© Terra Antartica Publication · logs. Well logging of CRP-3 has provided a complete and comprehensive data set of in situgeophysical measurements down to nearly the bottom of the

© Terra Antartica PublicationTerra Antartica2001, 8(4), 491-505

Statistical Analysis of Wireline Logging Data ofthe CRP-3 Drillhole (Victoria Land Basin, Antarctica)

C.J. BÜCKER1§*, R.D. JARRARD2, F. NIESSEN3 & T. WONIK1

1GGA, Leibniz Institute for Applied Geosciences, Stilleweg 2, 30655 Hannover - Germany 2Dept. of Geology & Geophysics, 717 WBB, Univ. of Utah, Salt Lake City, UT 84112-0111 - USA

3Alfred Wegener Institut für Polar- und Meeresforschung, Columbusplatz, 28575 Bremerhaven - Germany§Present address: RWE-DEA AG, Ueberseering 40, 22297 Hamburg - Germany

Received 16 January 2001; accepted in revised form 13 November 2001

Abstract - Drillhole CRP-3 in northern McMurdo Sound (Ross Sea, Antarctica)targeted the western margin of the Victoria Land basin to investigate Neogene toPalaeogene climate and tectonic history by obtaining continuous core and downholelogs. Well logging of CRP-3 has provided a complete and comprehensive data setof in situ geophysical measurements down to nearly the bottom of the hole (920 mbelow sea floor (mbsf)).This paper describes the evaluation and interpretation of the downhole logging datausing the multivariate statistical methods of factor and cluster analysis. The factorlogs mirror the basic geological controls (i.e., grain size, porosity, clay mineralogy)behind the measured geophysical properties, thereby making them easier to interpret geologically. Clusteranalysis of the logs delineates individual logging or sedimentological units with similar downholegeophysical properties. These objectively and independently defined units help differentiate lithological andsedimentological characteristics (e.g. grain size, sediment provenance, glacial influence).For CRP-3, the three factor logs derived from the downhole measurements reflect sediment grain size (proxyfor lithology), the occurrence of diamict and/or conglomerate (glacial influence), and sediment provenance.It is possible to divide the borehole into three main sections on the basis of cluster analysis of the threefactor logs. The top section down to about 200 mbsf is dominated by mudstone with clearly differentphysical properties from the mudstones occurring below this depth. Beneath 200 mbsf sandstones dominatethe lithology. Two types of sandstones were characterised in the early Oligocene/late Eocene sequence, witha division between the two types occurring at about 630 mbsf. These two types of sandstones, which aredifferentiated mainly on the basis of their magnetic and radiogenic properties, can be correlated by detritalmode provenance analysis. Comparison of the results of the factor and cluster analyses with the reflectionseismic profiles shows that the major change in sediment source from the Victoria Group to the TaylorGroup at 630 mbsf (earliest Oligocene) is not seen by seismic sequence analysis. This observation may haveimportant consequences for the entire Ross Sea seismic stratigraphy.

INTRODUCTION

The Cape Roberts Project (CRP) was designed todocument past variations in Antarctic ice cover andclimate and to reconstruct the early uplift history ofthe nearby Transantarctic Mountains (TAM). Inaustral spring 1999, the third and final drillhole(CRP-3) of the Cape Roberts Project was successfullycompleted. The three CRP drillholes CRP-1,CRP-2/2A, and CRP-3 are located in a sedimentarybasin about 20 km offshore from Cape Roberts,which gave the name for the project. A detaileddescription of the project, the geological and timesetting, and preliminary results, together with alithology profile, have been published by the CapeRoberts Science Team (1998, 1999, 2000). At a waterdepth of 299.7 m, drillhole CRP-3 reached a finaldepth of 939.4 m below sea floor (mbsf). All of the

downhole logging tools were run almost to thebottom of the hole. The downhole logs are essentialto achieve the aims of the project, in particular inlinking seismic reflection data to the drillhole geologyand for seismic sequence analysis. This paperdescribes the interpretation of the downhole logs andthe relationship to geological, sedimentary andseismic results. Downhole logging in CRP-3 wasundertaken in four phases depending on the drillingprogress. The first two phases were carried out afterdrilling to a depth of 346 mbsf with an HQ drillstring(96 mm diameter). Due to a fault zone at 257 –261 mbsf, the hole was logged down to 346 mbsffirst with the drillstring above 261 mbsf. Then thedrillstring was pulled up and the upper part logged.After this second phase of logging, the HQ drillstringwas cemented and drilling continued with an NQdrillstring (76 mm diameter). The third phase of

*Corresponding author ([email protected])

C.J. Bücker et al.492

logging was conducted when NQ coring had reached774 mbsf. The fourth and final phase of logging wasundertaken when NQ coring reached the final totaldepth for CRP-3, 939.42 mbsf. Except for thetemperature logs, all of the logs were recorded onlywhile the tool was hoisted from the bottom of thehole.

The downhole logging tools used in CRP-3 arethe same as those used in CRP-2/2A (Cape RobertsScience Team, 1999), with some additional tools. Adetailed description of the logging tools andtechniques is given in the Initial Report on CRP-3(Cape Roberts Science Team, 2000). A total of 9 toolswere used, yielding values for 15 physical andchemical parameters. Additionally to theseconventional downhole tools, a borehole televiewer(BHTV) was run and a vertical seismic profiling(VSP) experiment with two offset measurements wascarried out. BHTV and VSP results are presented byJarrard et al. (this volume) and by Henrys et al. (thisvolume), respectively. Downhole temperature profileswere collected several times; the results are publishedby Bücker et al. (this volume).

Each logging phase consisted of running thefollowing logging tools: temperature (first and last runof each phase), spectral gamma ray (K, Th, U), duallaterolog (deep and shallow resistivity), magneticsusceptibility, array induction (resistivity at fourpenetration depths), neutron porosity, boreholeteleviewer (with magnetic field measurements), sonicvelocity, dipmeter (with magnetic fieldmeasurements), vertical seismic profile. In addition tothis standard suite of CRP-3 downhole measurements,the fourth phase included a density log over the entiredrillhole and offset vertical seismic profiling.

The downhole measurements were interpretedusing multivariate statistics in a similar approach tothat used in CRP-2/2A (Bücker et al., 2000b). Theapplication of multivariate statistical tools proved tobe very quick, objective and reliable and resulted in anew integrated presentation of logs. Factor and clusteranalyses are excellent methods for handling a largeamount of logging data and meeting the demand for afast, reliable and objective evaluation andinterpretation. Again, as for CRP-2/2A, factor analysiswas used to rescale and reduce the original data set,and cluster analysis was used to block log data anddefine sedimentological characteristics such as grainsize and provenance changes as objectively aspossible.

DOWNHOLE LOGGING DATAAND QUALITY CONTROL

A complete set of downhole logs was recorded inCRP-3 using tools from the Leibniz Institute forApplied Geosciences (GGA-Institut, Germany). All ofthese tools were manufactured by ANTARES GmbH(Bremen, Germany) and are slimhole tools with a

diameter of not more than 52 mm. They can becompared to commercial wireline logging tools (e.g.,Schlumberger, Western Atlas). A description of themeasurement principles and techniques has beenpublished by the Cape Roberts Science Team (1999,2000). Comparisons with the results of the coremeasurements and cross correlations show the data tobe of generally good quality.

