July
13 July
13 July14 July14 July
15 July
26 October
22 October
22 October
23 October23 October24 October24 October
13 December
14 December
15 to 16 December
16 December
16 December
5 October
5 to 6 October6 October
7 October
Run
Sonic velocityNatural gammaCaliperCompensated gamma densityNatural gammaCaliperBorehole televiewerDownhole magnetometerTemperatureWater samplerFocussed resistivityNeutron porosityNatural gamma
Compensated gamma densityNatural gammaCaliperTemperature
Compensated gamma densityNatural gammaCaliperTemperatureFocussed resistivityNeutron porosityNatural gammaDownhole magnetometerBorehole televiewerDownhole magnetometerTemperatureSelf potentialWater sampler
Sonic velocityNatural gammaCaliperTemperatureWater samplerCompensated gamma densityNatural gammaCaliperTemperatureFocussed resistivityNeutron porosityNatural gammaTemperatureWater samplerFocussed resistivityNeutron porosityNatural gamma
Compensated gamma densityCaliperNatural gammaTemperatureDownhole magnetometerFocussed resistivityNeutron porosityNatural gammaTemperatureWater sampler
RecordQuality
UnusableExcellentExcellentGoodGoodGoodGoodGoodExcellentPoorExcellentExcellentUnusable
GoodGoodFailedExcellent
GoodGoodGoodExcellentExcellentExcellentUnusableFailedGoodGoodExcellentFailedExcellent
ModeratePoorExcellentExcellentExcellentFailedPoorGoodExcellentFailedPoorPoorExcellentExcellentExcellentExcellentUnusable
GoodGoodGoodGoodGoodGoodGoodPoorPoorPoor
All of the runs in Hole 504B during Leg 69 logged only the upper 200 m of the562 m finally drilled in the basement section.
The same structure can be seen in the compensated gam-ma density log of Hole 504A (Fig. 3). The logs also giverecords for the chalk section above the cherts, but be-cause of the large hole size their accuracy is open toquestion. However, the values of compensated gammadensity are closely comparable to measurements madeon samples taken from the hydraulic piston core ofnearby Hole 504 (Wilkens and Langseth, this volume).
Immediately above the basalt in Holes 501, 504A,and possibly 504B (Figs. 2, 3, and 4) there is a high-po-rosity unit with a high natural gamma count. The largepeak in the downhole magnetometer record at this levelin Hole 501 (Fig. 2 and Ponomarev and Nekhoroshkov,
283
J. R. CANN, R. P. VON HERZEN
3650
αQ
3700
3750
Basalt'contact"
- E
1A V P
1B V P
2ACP
2B C P
3P P
5 PP
1.0
100
2.0
CompensatedDensity
(Mg m-3)
50 15 0
Porosity (%)
3.0 0 500 1000 1500 600 300 0
Neutron FocussedPorosity Resistivity
(API units) (milli-ohms/m)
LithologicLog
Jl
3650
3700
3750
0 20 40 9000 12,000 15.000 25 50
Natural Downhole Temper-Gamma Magnetometer ature
(API units) (vertical component) (°C)(nT)
Figure 2. Logs run in Hole 501. Depths are meters below rig floor. The seafloor was at 3467 m, and the top of the basalt section was at 3731 m.Dotted lines are for reference. The caliper was fully extended at all depths shallower than 3693 m. The electrical conductivity log was de-rived by computation from the focussed resistivity measurements. Cross-hatched block in lithologic log shows massive flow unit. Inlithologic log, first letter stands for basalt type based on phenocryst assemblage, as follows: C, chromite bearing; P, clinopyroxene bear-ing; V, variably bearing. Second letter refers to basalt form, as follows: P, pillow basalts; F, flows.
284
DOWNHOLE LOGGING
3600 r
3650
Q.
Q
3700
Basalt
3600
3650
3700
1.0
100
1.5 2.0 2.5Compensated
Density(Mg m~3)
50 15Porosity (%)
3.0
0
LithologicLog
0 20 40NaturalGamma
(API units)
0 25 50 75Temperature
(°C)
Figure 3. Logs run in Hole 504A. Depths are meters below rig floor. The seafloor wasat 3468 m, and the top of the basalt section was at 3732 m. Dotted lines are forreference.
this volume) is probably associated not with anomalousmagnetization at this horizon, however, but with themagnetic edge effect between nonmagnetic sediment andmagnetic basalt. The nature of this horizon is not cer-tain, since no material was recovered from it; the imme-diately overlying hard chert required drilling with fullcirculation. It is probably a hydrothermal sedimentarylayer that is forming just above the basalt near theridgecrest.
Within the basement section, the most striking fea-tures are the massive flow units, which are several me-ters thick. The rate of recovery for these intervals washigh, and the core was sometimes recovered as continu-ous sticks. The uniformly coarse-grained nature of the
basalt in these intervals leaves no doubt that they aresingle outpourings or injections of basalt, although it isnot clear whether they are truly massive flow units orsills. Within the massive flow units, all the porosity-related logs show low, uniform porosity. The boreholeteleviewer records show the uppermost massive unit inHole 501 to be extremely uniform, with sparse randomcracking (Anderson and Zoback, this volume). The logsare particularly useful in defining the upper and lowerboundaries of the units, which are not always evidentfrom the cores recovered.
Between the massive units the porosity-related logsshow an oscillation between high and low porosity witha wavelength of 1 to 3 meters. It never settles on a con-
285
J. R. CANN, R. P. VON HERZEN
3750
3800
3850
3900
3950
1 VP
2ACP
2BCP
2CCP
2D CF
3A VP
3B V P
3CVP
4 C P
5 V P
8 P P
9 VP
16PP
17 V P
18C D19 PP
1.5 2.0 2.5 3.0 500 1000 1500 1 10 100 0 10 20
Compensated Neutron Focussed Lithologic CaliperDensity Porosity Resistivity Log (in.)
(Mg m"3) (API units) (milli-ohms/m)
-3750
-3800
- 3850
-3900
50 15 0
Porosity (%)
0 20 40 12,000 15,000 18,000 0 25 50 75
Natural Downhole TemperatureGamma Magnetometer (°C)
(API units) (vertical component)(nT)
3950
Figure 4. Logs run in Hole 504B on Leg 69, covering the upper part of the basement section. Depths are meters below rig floor. Theseafloor was at 3474 m, and the top of the basalt section was at 3748 m. The hole was cased from the mudline to 7 m into the ba-salt. Dotted lines are for reference. Units in lithologic log are identified as in Figure 2, with D standing for depth.
286
3750
3800
3850
3900
3950
BasaltcontactCasing shoe
200 100Sonic
Velocity(µsec/ft)
0 500 1000 1500 500
NeutronPorosity
(API units)
250FocussedResistivity
(milli-ohms/m)
1 V P
2ACP
2BCP
2CCP
3A V P
3B VP
3C V P
4 C P
5 V P
8 P P
9 VP
16PP
17 VP
18 C D.19 PP
LithologicLog
DOWNHOLE LOGGING
3750
3800
3850
3900
10
Caliper
395020 0 20 40
NaturalGamma
(API units)
Figure 5. Logs run in Hole 504B on Leg 70, covering the whole of the basement section. Depths are as for Figure 3. The electrical con-ductivity log was derived by computation from the focussed resistivity measurements. Dotted lines are for reference. Units in litho-logic log are identified as in Figure 2. (A) 3750-3950 m, (B) 3950-4150 m, (C) 4150-4300 m.
287
J. R. CANN, R. P. VON HERZEN
B3950
4000
4050
4100
4150.200 100
SonicVelocity(nsec/ft)
19P P
20 V P
norecovery
21 P
22 P
23B C P
24 P F
25 CP
28 V P
29B V P
30A V P
30C V P
33A PP
33B P P
/ 3 4 P F/
0 500 1000 1500 500 250 0 0
Neutron Focussed LithologicPorosity Resistivity Log
(API units) (milli-ohms/m)
Figure 5. (Continued).
3950
4000
4050
4100
10Caliper
(in.)
20 0 20 40
NaturalGamma
(API units)
4150
288
4150 r
4200
4250
4300200 100
SonicVelocity(µsec/ft)
0 500 1000 1500 500 250
Neutron Focussed
Porosity Resistivity
(API units) (milli-ohms/m)
36 P P
37 V F,////
38 P D
40 VP
45 PP
47 V P
48 V F
DOWNHOLE LOGGING
η4150
4200
4250
Lithologic
Log
10
Caliper
(in.)