Sample interval, radius of investigation, verticalresolution and a summary of tool responses of eachlogging tool have been published by the Cape RobertsScience Team (2000). Figure 1 shows a compositeplot of all downhole measurements used in this study,together with a simplified lithological log derived byvisual core descriptions (Cape Roberts Science Team,2000). Depth shifts of as much as 1 m can occuramong different logging runs, caused mostly by cablestretch or slip. Prior to this presentation, all downholelogs were first matched to a reference log (electricalresistivity) and then to core depth, thus makingdownhole log and core measurements comparableeven at a small scale. In general, depth offsets wereless than a few decimetres, but sometimes up to onemetre.

Log quality and reliability are judged to beexcellent for nearly all of the CRP-3 logging toolsexcept the sonic tool. This conclusion is based oninternal consistency and calibration tests for sometools, replicability as observed in short intervalslogged twice with the same tool (repeat sections), andsimilarity of log character among different loggingtools that use different physical properties to detectthe same geological variable. All the followingdescriptions of downhole logs refer to figure 1.

BOREHOLE GEOMETRY

Borehole calliper was measured with the dipmetertool, which uses the four resistivity pads as a four-arm calliper tool. Hole deviation was measured withdipmeter and televiewer tools, based on 3-axisfluxgate magnetometer and 2- or 3-axis accelerometerrecordings. During all downhole measurements amagnetic field station was run on the ice to have acontrol for magnetic storms. This was important asthe Cape Roberts drillhole location is close to themagnetic southpole. The four-arm callipermeasurements together with the dip-angle of theinclination of the borehole are given in Figure 1. Ingeneral, the hole shows a good calliper with only afew larger breakouts and washouts, which do notexceed 250 mm. Calliper and dip-angle measurementscould not be obtained in the fault zone at 255 –275 mbsf where the drillstring was in place. Hole sizehad two effects on CRP-3 logging. First, a bridge,presumably due to sticky clays at the top and bottomof the volcanic intrusion at 900 mbsf, prevented thetools from reaching the bottom 18 – 39 m of thehole. Only the temperature tool, which was the first

Statistical Analysis of Wireline Logging Data of the CRP-3 Drillhole 493

Fig. 1 - Core derived lithology and downhole logs for hole CRP-3. From left to right, the following parameters are shown: first column: calliper of the borehole (cal1 – cal4); cal 1 and cal2 are the calliper logs for the HQ borehole (cal1 left part, 110 – 140 mm,cal2 right part 140 – 110 mm), cal3 and cal4 are the calliper logs for the NQ borehole (cal3 left part, 90 – 110 mm, cal4 right part, 110 –90 mm); second column: angle of inclination of the borehole (0° (left)– 3°(right)); between second and third column: depth scale 1:4500and depth in metres below sea floor; third column: core-derived lithology, legend see Barrett et al., (this volume) fourth column: spectralgamma ray log (sgr, 0 – 150 API); fifth column: potassium (K, 0 – 4 %), uranium (U, 0 – 8 ppm), thorium (Th, 0 – 16 ppm); sixthcolumn: susceptibility log (sus, 0 – 100×10-3 SI), anomaly of vertical component of magnetic field delta_z (-500 – 500 nT); seventhcolumn: electrical resistivity log (RShallow, RDeep, 0.5 – 500 ohm·m), logarithmic scale; eighth column: density log (rho, 2.0 – 2.7 g/cm3),porosity log (poro, 0 – 40 p.u.); ninth column: whole-core P-wave velocity (Vpc, 1.5 – 5.0 km/s), thorium/potassium ratio (Th/K, 1 –6×10-4) (for lithology legend, see Barrett et al., this volume).


log run at this depth, reached 921 mbsf. To preventtool losses, subsequent logging runs were stopped atshallower depths of 900 – 919 mbsf.

Second, an enlarged hole can degrade the accuracyof most log types. As can be seen in the overall goodcalliper log in figure 1, hole conditions are not asignificant source of error for the CRP-3 logs. Abouta dozen washouts were detected in the dipmetercalliper log, most of them caused by unstable holeconditions associated with loose or unconsolidatedsands.

Measured hole deviations are remarkably small.Over the entire borehole, the angle of inclination isless than 3°, in the lower part less than 2°, and sharpcurve inflections were not observed. In the top 350metres the hole deviation is 1.0° – 2.5° to thesouthwest, and about 1.5° to the southeast in thelower part of the hole. This means that the boreholeis nearly vertical and the hole deviation does not haveany influence on the downhole measurements.

SPECTRAL GAMMA RAY

The spectral gamma ray tool was run down to918 mbsf and reached the volcanic intrusion. Rarespikes caused by artefacts were observed on the rawgamma ray logs. These spikes resulted from noise-induced incorrect partitioning of the spectrum ofgamma-ray energies and could be detected as a sharppositive spike in one elemental concentration andcorresponding negative spike in one of the other twoelemental concentrations. These spikes were editedand removed.

Gamma ray measurements were also made insidethe drillstring (0 – 25 mbsf, 255 – 275 mbsf), and theattenuated values were corrected by using the ratio ofthe measurements inside and outside of the drillstring.No definite explanation can be given for the highuranium values in the Quaternary section of theborehole down to 20 mbsf. These may be attributedto a higher organic content.

Gamma ray values (from the spectral gamma raylog, sgr) and the potassium (K), thorium (Th), anduranium (U) contents show a general decreasedownhole to the top of the doleritic shear zone at790 mbsf. The highest sgr and thorium values wereobserved for the Beacon Sandstone in the depthsection between 820 mbsf and 900 mbsf. The BeaconSandstone can be divided into two sections mainlyusing the thorium log, which shows very high valuesin the lower part of the Beacon Sandstone below850 mbsf. No other physical property shows adifference for different parts of the Beacon Sandstone.The difference in the thorium log may indicatedifferent alteration stages.

In the uppermost part of the borehole, sgr showsmore or less constantly high values up to 100 APIalthough the lithology log shows alternatingsandstones, mudstones and diamictites. The gamma

ray log is traditionally used as a shale/sand indicatorand to estimate shale content (Rider, 2000). Howeverthe expected distinction between mudstone andsandstone was not observed in the CRP-3 boreholeabove 120 mbsf. The high gamma ray values in thisuppermost section of the borehole indicate a highermud content in the sand and diamictite sections thanthe lithology log indicates. Below about 144 mbsf,the sgr log exhibits a good correlation with thelithological log, with high values for mudstones andlow values for sandstones. The conglomerates do notshow any extinct gamma ray values. Below about460 mbsf the gamma ray data show the lowest valuesin the entire borehole, with only small fluctuationsthat clearly indicate sandstones. From about 630 mbsfto 790 mbsf the gamma ray values are fairly constant,with a slight increase towards the top of the doleriteshear zone at 790 mbsf. However, the traditionallyexpected gamma-ray pattern for coarse and fine-grained sediments is obscured in formations withimmature unclean sands that contain substantialquantities of radioactive minerals, such as potassiumfeldspar and micas, and which contain a considerableamount of clay.

The overall pattern of the thorium/potassium ratio(Th/K, Fig. 1) within the Tertiary section of theborehole down to 790 mbsf indicates threepetrophysical units. This ratio shows high andconstant values down to 200 mbsf, and then drops tolower values varying from 200 to 630 mbsf, andbelow 630 mbsf the Th/K ratio is again high andvarying. As the Th/K ratio is not influenced by theabsolute radionuclide concentrations, these differingratios may indicate different sediment sources. Thebroad pattern of downhole sgr variations reflects thecombination of decreasing mud component andincreasing Beacon sandstone source downhole.