20 0 20 40
Natural
Gamma
(API units)
4300
Figure 5. (Continued).
stant porosity or distinguishes units on a larger scale.Recovered basalt and borehole televiewer images of theupper part of the hole (Anderson and Zoback, this vol-ume) show that these intervals are mostly pillow basalt,with some thin flows. Sections of logs can sometimes bevery precisely matched with one another, with individ-ual highs and lows clearly correlated. In such intervalscorresponding points can be readily identified. In otherintervals the correlation is not so good, and the reason isnot always clear. Ship's heave or the tools' snagging onthe walls of the hole can degrade a record that wouldotherwise match. On the other hand, the two logs recorddifferent variables, and they might give different valueseven under ideal conditions. For convenience and con-
sistency, the first explanation was assumed to be cor-rect, although it was fully understood that both factorswere likely to have operated.
It is not easy to identify highly fractured intervals ofthe section from the porosity-related logs. The intensityof fracturing, although very important to the permeabil-ity of the formation, is not necessarily directly related toporosity. The intensity of fracturing might be easier toinfer from the caliper log, because the sides of a holeoften break away along closely spaced fractures. Figure7 is a condensed caliper log for Hole 504B, which wassampled at 1-meter intervals. Some parts of the hole are2 in. or so larger than bit size, and the enlargements areoften accompanied by irregularities in the walls. One
289
J. R. CANN, R. P. VON HERZEN
3620 r
"• 3650 -
£
OCDU
^CO
uα>
h
"cöCDCO
03N
ootno
occCDc
I0
o
c
—'ö.
1.0 1.5 2.0 2.5 3.0Compensated
Density(Mg m-3)
r—i—i i i i i—| | i | |50 15 0Porosity (%)
500 1000 1500 1 10 100Neutron Focussed
3620
3650
Porosity(API units)
Resistivity(ohm m)
LithologicLog
12,000 15,000 18,000 0 25
Downhole Temper-Magnetometer ature
(vertical component) (°C)(nT)
Figure 6. Logs run in Hole 5O5B on Leg 69, covering the basalt sediments and the short basement section. Depths arein meters below rig floor. The seafloor was at 3517 m, and the top of the basalt section was at 3653 m.
would expect such intervals to be zones of high permea-bility, but this does not seem to be the case. The thermalhistory of the hole (Becker et al., this volume) suggestsvery strongly that there is a zone of high permeability atsub-bottom depths of 90 to 100 meters (about 3850 mbelow the rig floor) through which water is flowing intothe surrounding rock. There is no particular anomaly inthe caliper log at this depth. Conversely, zones that areparticularly anomalous in the caliper log, such as thoseat 3800 and 3975 meters below the rig floor (50 and 225m into the basement) fail to show any indication ofoutflowing water in the thermal log.
One feature of the caliper log that is worthy of note isthe apparent periodicity of the variations in hole diam-eter with increasing depth. The periodicity is apparentlyunrelated to the porosity logs (see material following) orto the presence of massive flow units in the section(shaded bars in Fig. 7). Between five and six cycles inhole diameter occur between 3800 and 4200 meters,which gives a wavelength of 70 to 80 meters. Such pe-riodicity is similar to that reported by Francis (1982),although it is of somewhat shorter wavelength. Francisattributes the periodicity to the oscillation of the drillstring in the hole, which in turn is related to the changesin the dynamics of the drill string as the length of thestring increases. He believes that the hole is widened bybeing reamed out by the upper edge of the bottom holeassembly. The operation of such a factor is very possiblehere, especially if the caliper of the lower part of thehole is compared with the profile given by Francis(1982, fig. 5) of the lower part of drill string at an ap-propriate horizontal exaggeration.
POROSITY-RELATED LOGS IN HOLE 504B
We analyzed the porosity-related logs of Hole 504Bfurther to improve our understanding of the porosity ofthe section. Corresponding pairs of points were readfrom the Leg 70 neutron and resistivity logs and fromthe Leg 69 compensated gamma density log, althoughthe compensated gamma density log covered only thetop 200 meters of the basement section. To identify cor-responding points, parts of the curves that were similarin shape were chosen where possible. An overall shift of8 meters in the Leg 70 logs relative to the Leg 69 logsproved to be necessary to match the massive units thatappeared in both legs. This adjustment is incorporatedthroughout the analysis discussed in this chapter.
The neutron density logs give values on the API scalethat were converted to porosity by calibration curves.No curves were available for the precise combination oftool spacing and hole diameter used here, so a curve wasinterpolated between those given for different combina-tions of spacing and hole diameter by Schlumberger(1974) and Gearhart-Owen (1976). Individual values ofporosity are plotted as <f>N in Figure 8A. The effects ofvariations in temperature and hole diameter were as-sumed to be minor in comparison to the inaccuraciesotherwise present in the analysis. This can be confirmedfor low porosities by comparing the neutron porositiesfor the massive flow units at different depths in thehole. For all of the massive flow units, calculated po-rosity is 3 to 6%, with little variation with either depth(and hence temperature) or hole diameter (see Fig. 7 anddiscussion thereof). The massive units are physically
290
3750
3800
3900
5 4000
IQ
4100
4200
4300 L-
10 11
Hole Diameter (in.)
12 13T
size
14
—r
DOWNHOLE LOGGING
15 16
Figure 7. Caliper log for the Hole 504B basement section (Leg 70). Points are sampled from the log at 1-m intervals and plottedin groups of five at 5-m intervals. Bars at left show massive flow units.
291
J. R. CANN, R. P. VON HERZEN
Ao
3750
3800
0.1 0.2 0.3Bo 0.1 0.2 0.3
3900
Jli-
DC
O
Q.
4000
4100
4200
4300 L * * . *
Figure 8. Porosity values calculated from (A) neutron log and (B) focussed resistivity log for Hole 504B basement section.Points are sampled from correlated peaks and troughs, and thus correspond from one leg to the other. Porosity is expressedas a fraction. Bars show massive flow units.
292
DOWNHOLE LOGGING
very uniform, so such uniformity would be expected.For higher porosities, a comparison of the differentcurves suggests that the effect of variations in hole di-ameter should be much less marked than the effects ofporosity variation, and this was assumed to be the casehere.
In relating resistivity to porosity the empirically basedArchie's Law was used:
where Φ is porosity, q is an empirical exponent that de-pends on the connectedness of the cracks, Rw is theresistivity of the seawater that fills the pores, and Rf isthe observed resistivity of the formation (Archie, 1942).Since the resistivity of seawater depends critically ontemperature, a temperature correction must be appliedin entering Rw. This correction was determined by usinga temperature log that was run on the same day as theresistivity log (16 Dec). Analyses of water samples(Mottl et al., this volume) indicated that the water in thehole and in the formation had the same salinity asseawater, so it was possible to compile curves showingthe change in Rw with increasing depth in the hole byusing the resistivity-temperature-salinity curves in theGearhart-Owen data handbook (1976). See also Schlum-berger (1979, chart Gen. 9).
The next step in finding porosity requires some infor-mation about q, which must be assigned a constantvalue (even though the connectedness of cracks is likelyto change throughout the section) to make it possibleto derive semi-independent information about porosityfrom resistivity. Often assumed to be 2, q was derivedfor this study as follows. Archie's equation was solvedfor q for each point by using neutron porosity as a mea-sure of Φ, the observed value of Rf, and the value of Rw
appropriate for the temperature at the appropriatedepth. The median value of all values of q calculated inthis way was 2.1, with a spread from the twentieth per-centile to the eightieth percentile of 1.9 to 2.4. This val-ue was then used throughout the hole to solve Archie'sequation with the observed values of Rfa.ná appropriatevalues of Rw as described previously to give a semi-in-dependent set of porosity values. The values of porositycalculated from resistivity in this way are shown as ΦR inFigure 8B.
The effect of changing the estimate of q can be seenfrom Figure 9, a plot of ΦR against ΦN t n a t shows thecurves that result when three values of q are used tocalculate the equation
ΦR221 ΦN* = RJRf
There is considerable scatter about the ideal line (thatfor which q = 2.1), but in most cases the scatter corre-sponds to only a few percent in porosity.