Below 200 mbsf, changes in grain size arereflected in the sgr, and the tool can be used as afacies indicator. Due to this relationship, the sgr logshows a close correlation with the core-basedpalaeobathymetry curve (Cape Roberts Science Team,2000).

MAGNETIC SUSCEPTIBILITY AND ANOMALYOF THE VERTICAL INTENSITY OF THEMAGNETIC FIELD

The magnetic susceptibility tool was run over theentire borehole down to 902 mbsf. The gap in thefault zone from 255 to 275 mbsf was filled by crosscorrelation with downcore velocity measurements(correlation coefficient 0.65).

The most prominent feature of the susceptibilitylog (Fig. 1) relates to the Beacon Sandstone, whichshows more or less zero susceptibility values and insome cases negative ones due to properties of purequartz (quartz has a low negative susceptibility due toits diamagnetism). Delta_z, the anomaly of the


vertical intensity of the magnetic field, also shows nochanges for the Beacon Sandstone section and thusindicates the Beacon Sandstone is effectively non-magnetic in the depth interval 820 – 900 mbsf. Thisis remarkable, because the sgr log shows cleardifferences within the Beacon Sandstone. The non-magnetic behaviour of the Beacon Sandstone mayalso provide new information for the interpretation ofthe aeromagnetic anomaly observed between the CapeRoberts drillhole locations and the coastline (Bozzo etal., 1997). This anomaly may be explained by anabsence of the Beacon Sandstone in this regionresulting in an unroofing of the underlying volcanicintrusion.

Grain size affects both the magnetic susceptibilityand spectral gamma ray logs. Fine-grained sediments(mud, clay) often have much higher amounts of bothmagnetic and radiogenic minerals than coarse-grainedsediments like sandstones (Bücker et al., 2000a).Therefore, the combination of sgr and susceptibilitylogs usually provides a robust indicator of sandstoneversus shale. However, the susceptibility also shows aresponse to the conglomerates. Most of theconglomerates below 200 mbsf show a marked peakin susceptibility, indicating elevated magnetite content.

In most cases, changes in the magnetic anomalyvalue (delta_z) are closely related to susceptibility.The magnetic anomaly curve reflects the remanentand induced magnetism of the sediments and shows ahigher vertical resolution in signal strength than thesusceptibility curve, which responds only to theinduced magnetisation. Remanence values can becalculated from the values of the magnetic field andsusceptibility measurements and can be used toconstruct a more detailed magnetic anomaly model ofthe aeromagnetic anomaly west of the Cape Robertsdrillsites (Bozzo et al., 1997).

The magnetic susceptibility and magnetic anomalylogs show a pattern similar to the gamma ray andthorium/potassium ratio logs. Susceptibility isuniformly high down to about 200 mbsf anduniformly low in the interval 444 – 630 mbsf. Below630 mbsf down to 790 mbsf the susceptibility againshows high values. The zone from 200 to 444 mbsfshows an alternation between these two levels. Theoverall pattern in the magnetic susceptibility andmagnetic anomaly logs, more than that of the gammaray log, appear to be more closely related toprovenance changes than to variations in grain size.Thus, the low susceptibility values in the depthsection 200 – 630 mbsf indicate a mainly Beaconsource, which consists of nonmagnetic sandstones.

Diamictites and conglomerates can be detectedover the entire borehole section below 144 mbsf dueto their enhanced susceptibility and magnetic anomalyvalues. This response is particularly characterizedbelow 440 mbsf, where each conglomerate sectionand even relatively large lonestones were detected onthe basis of susceptibility and magnetic anomaly

variations.ELECTRICAL RESISTIVITY

Electrical resistivity shown in figure 1 wasmeasured with the dual-laterolog tool, which gives aten times higher vertical resolution than the arrayinduction tool. The data gap in the fault zone from255 to 275 mbsf was filled by cross correlation withcore p-wave velocity (correlation coefficient 0.63).With the dual-laterolog, electrical resistivity wasmeasured at two depths of investigation: “RDeep” and“RShallow”. Both curves show a very highcorrelation, and differences in these two curvesindicate rock fracture in this case where this isabsence of invasion of a borehole fluid of anomalousresistivity.

Electrical resistivity mainly responds to formationporosity, as the pore fluids rather than the rock matrixaffect electrical conductivity. Resistivity can also beinfluenced by clay minerals, and importantly clayeffects depend on the salinity of the formation fluid.Clay conducts electricity, and thus decreasesresistivity in low-porosity rocks (Waxman & Smits,1968); on the other hand, clay increases tortuosityand therefore increases resistivity in high-porosityrocks (Erickson & Jarrard, 1998). However, claymineral concentration in CRP-3 is low (Neumann &Ehrmann, this volume), and its influence on bothneutron porosity and resistivity is probably minor.

The Beacon Sandstone shows a very homogeneouselectrical resistivity. The highest resistivities werefound in the doleritic shear zone from 780 to820 mbsf, indicating low porosities in this depthinterval. Over the entire Tertiary section of theborehole, resistivity shows a highly varying characterwith peaks and differing baselines. In particular below440 mbsf, the peaks in the resistivity curves can beclearly attributed to conglomerates. In the uppermostpart down to 100 mbsf, resistivity continuallyincreases due to sediment compaction. From 100 to200 mbsf the resistivity baseline is high, with severalpeaks up to 500 ohm·m. The section from 200 to440 mbsf is characterized by alternating resistivities,also showing high values for muddy sections (i.e., at308 – 316 mbsf). The lowest resistivity baseline isfound from 440 to 630 mbsf with again high valuesfor conglomerates and lonestones. Similar tosusceptibility and magnetic anomaly baselines, theresistivity baseline is relatively high from 630 mbsf tothe top of the doleritic shear zone. In general, theelectrical resistivity curve reflects the porosity, but theresistivity baseline may indicate differences in theformation factor and thus in sediment source.

DENSITY, NEUTRON POROSITY AND VELOCITY

Density logs were obtained for the open-hole NQpart of the hole, whereas through-pipe logs were runin the HQ part (above 345 mbsf). The through-pipedensity values were corrected by comparing average


density values in the 100 m above and below thechange from NQ to HQ drillstring. The whole-coredensity logs demonstrate that the average densities forthese two intervals are about the same. In general,core and log measurements of bulk density areconsistently very similar, indicating that both arereliable. No systematic difference between bulkdensity core and log measurements was observed,showing that there was no measurable stress releasein the cores.

The raw neutron log is a ratio of counts measuredby a far (philong) and a near (phishort) detector. Tomake the measured neutron porosities comparable toformation porosity, a porosity log (poro) wascalculated from the measured counting ratio of theneutron porosity tool derived from cross correlationwith core-derived porosities using the followingformula:

poro = 22 + 31.7 ln(philong/phishort).

Initially, the HQ interval (above 345 mbsf) waslogged open-hole with the neutron tool. During thethird logging phase, this interval was reloggedthrough-pipe, and this latter log is presented here. Thetwo logs show a very high correlation. Only a slightpipe effect appears to be present, but this is so smallthat no pipe correction was applied. However, therelationship between porosity and the ratio ofcountrates from long and short detector is not linearresulting in higher errors at high porosities.