The value of 2.1 used as the overall Archie's Law ex-ponent is rather greater than that calculated from mini-core samples by Karato (this volume). Using direct mea-surements of porosity and resistivity, Karato found a
value for q of 1.67±0.27, which suggests that the mini-cores display a greater degree of pore connectednessthan the rock in the hole overall. Since a dispropor-tionately large number of the minicores are from mas-sive flows and pillow interiors (a fact underscored by theminicores' lower overall values of porosity), the greaterdegree of pore connectedness would be expected.
The value of q is also rather larger than that calculat-ed by Kirkpatrick (1979), who used resistivity and gam-ma density logs from Hole 396B.
In both Figures 8A and 8B, porosity decreases withincreasing depth, although there is some scatter, espe-cially in ΦN The trend is better brought out in Figure 10,where 10-meter median values of porosity are plottedrather than individual points. The two curves in Figure10 are remarkably similar in shape. The smaller degreeof variation in ΦR is probably related to the resistivitytool's ability to penetrate the formation deeper than theneutron tool. Both curves show a general decrease inporosity from about 13% near the top of the basement(3748 m below the rig floor) to about 6% 250 meters in-to the basement (3998 m below the rig floor) and aroughly constant value below that depth. The long-spacing resistivity experiment (Von Herzen et al., thisvolume) resulted in a very similar porosity variation.The massive flow units stand out as having a porosity ofabout 5%, regardless of depth in the section. The upperunit, which is near a below-rig depth of 3800 meters, is,for this reason, the most striking, but the others standout clearly as well.
Figure 11 further examines the Archie's Law expo-nent q, which is calculated from neutron porosity asexplained previously. Although the general level of qtaken for all points remains more or less constant withincreasing depth, this is not the case for points for whichΦR or ΦN calculates as being greater than \O°7o. For theseintervals of higher porosity, which exclude the massiveflow units, q gradually increases with depth from about2 at the top of the basement nearly to 3 toward the bot-tom of the hole. This change suggests that the degree ofconnectedness of the pores in the high porosity zoneschanges with depth, since q is related to connectedness,and neutron porosity, from which q is calculated, is not.The direction of the change would be from more con-nected pores at the top of the basement, with pores inthe high porosity zones forming an interconnected net-work of cracks, to less connected pores deeper in thecrust, where pores in the high porosity zones would berelatively isolated from one another.
This kind of change is consistent with the increasingdegree of alteration with increasing depth in the hole,which itself is probably related to the increasing in situtemperatures (Honnorez et al., this volume). Alterationoften results in the filling of the pores with minerals andthe eventual isolation of the pores from one another,with a consequent change in the connectedness of thepores.
Figure 9, where ΦR is plotted against ΦN* amplifies theevidence in Figure 11. The line of equal porosities in thisplot corresponds to a value of q of 2.1, and the data forwhich ΦN is greater than ΦR correspond to values of q
293
J. R. CANN, R. P. VON HERZEN
0.40 r
0.35 -
0.35 0.40
Figure 9. Porosity calculated from neutron log plotted against porosity calculated from resistivity log. Lines show valuesof Archie's Law exponent q. Hole 504B basement section.
greater than 2.1. Although the plot shows broad concor-dance between the values, as would be suggested byFigures 8 and 10, there are marked deviations from this.At low porosities, for example, ΦR is greater than ΦN o n
the average, suggesting that the pores are more con-nected in zones with a neutron porosity of less than 7%,which are more common towards the bottom of thehole. It is in the region of such low porosities, however,that the neutron log is most dependent on such factorsas hole diameter, which have not been accounted forhere. The prevalence of values of q greater than 2.1 athigh porosities shows clearly, as in Figure 11.
Comparison of the neutron and resistivity logs withthe compensated gamma density log was hampered bythe lack of a compensated density log for the bottompart of the hole. However, porosity was calculated fromthe gamma density data available, and the resultingvalues, ΦD> a r e plotted against neutron porosity in Fig-ure 12. Although there is good agreement in the valuesof porosity up to 15%, at higher porosities there is pro-
gressive deviation towards proportionately higher val-ues of ΦD The values of ΦD reach levels greater than 50%if grain density is assumed to be 2.95 Mg m~3. The ex-planation for the deviation may be that the more porouszones have lower grain density, but the most likely rea-son for lower grain density would be hydration, whichwould give similarly high neutron porosities. Apparent-ly the compensated density tool cannot be relied on togive realistic values of porosity, at least not in a basalticformation, although it does identify the position ofzones of high and low porosity in the hole.