Neutron porosity measures the total hydrogencontent of the formation, including water bound inclays plus free water in pores. Thus, neutronporosities can be too high in formations that are richin clay minerals. On the other hand, this effect can beused in combination with density measurements todetermine clay content and clay type (Bücker et al.,2000b). The downhole porosity log shown in figure 1was corrected to core porosity by using the formulagiven above.

The sonic velocity tool was the only tool that didnot provide a good quality log. This tool measures p-wave velocity based on picked travel-times ofrefracted waves from a sound source to two receivers.The strength of this refracted wave was generally tooweak to be reliably detected by the far source-receiver combination. After returning home werecognised that this weak signal was due to a toolfailure. Because of the high mud salinity, the materialof the receivers was swollen and this obscured thereceived signal. The velocity log shown here (Fig. 1)was measured on cores and is thus indicated by ‘c’.

Down to 100 mbsf, the porosities show acompaction trend. Below this depth, the porositiesshow a good correlation with the neutron, density andcore-derived sonic-velocity logs. The highestvelocities and densities and, accordingly, lowestporosities were found for the doleritic conglomeratefrom 780 to 820 mbsf. Conglomerates can be clearlydetected by their high velocities of up to 6 km/s and

densities up to 2.6 g/cm3. Average velocity in thelowermost sedimentary section from 630 – 780 mbsfis about 3.7 km/s. In general, velocities in CRP-3 areabout 3.2 ± 0.6 km/s, apart from the first 100 m ofcore, where velocity is less than 2.0 km/s, and thedolerite shear and conglomerate zone, where velocityis greater than 4.5 km/s. The average velocity for theCRP-3 core is much lower than previously thought(Bücker et al., 1998). Cooper & Davey (1987) usedstacked velocities from reflection seismic processing.This finding has a considerable effect on theinterpretation of the offshore seismics of the entireRoss Sea and is discussed in detail by Henrys et al.(this volume).

Porosity is the dominant variable affecting neutronporosity, density, sonic velocity, and resistivity.Therefore, these logs are often referred to as porositylogs. Porosities of most siliclastic sediments dependon grain size and compaction history. Sea-floorporosities of well-sorted sands are about 30 – 40 %,whereas clays show porosities of up to 80 % or more(e.g., Hamilton, 1971). Initial porosities aresubsequently decreased both by mechanicalcompaction and by chemical diagenesis. Thecarbonate content, which is the main cementationmineral in the CRP-3 core, is discussed by Dietrich(this volume).

Small-scale variations in both the sgr andmagnetic susceptibility logs show a positivecorrelation with variations in density and electricalresistivity throughout most of the Tertiary sediments.This relationship is similar to that commonly seen forhighly compacted sandstones and shales. Silty andmuddy beds are less porous, higher in density andvelocity, and higher in both radioactive elements andmagnetic minerals than are sandy beds. Comparisonwith the core-derived lithology log confirms that thispredicted lithological association is present.

Figure 1 shows plots of three porosity-sensitivedownhole logs resistivity, density, and velocity. Asexpected, the broad trends in all three logs are verysimilar, except for a gradual decrease in resistivity,which is expected to occur independently of porositychange. Rock resistivity depends on both porosity andpore-fluid resistivity. Pore-fluid resistivity decreasesgradually downhole due to the thermal gradient,causing an associated downhole decrease in observedformation resistivities that is superimposed on thepattern of porosity changes.The changes in neutron porosity, resistivity, velocityand density with depth do not follow a simplecompaction trend often found in siliclastic sediments(Hamilton, 1976). No systematic depth-dependentdecrease in porosity is evident below 200 mbsf. Thisfinding for the CRP-3 drillhole differs from the strongdownhole porosity decreases observed in CRP-1(Niessen & Jarrard, 1998) and CRP-2 (Brink et al.,2000). Apparently, diagenesis and grain-sizefluctuations affect present porosity much more than


the early mechanical compaction and cementationhistory does (Jarrard et al., this volume; Jarrard et al.,2000; for CaCO3 content see Dietrich, this volume).

Table 1 gives average values for the physicalproperties of some lithological units in CRP-3 withthe highest values for each property shown in boldprint. The mudstone from the depth section 125 -140 mbsf, which is below the compaction-influencedfirst 100 m of the core, shows the highestsusceptibility and magnetic anomaly values. Theporosity is also very high (24 %). The BeaconSandstone from 825 to 900 mbsf has considerablydifferent physical properties than other sandstones, i.e.in the depth sections 185 - 200 mbsf, 480 - 510 mbsfand 645 - 660 mbsf. In particular, the porosity ismarkedly lower in the Beacon Sandstone (7 %),although densities and velocities are comparable.However, due to the distinctly lower porosity andsomewhat higher velocity, seismic offshoremeasurements should clearly detect the BeaconSandstone. The other three sandstones can be clearlydistinguished by their physical properties. SandstoneI, in the depth section 185 - 210 mbsf, has thehighest resistivity; sandstone II, in the depth section480 - 510 mbsf, has the lowest magnetisation; andsandstone II, in the depth section 645 - 660 mbsf, hasthe highest susceptibility.

VISUALLY DERIVED LOG-BASED UNITS

In the Initial Report on CRP-3 (Cape RobertsScience Team, 2000), the interval 0 - 901 mbsf wasdivided into five logging units on the basis of logs.After full processing of the downhole logging dataset, the changes to the limits of these logging unitsare only minor.

Log Unit I, 0 - 144 mbsf, is characterised bygenerally very homogeneous log responses,particularly for the gamma-ray, magnetic

susceptibility, and K/Th logs. Resistivity, density andvelocity show a downward increase, whereas porositygenerally decreases. There is no clear response for thediamictites (i.e., 85 - 95 mbsf), possibly due to highmud content.

Log Unit II, 144 - 630 mbsf, is characterised bybimodal responses for all the logs, with one modesimilar to log Unit I. Alternations on a 10 – 30 mscale are due to sedimentary sequences (Naish et al.,this volume). A basal conglomerate marks mostsequence boundaries with high resistivity and density,low porosity, and high susceptibility and magneticvalues. The gamma ray responses of theconglomerates change downhole from low to highervalues, and the characteristic low K/Th signaturebecomes more evident downhole. The conglomeratesare clearly seen as spikes on the resistivity curve,because of the combination of poor sorting and highcementation. Some fining upward sequences fromclean sand to muddy sand and silt are visible in thelog responses.

Log Unit III, 630 - 790 mbsf, is lithologicallyrelatively homogeneous, with low gamma-ray values,but enhanced Th/K values and high susceptibilitiesand magnetisation. Porosity, density, and velocity areheterogeneous, with variations similar to those in logUnit II: low porosities in the conglomerates and well-cemented sandstones, with high-porosity sandstonesalso present. Except for the conglomerates, grain-sizechanges are not as evident in the gamma ray andsusceptibility logs as in log Unit II. This finding isconsistent with core descriptions of this unit asmostly greenish sandstone. Fining and coarseningupward sequences are also seen in the susceptibilityand gamma ray logs.

Log Unit IV, 790 - 823 mbsf, is characterized byvery low porosities, very high susceptibilities, andrelatively low gamma ray values. This unit consists of

Lithology clay sand-stone I sand-stone II sand-stone III Beacon SandstoneIgneous

Intrusion

Depth (mbsf) 125-140 185-200 480-510 645-660 825-900 900-910

K (%) 2 2 1 1 3 4U (ppm) 2 2 1 1 2 3Th (ppm) 7 6 2 3 11 5Th/K 3.5 3.2 1.8 1.5 4.1 1.5Rdeep (Ohmm) 8 17 4 8 7 6sus (10-3 SI) 25 23 3 28 0 4delta_z (nT) -80 60 -10 10 0 -4rho (g/cm3) 2.2 2.3 2.4 2.4 2.4 n.v.poro (%) 24 19 20 22 7 19vpc (km/s) 2.8 3.2 2.6 3.1 3.3 2.7

Tab. 1 - Average values for the physical properties of some characteristic lithological units in CRP-3 (abbreviationssee figure caption of figure 1). The highest values for each property are shown in bold. The mudstone from 125 –140 mbsf shows the highest porosity values. The Beacon Sandstone from 825 – 900 mbsf has considerably differentphysical properties than the other three sandstones, i.e., in the depth sections 185 – 200 mbsf, 480 – 510 mbsf, and645 – 660 mbsf. The porosity is markedly lower in the Beacon Sandstone. Thus, seismic measurements should clearlydetect the Beacon Sandstone. Sandstones II and III show nearly the same thorium/potassium ratio, but theirsusceptibility values are distinctly different. Density measurements could not be made for the igneous intrusion below900 mbsf due to material falling to the bottom of the hole (n.v.: no values).


a fault zone (790 - 803 mbsf), with elevated gamma-ray values and the highest susceptibility values of theentire borehole, and Tertiary doleritic sediments (803- 823 mbsf).

The transition from log Unit IV to V is very sharpand corresponds to the major unconformity betweenTertiary and Devonian sedimentary rocks.

Log Unit V, 823 - 901 mbsf, is the upper part ofthe Beacon Sandstone. As mentioned earlier, theBeacon Sandstone is non-magnetic, visible in thesusceptibility and magnetic anomaly logs. Porosity ishigher than in the overlying doleritic conglomeratebut lower than in the entire Tertiary section of thehole. The gamma ray log shows a remarkablevariation in the Beacon Sandstone, with lower valuesin the upper part and the highest values in the lowerpart, dividing the Beacon Sandstone at about855 mbsf into two parts. The high gamma ray valuesare supported mainly by elevated thorium andpotassium contents. These high values areincompatible with expectations based on themineralogical maturity of analogous Beacon outcrops,suggesting that diagenetic precipitation has enrichedthese sandstones in K and Th. For detailed listings ofthe physical properties in the log units see alsotable 1.

Unfortunately, because of bridges and materialfalling to the bottom of the borehole, only a fewvalues could be obtained for the igneous intrusionbelow 900 mbsf.

It was time intensive to derive these visuallyidentified logging units; they are very subjective anddepend mainly on the observer ’s experience.Objective statistical methods are faster and muchmore reliable and better suited to establish loggingunits (Bücker et al., 2000) and are described in thefollowing sections.

APPLICATION OF MULTIVARIATESTATISTICAL METHODS

The multivariate statistical procedure used in theevaluation and interpretation of the downhole logs isdescribed in detail by Bücker et al. (2000a). TheCRP-3 downhole logs were treated similar to theCRP-2/2A logs. The data set has to be prepared priorto the factor and cluster analyses. The first step is anintensive quality control and editing of spikes etc.from the entire data set. The next step consists offilling all gaps in the downhole measurements dataset by cross correlation with other physical propertydata that do not have gaps. Gaps in the electricalresistivity and magnetic susceptibility curves, whichcould not be measured through the drillpipe, werefilled by correlation with the downcore velocity log(correlation coefficient R in the depth section 150 –350 mbsf: R = 0.57, and R = 0.65, respectively). Themagnetic anomaly log delta_z was filled by

correlation with the susceptibility log (R = 0.65). Thenext data preparation step consisted of a test fornormal, i.e., Gaussian distribution within each dataset. Electrical resistivities and magnetic susceptibilitiesexhibit a log-normal distribution, and an applicationof a logarithmic transform to these logs yieldedobservations that are normally distributed.

Finally, the observational data were standardisedby subtracting the mean and dividing by the standarddeviation. The resulting logs are dimensionless, eachwith a mean of zero and a standard deviation of 1.This procedure permits comparison of all the originaldownhole logs, regardless of their original scaling.

FACTOR LOGS

With factor analysis, interrelationships among a setof variables are examined. This technique is used toderive a subset of uncorrelated variables called factorsthat explain the variance in the original observationaldata set without losing important information. Withthis procedure, the dimensionality of the original dataset is reduced, resulting in two or three factors thataccount for nearly all the variance in the original dataset. This makes the interpretation of the huge amountof logging data much simpler, as the factor logsrespond to important sedimentological features suchas grain size, provenance or glacial influence (Bückeret al., 2000b).

Factors and factor loadings were calculated fromthe standardised logging curves using standard R-mode factor analysis procedures (Davis, 1986;Backhaus et al., 1986; Tabachnick & Fidell, 1991).A Kaiser Varimax factor rotation of the matrix offactor loadings resulted in a simplification of thefactor co-ordinate system. The resulting factor modelis simply a linear combination of underlying physicaland/or chemical properties. A factor is taken as beingsignificant for an underlying property, if itseigenvalue is greater than 1. Theoretically, the factorlogs are uncorrelated. This is not always the case,because the borehole geophysical tools were initiallydesigned to respond primarily to porosity andlithology (Elek, 1990). Consequently, the first twofactors derived from the factor analysis are alsorelated to porosity and lithology.

One of the main advantages of the factor logs isthat they are - by definition - not interrelated. Theambiguity of the downhole logs is strongly reducedand the factor logs can be interpreted directly interms of the controlling background variables.However, as the rock properties themselves are notuncorrelated in all cases, some correlation betweenthe factor logs is possible.

All of the downhole logs of figure 1 wereincluded in the factor analysis of the Tertiary sectionof the hole down to 790 mbsf. Only the shallowresistivity was not used because it shows a strongcorrelation with the parameter RDeep. For this data


set, 76 % of the variance observed in the inputvariables can be described by the first three factors,although the number of variables has been reducedfrom 11 to 3. This means that the factor extractionresulted in a reduction of the logs by a factor of four,whereas more than three-quarters of the originalinformation is kept. The results of the factor analysis,together with communalities, factor eigenvalues andfactor loadings, are listed in table 2. The factor logsare displayed in figure 2, together with the lithologycolumn, the gamma ray and magnetic susceptibilitylogs and the multivariate cluster log. The cleargrouping of variables (Tab. 2) and different patternsof the factor logs (Fig. 2) confirm that in this case,the relationship between factors and sharedbackground information is negligibly small.

Factor loadings greater than ± 0.5 are taken assignificant and shown in bold print in table 2.

CLUSTER LOG

In general, cluster analysis is an ideal techniquefor grouping individuals or samples into a prioriunknown groups. The objective of the cluster analysisis to form groups on the basis of measuredcharacteristics with the aim of maximising thedifferences between the groups. The Ward method isthe most commonly used method that uses aEuclidean norm for complete-linkage hierarchicalclustering (Davis, 1986) and was used in this study.The factor logs were used as input logs for thecluster analysis, and statistical electrofacies weredefined. To decide how many clusters are significantand useful, a cluster dendrogram and the so-calledelbow criterion were used. The resulting statisticalelectrofacies, or logging units, which stand for distinctcombinations of rock physical and chemical properties(e.g., Serra, 1986; Doveton, 1994), are shown infigure 3. For the CRP-3 site, five significant clusterswere obtained for the Tertiary section of the holedown to 790 mbsf on the basis of multivariateanalysis of the three factors. Taking more clusters intoconsideration would simply result into a subdivisionof these five clusters, complicating the interpretation.However, it may be helpful for refined interpretationin certain depth sections.