The values of porosity derived here are significantlylower than those derived in other ways by Kirkpatrick(1979) and Salisbury et al. (1979). The reason for thedifference is not clear. It may be related to the methodsused to derive porosity or to the geological setting of theother sites. Sites 395 and 396 (Kirkpatrick, 1979) are insediment ponds on rifted crust that is much more intense-ly faulted than the crust at Site 504. Site 417 (Salisburyet al., 1979) is in old Atlantic crust that is relatively rift-
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Figure 10. Porosity values for Hole 504B basement section calculated from (A) neutron log and (B)focussed resistivity. Values are 10-m median values derived from the points in Figure 8. Porosityis expressed as a fraction. Bars show massive flow units.
ed and has also had a long history of low temperaturealteration. The significance of these differences will haveto be evaluated after logging runs are made elsewhere.
At first sight, the sonic velocity log is very noisy andfails to provide much information. Some of the transittimes measured correspond to a velocity lower than thatin seawater or higher than that in peridotite (Fig. 5). Aremeasurement of the bottom 50 meters of the hole pro-
duced values that showed little similarity, on a 1-meterscale, to the values acquired when the entire section wasmeasured. Both sets of values contain sudden transi-tions in transit time that suggest skipping. However, av-eraging the transit times over several meters smooths therecord markedly, and a signal appears to emerge. Figure13 is a plot in which 10-meter mean transit times havebeen converted to velocity, Vp. The figure also shows
295
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Figure 11. Archie's Law exponent for Hole 504B basement section calculated from neutronporosity and observed resistivity plotted against depth for the points shown in Figure 8.Circles surround points for which either the neutron porosity or the resistivity-basedporosity or both are greater than 10%. Bars show massive flow units.
100-meter means. The plot shows a fairly steady in-crease in sonic velocity for the top 300 meters or so ofthe basement, with a rate of increase of about 5 (km/s)/km over that interval. Over the lower 300 meters of thebasement, Vp becomes much more uniform at about 4.6km/s. Local high velocities correspond to massive flowunits, which are also apparent in the resistivity and
neutron plots in Figure 10. Indeed, there is a very closeinverse relationship between sonic velocity and porosity.Figure 14 shows Vp plotted against 10-meter means ofporosity calculated from resistivity, and the near lineari-ty of the inverse relationship between these independentmeasurements is strong support for the idea that thesmoothed velocity log is giving a clear signal. The mini-
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Figure 12. Porosity calculated from neutron log plotted against porosity calculated from compensated gamma den-sity log. Assumed grain density: 2.95 Mg m~3. Hole 504B basement section.
core samples examined by Karato (this volume) pro-duced lower values of sonic velocity for a given level ofporosity, possibly because the minicore measurementsof sonic velocity are less affected by cracks. Narrowcracks have little effect on porosity but markedly de-crease sonic velocity. Another factor that may have con-tributed to the discrepancy is that the scale of measure-ment is different for the logging tool, which samplesmuch larger volumes of material. This analysis thus in-dicates that despite small scale random variations, someof which must represent noise, the sonic log contains in-formation at a larger scale that can be used to provideconstraints on pore geometry and that can provide away to relate marine seismic measurements (Stephen,this volume; and Langseth et al., this volume) to therocks recovered.
CONCLUSIONS
The logs obtained at Sites 501, 504, and 505 provedto contain important information about the structure ofthe sections drilled. In the sediments of the high heatflow area (Holes 501 and 504A), the appearance in thelogs of layers of chert separated by bands of limestone isvery clear. An oscillation in the porosity-related logs ona scale of 1 to 3 meters in the basement at Holes 501,504B, and 5O5B is related to the presence of pillow ba-salts or perhaps thin flows. Thick, massive flow unitsappear as sharply bounded zones of uniformly low po-rosity; they are scattered through the sections at ran-dom.
Over the long section of the deep hole at Hole 504B,neutron porosity was evaluated from standard calibra-tion curves. Values of the Archie's Law exponent werecalculated by using measured formation resistivity, sea-water resistivity at the temperatures determined by log-ging, and values of neutron-derived porosity. The me-dian value of the exponent calculated in this way is 2.1,and resistivity porosities were calculated from observedresistivities using this exponent. Both measures of po-
rosity show a decline from about 13% at the top of thebasement to about 6% 250 meters into the basement.Below that depth porosity is essentially constant at 6%.