RESULTS AND DISCUSSION

FACTOR AND CLUSTER LOGS

The factor analysis shows that the first two factorsare equally weighted, and each of them contributes30 % to the variance. The third factor contributes halfof the variance of the first two factors (Tab. 2). Thefactor analysis revealed the most discriminatingvariables to be gamma ray, potassium, and velocity,each with a factor loading greater than 0.9. Factor 1is loaded by the radiogenic properties gamma ray,

potassium, thorium, and uranium (Tab. 2). Factor 2 isloaded by the index properties velocity, resistivity,density and porosity, and to a small amount by themagnetisation delta_z. Factor 3 is loaded by the

Factor Analysis valid cases: 769

Communalitiesestimated Analysis

sgr 1 0.960

K 1 0.847

U 1 0.573

Th 1 0.883

rho 1 0.819

poro 1 0.629

Vpcore 1 0.841

delta_z 1 0.624

logRDeep 1 0.803

logsus 1 0.726

ThK 1 0.654

Eigenvalues

Factor Eigenvalue VariancePercent

Percentcumulated

1 3.72 33.82 33.822 3.43 31.20 65.033 1.21 10.99 76.014 0.84 7.66 83.67

5 0.54 4.88 88.55

6 0.41 3.73 92.28

7 0.30 2.73 95.01

8 0.23 2.11 97.12

9 0.17 1.55 98.66

10 0.11 1.01 99.68

11 0.04 0.32 100.00

Varimax Factor Loadings

Factor 1 Factor 2 Factor 3 communality

sgr 0.96 -0.08 0.20 0.96

K 0.90 -0.03 -0.18 0.85

Th 0.83 -0.12 0.42 0.88

U 0.75 -0.02 0.13 0.57

Vpc -0.03 0.91 0.12 0.84

logRDeep 0.17 0.84 0.26 0.80

rho -0.39 0.82 0.02 0.82

poro 0.04 -0.78 0.16 0.63

ThK 0.12 -0.10 0.79 0.65

logsus 0.37 0.34 0.69 0.73

delta_z -0.13 0.54 0.56 0.62

Square sum 3.33 3.25 1.78 8.36

Percentage of

Variance30.30 29.55 16.16 76.01

Tab. 2 - Results of factor analysis of downhole logs from CRP-3.The number of valid cases is the number of values in eachdownhole log used for the analysis. The upper part of the Tablepresents communalities, eigenvalues and proportion of explainedvariance. The eigenvalues are assumed to be significant if they aregreater than or equal to 1; these are in bold print here, shown infigure 2, and included in the subsequent cluster analysis. The lowerpart of the Table gives the factor loadings, the communality, andthe total amount of explained variance. Factor loadings greater than0.5 are shown in bold. The sum of the factor loadings squared isequal to the eigenvalue, which is the variance explained by afactor. Three factors have an eigenvalue greater than 1. The totalvariance explained by these three factors is 76%. (An explanationof the variables is given in Figure 1.) Factor 1 and Factor 2 shownearly the same percentage of explained variance (30%).


thorium/potassium ratio, the susceptibility and to asmall amount also by the magnetisation delta_z.Thorium also contributes to factor 3, but the factorloading is only 0.42. The factor logs are shown infigure 2 down to the base of the Tertiary section ofthe hole at 790 mbsf, together with a simplifiedlithology column and the gamma ray and

susceptibility logs. The specific loadings of the factorlogs lead to the following interpretations.

Gamma ray and susceptibility are influencedindirectly by sediment grain-size distribution, at leastfor these formations. Thus Factor1 is a good proxyfor lithology and grain-size variations (Fig. 2). Fine-grained, mud-dominated lithologies are found in the

Fig. 2 - Core-derived lithology, downhole logs for gamma ray (sgr) and susceptibility (sus), factor logs, and cluster log for the Tertiarysection of the hole. The downhole logs are the same as in Figure 1. Factor1, Factor2, and Factor3 are the factor logs as derived by factoranalysis. Colored areas denote factor values greater than zero. The multivariate cluster log (right column) was calculated on the basis ofthese three factor logs.The Factor1 log is mainly related to grain size (high loading of sgr, K, U, Th), whereas the Factor2 log is related to effective porosity(high loading of porosity, density, sonic velocity, resistivity) and, to a lesser extent, to delta_z.The green dots plotted together with Factor1 represent measurements of silt content by Ehrmann (this volume). Factor2 may be viewed asa glacier and/or ice-rafting indicator, positive values represent conglomerates or diamictites. The Factor3 log (loaded by the ratio Th/K, thesusceptibility, and the magnetic anomaly delta_z) indicates sediment source. The red dots (Qtotal) represent the total quartz content derivedby Smellie (this volume) on a reversed scale. The observed anticorrelation between factor3 log and quartz content confirms the Factor3 logas provenance proxy. The multivariate cluster log (right column) can be divided into three main sections: 0 – 200 mbsf, 200 – 630 mbsf,630 – 790 mbsf. The colours represent each of the five clusters (cf. Tab. 3). Clusters 1 (grey) and 3 (green) represent two mudstones withdifferent sediment sources, clusters 2 (red) and 5 (blue) represent two different sandstones, and cluster 4 (dark blue) represents mainlyconglomerates and to a smaller extent diamictites.


upper section of the borehole down to about450 mbsf (black areas in figure 2). No mudstonesappear below this depth; the lithology consists ofsandstones and intercalated conglomerates. Infigure 2, the silt content (Ehrmann, this volume) isplotted together with the Factor1 log and shows agood correlation to Factor1 (correlation coefficient0.58). Only in the uppermost section of the boreholedown to 80 mbsf does a noticeable difference occurbetween Factor1 log and silt content. This differencemay be a result of the undercompacted sediments inthis depth section. After calibration, the Factor1 logmay be viewed as a high-resolution grain-size log.Factor1 is to a smaller extent (-0.39, see Tab. 2)negatively loaded by the density. The negative loadingmeans that there is an anticorrelation with the gammaray values, which may indicate fine-grained sedimentsthat have a lower density than the coarser grainedsediments. But the loading of density to Factor1isonly small and may show only a trend for thisrelation.

Factor2 is loaded by all the physical propertiesthat are influenced by porosity (velocity, density,resistivity, neutron porosity). Thus, the Factor2 log

may be viewed as a true porosity log and delineatesmainly the low-porosity and high-densityconglomerates and to a lesser amount diamictites inthe upper portion of the hole. Sections with Factor2values greater than zero are shaded green in figure 2and mark the high-density/low-porosity conglomeratesand diamictites. Diamictites are often mixed with aconsiderable amount of clay, thus preventing themfrom being differentiated from mudstones. The upperpart of the borehole down to about 100 mbsf showsvery low Factor1 values and porosities, confirmingthe assumption made for Factor1 that this part isunderconsolidated and thus masks the actual physicaland chemical properties. As the diamictites andconglomerates are mainly related to glaciers and/orice rafting, the Factor2 log acts as an ice indicator forclimate interpretation. However, as pointed out earlier,it does not seem to be possible to clearly distinguishbetween diamictites and conglomerates. Factor2 isalso loaded by the magnetic properties delta_z and toa smaller extent by the susceptibility, indicating ahigher proportion of magnetic minerals, such asmagnetite, in the high-density lithologies. Below144 mbsf presence of diamictites and conglomerates

Fig. 3 - Results of cluster analysis of the downhole logs. The “box-and-whisker” plots show the average values for each parameter in eachcluster with 95 % (boxes) and 50 % (whisker) confidence limits. The vertical axis in each diagram is the physical parameter in thecorresponding unit. The two sandstones (red and blue) show nearly identical gamma ray, density, and porosity values. But the susceptibilityand Th/K ratio differentiates between these two lithological units. The patterns shown in these plots can be viewed as fingerprints of thesediments. From this figure it becomes clearly visible that each cluster (lithological unit) has a characteristic set of physical properties: thefingerprint. For a more detailed interpretation of these fingerprints with respect to sediment source, it is necessary to use reference valuesof physical properties of rocks from the Transantarctic Mountains, which are not available at the time.


can be clearly detected on the basis of their elevatedsusceptibility, magnetic anomaly and resistivityvalues. Especially below about 440 mbsf, eachconglomerate section and even relatively largelonestones are detected by the Factor2 log.

Loading of Factor3 is due mainly to propertiesthat are not directly related to grain size or lithology.The thorium/potassium ratio (Th/K) is independent ofthe absolute values, which makes this parametervaluable as a sediment source indicator. To a lesserextent, the absolute thorium content also shows apositive correlation with Factor3 (0.42, see table 2).Magnetic susceptibility and magnetic anomaly areparameters, which are influenced by the amount ofmagnetic minerals like (titano) magnetite series orhematite. The first d emagnetisation tests on CRP-3samples indicate that magnetite is the predominantmagnetic mineral in these specific horizons (CapeRoberts Science Team, 2000). In figure 2, the Factor3log is plotted together with the total quartz content(Smellie, this volume) on a reversed scale. Theobserved anticorrelation between the Factor3 log andthe total quartz content indicates that Factor3 acts asa provenance indicator. The Tertiary section of theCRP-3 drillhole down to 790 mbsf can be dividedaccording to Factor3 into an upper, middle and lowerpart (blue areas in figure 2 denote Factor3 valuesgreater than zero). The upper and lower part showhigh Factor3 values and low quartz contents, whereasthe middle part is characterized by low Factor3 valuesand higher and varying quartz contents. As theuppermost section of the borehole is dominated bymudstones, the low quartz content is reasonable. Butthe low quartz content in the section from 630 –790 mbsf, where sandstones are observed that aresimilar to the sandstones above this section, the highFactor3 values and low quartz contents indicate adifferent sediment source. Following thisinterpretation, the Factor3 log acts as a provenanceproxy, and is not just reflecting mud content.

The multivariate clusterlog was calculated byusing the three factor logs as input variables (rightcolumn in figure 2). The advantage of using the threefactor logs instead of the eleven downhole logs is thereduction of variability and fine scale variations,which could complicate interpretation. The mean andstandard deviation of the calculated physical

properties of each cluster are given in table 2 andgraphically presented in figure 3 as so-called box-and-whisker plots. The different physical propertiesrepresented by the clusters are depicted as averagevalues with 95 % (boxes) and 50 % (whisker)confidence limits. This graphical representation makesthe grouping of physical properties clearly visible.The arrangement of the coloured boxes in figure 3 ischaracteristic for each cluster and acts as a physicalfingerprint for the drilled sediments. By dendrogramevaluation, five clusters were derived as characteristicof the Tertiary section of the hole. Taking moreclusters into account would result in a subdivision ofthe five cluster solution and might be helpful forobservations at finer depth scales, but wouldcomplicate the general interpretation. Each cluster hasa different combination of physical properties and canbe seen as a statistically determined electrofacies orpetrofacies as defined by Serra (1986). For example,clusters 1 and 3 (grey and green, respectively, in thecluster log in figure 2) have identical sgr values,suggesting mudstone as the petrofacies. But they havesignificantly different Th/K ratios and susceptibilities,indicating different sediment compositions and thusdifferent rock sources. The same is true for the twotypes of sandstones, which are marked in blue andred in the cluster column in figure 2. These twosandstones have comparable radioactive properties andphysical properties such as porosity. But the magneticsusceptibility, magnetic anomaly delta_z, and theTh/K ratios are totally different. These source–rock-dependent properties again indicate different sedimentsources for these two sandstones in the borehole.Diamictites and conglomerates are marked blue,which represents high index properties, and also thehighest magnetic values. The conglomeratesapparently consist of rocks enriched in magnetite,which may also indicate a specific rock source in theTransantarctic Mountains.

In general, the cluster log in figure 2 can bedivided into three main sections; the first unit isdominated by the grey mudstones down to about200 mbsf. The blue sandstone is clearly the mostprominent feature in the depth section from 200 –630 mbsf. In the upper portion, this sandstone isintercalated with mudstones (shown in green), whichshow different properties from the mudstones (shown

cluster # sgr K U Th rho poro vpc delta_z Rdeep sus Th/K lithology

1 (grey) 77 2 2 6.7 2.21 25 2.5 -55 4.7 24 3.4 mudstone 12 (red) 44 1.3 1.1 3.6 2.39 20 3.16 117 9.1 25.6 3 sandstone 13 (green) 76 2.6 1.9 5.5 2.36 13 3.36 -36 13.2 12.5 2.2 mudstone 24 (blue) 45 1.3 1.3 3.5 2.53 9 4.38 179 31.6 30.1 2.7 conglomerate5 (light blue) 34 1.2 0.8 2.2 2.36 19 2.81 -62 4.3 3.8 1.8 sandstone 2

Tab. 3 - Calculated physical properties of clusters of the multivariate cluster analysis shown in Figure 2. Numbers in bold print representthe highest value of the property and illustrate its respective cluster (see Figure 1 for units). Each cluster has a different combination ofphysical properties. For example, clusters 1 and 3 have identical sgr values, but they have significantly different Th/K ratios andsusceptibilities, suggesting different sediment compositions and thus, different rock sources. (An explanation of the variables is given infigure 1).


in grey) in the uppermost section of the borehole. Thelowermost Tertiary section of the borehole from 630– 790 mbsf is dominated by the red sandstone withthe higher magnetite component than the bluesandstone. However, some of these red sandstones arealso indicated at 200 mbsf depth, which may indicatea short-term change in sediment source at this depth(time).