Although when all points are taken together the Ar-chie's Law exponent is consistent through the depth ofthe hole, when only points corresponding to a porositygreater than 10% are considered, there is a systematicincrease in the exponent from 2 to 3 from the top to thebottom of the basement section. This seems to representa greater degree of pore connectedness at the top of thebasement than at the bottom of the hole; thus, the po-rosity is more cracklike at the top and more porelike atthe bottom. This in turn is probably related to the in-creased hydrothermal alteration caused by the highertemperatures in the deeper part of the section, with theconsequent sealing of cracks by precipitating minerals.
The sonic log in Hole 504B was noisy on a small scalebut appeared to give meaningful results when values av-eraged over intervals of 10 meters or more were judgedagainst the independently measured resistivity log. Thevariations observed in the smoothed record of sonic ve-locity are related to the contrast between massive flowsand pillow units. The log also shows a strong overall in-crease down the hole, with a gradient of about 5 (km/sec)/km over the top 300 meters. In the lower part of thesection this gradient declines substantially, and a meansonic velocity of about 4.6 km/sec is apparent. Com-pensated gamma density results were obtained for theupper part of the hole but gave unrealistically high val-ues for the high porosity units, perhaps as a result of thesmall depth of penetration of the gamma radiation. Thecaliper log functioned well, but variation in hole sizeseemed to be related to neither the porosity nor the per-meability of the basement. The message carried by thecaliper log is probably significant but is not yet clear.
Despite the difficulties encountered, the operation asa whole was illuminating. The logging results, in partic-ular those of the deep basement site, substantially sup-plement the logging results previously derived and give a
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Figure 13. Sonic velocity plotted against depth below rig floor for the basement section of Hole 504B. Solidline gives velocity for 10-m intervals calculated from mean transit times. Asterisks show 100-m meanscalculated similarly. Bars show massive flow units.
much clearer picture of the nature of the ocean base-ment than any achieved before.
ACKNOWLEDGMENTSWe must fully acknowledge here the assistance and instruction of
the many people who have helped us to understand some of the rudi-ments of log interpretation and who have discussed the meaning ofthese results at great length. In particular we thank Larry Axline,
Mark Langseth, Roy Wilkens, Matt Salisbury, Gene Boyce, and RoyHyndman, who helped in their own ways to set a particular characteron this paper. We are also greatly indebted to Christine Cochrane fordrafting the figures so expertly. Tim Francis and Matt Salisburyreviewed the first draft of the paper and made many valuable sugges-tions.
REFERENCESArchie, G. E., 1942. The electrical resistivity logs as an aid in deter-
mining reservoir characteristics. /. Petrol. Tech., 5:1-8.
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Formation evaluation data handbook, 1976. Fort Worth (Gearhart-Owen Industries, Inc.).
Francis, T. J. G., 1982. Effect of drill string movement on shape of thehole and on the cored rocks at Hole 459B. In Hussong, D. M.,Uyeda, S., et al., Init. Repts. DSDP, 60: Washington (U.S. Govt.Printing Office), 835-840.
Kirkpatrick, R. J., 1979. The physical state of the oceanic crust: re-sults of downhole geophysical logging in the Mid-Atlantic Ridge at23°N. J. Geophys. Res., 84:178-188.
Log interpretation charts. 1974. New York (Schlumberger Ltd.).Log interpretation charts. 1979. New York (Schlumberger Ltd.).Lonsdale, P., and Klitgord, K., 1978. Structure and tectonic history of
the eastern Panama Basin. Geol. Soc. Am. Bull., 89:981-999.Salisbury, M. H., Donnelly, T. W., and Francheteau, J., 1979. Geo-
physical logging in Deep Sea Drilling Project Hole 417D. In Don-nelly, T., Francheteau, J., Bryan, W., Robinson, P., Flower, M.,Salisbury, M., et al., Init. Repts. DSDP, 51, 52, 53, Pt. 1: Wash-ington (U.S. Govt. Printing Office), 705-713.
0.05 0.10
0,0.15 0.20
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Figure 14. Sonic velocity plotted against porosity calculated fromresistivity log using Archie's Law exponent of 2.1. Hole 504B base-ment section.
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