At the left margin of figure 3, the time scale,simplified core-derived lithology, and the cluster logare shown. In the next column, the cluster log wassimplified into three main units. These three mainunits can be compared to the provenance analysis bySmellie (this volume). An excellent correlation with aTaylor Group sediment source can be seen for thedepth section from 200 – 630 mbsf, and with aVictoria Group sediment source from 630 to790 mbsf. This supports the above assumption thatthe cluster log mainly summarises rock sourceinformation, and to a lesser extent lithologyinformation. The results of the cluster analysis andthe relation to sediment provenance can also becompared with some other results. For example, thecarbonate distribution (Dietrich, this volume) shows apattern similar to that of the cluster log. From 0 to220 mbsf the carbonate content is low, with anaverage value of about 3 %; between 220 and630 mbsf it is higher on average and varies between1% and 10 %; and below 630 mbsf carbonate isagain low and fairly constant at 2 %. These differentcarbonate contents may reflect diagenesis stages andinfluence the mechanical properties velocity andporosity and also the magnetic properties, since thechange at 630 mbsf is not accompanied by a velocitychange. This missing sharp velocity change may bedue to a more gradual and progressive change inprovenance. Detailed magnetic studies (Florindo et al.,this volume) yielded results, which fit excellently tothe cluster analysis. Magnetic studies on core plugsalso showed a change at 630 mbsf with elevatedsusceptibilities and natural remanent magnetisations.Magnetic hysteresis experiments also detected thechange in the grain size of magnetite at 630 mbsf(Sagnotti et al., this volume). These elevatedsusceptibilities can be related to a change in oxidationconditions and thus to a warmer climate representedby the sediments below 630 mbsf in CRP-3. Theseelevated susceptibilities can also be related to theδO18 curve for the southern ocean, indicating thechange from Oligocene to Eocene and a warmerclimate (Zachos et al., 1996). Clay analysis (Neumannet al., this volume) reveals a higher smectite contentbelow 600 mbsf, again confirming a major change atthis depth. The downhole logging results from theCIROS-1 drillhole (Bücker et al. 1998) show that theearly Oligocene/late Eocene change is alsoaccompanied by a susceptibility change at about430 mbsf from lower to higher values (Sagnotti et al.,

1998, Wilson et al. 1998,). Summarising all thesefindings suggests that the early Oligocene/late Eoceneboundary is encountered in the CRP-3 drillhole at630 mbsf.

COMPARISON WITH SEISMIC STRATIGRAPHY

The cluster log summarises lithological andsediment source information and is thus excellentlysuited for a comparison with the seismic stratigraphy(see Henrys et al., this volume). The simplifiedcluster log of figure 3 is shown in figure 4 with theDevonian section added below 790 mbsf down to thebottom of the hole at 939 mbsf together with theseismic stratigraphy of Henrys et al. (this volume).The Ross Sea seismic units V3, V4, and V5 fromCooper & Davey (1987) drilled by the Cape Robertsdrillholes CRP-1, CRP-2/2A, and CRP-3 are alsoshown in figure 4. Drillhole CRP-2/2A crossed theV3/V4 boundary at about 80 mbsf and the V4/V5boundary at about 440 mbsf. The V4/V5 boundarywas not crossed by drillhole CRP-3, but a majorseismic reflector was encountered at 200 mbsf inseismic unit V5. The cluster log for drillhole CRP-2Awas taken from Bücker et al. (2000b). Most of theseismic reflectors can be correlated with the results ofthe cluster analyses, but some could not. Forexample, the change from the blue cluster to the redcluster in CRP-3, which marks two differentsandstone types and a major change in sedimentsource, but not lithology, is not recognised as aseismic reflector. This is comprehensible whencompared with Figure 3. There is no major differencein the index properties density and velocity betweenthese two sandstones. Thus there is no seismicimpedance contrast and, therefore, no seismicreflector.

A major seismic reflector in between seismic unitV5 was encountered at about 200 mbsf in CRP-3.Cluster analysis identified this reflector as a change inprovenance as well as in lithology. The change inlithology from mudstone to sandstone is accompaniedby a change in porosity and velocity, resulting inenhanced seismic impedance and thus a seismicreflector. The upper part of the cluster log fordrillhole CRP-2A is dominated by values, whichrepresent mudstones. The seismic boundary betweenseismic units V3 and V4 was crossed at 80 mbsf, andbelow this depth no more mudstones with specificcharacteristics shown in black colour were observed.A major change in cluster pattern from black andgreen-dominated depth sections to blue and red-dominated depth sections is observed at about300 mbsf. No prominent reflector can be related tothis depth, but the depth marks a change in the dip ofthe seismic reflectors (change to steeper dips) andthus a tectonic and/or glacial event. The seismicboundary at about 400 mbsf in drillhole CRP-2A(seismic sequences V4/V5 boundary) may be related


to a change from one sandstone (blue clusters) toanother sandstone (red clusters). These two clusters(blue and red for CRP-2A) differ mainly in porosityand velocity, producing the seismic impedancenecessary for the seismic reflector.

In summary, it seems reasonable to comparecluster analysis results with seismic stratigraphy. Itshould be pointed out that seismic analysis is blind tolithological or sediment source changes that are onlyassociated with changes in radioactive or magneticproperties. This is evident for the Ross Sea seismicdata and will have consequences for the entire RossSea seismic stratigraphy, which has to take intoaccount that sediment properties are not simplyreflected by velocity and density.

CONCLUSION

With the Cape Roberts Project, the most completeand comprehensive set of in situ physical propertiesdata has been obtained for Antarctica. The main aimof this paper is a geological interpretation by takinginto account all information contained in themeasured downhole logs. For this purpose amultivariate statistical procedure is used, which leadsto the following results:

The interpretation of the huge amount of loggingdata is simplified, as the factor and cluster logs

respond to important sedimentological features suchas grain size, provenance, or glacial influence.

Physical properties of lithological units are clearlyand reliably differentiated using cluster analysis.

The basic geological controls (i.e. grain size,porosity, clay mineralogy) affecting the measuredpetrophysical properties are identified using factoranalysis, thus making geological interpretations mucheasier.

Comparison of the clustering results with seismicstratigraphy, and geological correlations with seismicunits, link the downhole measurements and lithologieswith the seismic lines.

ACKNOWLEDGEMENTS - The Alfred Wegener Institutfür Polar- und Meeresforschung, the Bundesanstalt fürGeowissenschaften und Rohstoffe, and the German ScienceFoundation supported this work. Special thanks go to PeterBarrett and Franz Tessensohn for their consistentencouragement, enthusiasm, and helpful discussions. Thepaper benefited from excellent comments by reviewersJeroen Kenter and Mike Lovell, many thanks to them!Borehole measurements and their logistics are never simplein these “icy” environments, and we would like to thankeveryone who helped make these measurements successful:once more, thanks to Pat Cooper, Alex Pyne, JohnAlexander, Jim Cowie, and many others at Scott Base andCrary Lab MCURDO, Antarctica. In particular, we would liketo thank Peter Schulze (his personal and technical

Fig. 4 - Fit of cluster analysis results into the seismic section (Henrys et al., this volume). The cluster log below the CRP-3 drilling rig isthe same as in Figure 3, the cluster column for drillhole CRP-2A was taken from Bücker et al. (2000). Some of the seismic reflectors canbe correlated to the cluster analysis but not all of them. For example, the change from the blue cluster to the red cluster in CRP-3, whichmarks a major change in sediment source, but not lithology, is not seen as a seismic reflector. A major seismic reflector in seismic unit V5was encountered at about 200 mbsf in CRP-3. Log analysis identified this reflector as a change in provenance as well as lithology. Fordrillhole CRP-2A a major change in cluster pattern can be observed at about 300 mbsf. No prominent reflector can be related to this depth.But this depth marks a change in the dip pattern of the seismic reflectors and thus a tectonic and/or glacial event. At about 630 mbsf thephysical properties of the sediments are changing, this change cannot be identified in a seismic reflector. For the interpretation of seismicline NBP9601-89 see Henrys et al. (this volume). See text for more details.


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