Article history:Received 16 June 2011Received in revised form 10 June 2013Accepted 19 June 2013Available online 28 June 2013
Keywords:Oceanic lithosphereElectrofaciesDownhole loggingOcean drilling program
A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceaniclithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subductionzones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because ofthe difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution ofthe majority of geophysical remote sensing methods. Downhole logging data from drillholes through base-ment formations, when integrated with recovered core and geophysical remote sensing data, can providenew insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes,and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alterna-tive approach to determine the formation architecture and lithofacies of the oceanic sub-basement by usinglogging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS)imagery), which has the potential to become a key tool in deciphering the high-resolution internal architec-ture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulouslydeciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during OceanDrilling Program legs (1) advances our understanding of ocean crust formation and accretionary processesover both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholesto the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow theinvestigation of (i)magmatic lithology and architecture in the lower ocean crust and uppermantle; and, (ii) voidspace abundances in crustal material and the determination of complex lithology-dependent void geometries.
Oceanic lithosphere, which is formed at mid-ocean ridges, coversalmost 70% of the Earth's surface and facilitates chemical, heat, andwater cycles among the atmosphere, hydrosphere, lithosphere, andthe Earth's interior, contributing to the thermal evolution of the plan-et (e.g., Davis and Elderfield, 2004, and references therein). To under-stand the formation and evolution of the oceanic lithosphere, severaldecades of marine geological and geophysical research have been un-dertaken, in addition to extensive investigation of ophiolites (e.g.,Cann, 1974; Detrick et al., 1987; Kelemen and Aharonov, 1998;Maclennan et al., 2004; MacLeod and Yaouancq, 2000; Nicolas et al.,1988; Sinton and Detrick, 1992; Smewing, 1981; Wilson et al., 2006).
The evolution of research-vessel-based remote sensing technologyhas enabled the development of higher-resolution physical mapping,both at and below the ocean floor (Heezen and Tharp, 1977). Anumber of underwater technologies have been developed toconduct high-resolution mapping, remote sensing, and in situ sam-pling of outcropping vertical sections of oceanic lithosphere (Barkeret al., 2008; Ferrini et al., 2013; Heft et al., 2008; Karson, 1998; Karsonet al., 2002; Lissenberg et al., 2013; Pollock et al., 2009). Although theareal and depth coverage of shipboard geophysical remote sensingand the resolution of near-source observations by underwater vehiclesare excellent, these methods can only directly sample surficial featuresand indirectly observe sub-surface features, meaning that informationderived fromdirect sampling of sub-basement features is lackingduringthe integration of plane and cross-sectional views of the oceaniclithosphere.
To date, scientific ocean drilling is the only research tool that candirectly obtain in situ samples from oceanic sub-basement with signifi-cant vertical extent (e.g. Dick et al., 1999; Robinson et al., 1989; Wilsonet al., 2003, 2006). However, drilling into sub-basement material isoften challenging with core recovery rates that typically range from0% to 30%, and the material that is recovered is often lithologically bi-ased, with higher recovery of easily drilled sequences (e.g., Tominagaet al., 2009). As a consequence, recovered core pieces generally provideincomplete information on the succession that was drilled, meaningthat accurately reconstructing the in situ formation architecture andthe volcanic andmagmatic lithologies of drilled formations is extremelydifficult.
A complementary approach to drilling and coring of basement indocumenting the nature of in situ ocean lithosphere is downhole loggingof its physical properties (e.g., Fisher, 1998; Gaillot et al., 2007; Goldberg,1997), particularly using an in situ formation “imaging” tool, such as theSchlumberger Formation MicroScanner® (FMS). This tool measures themicroresistivity of the borehole wall in a “scanning” manner and is apowerful tool that can image the vertical section of the ocean crust at asub-meter-scale resolution. As an example, Fig. 1 shows a sectionthrough a sequence of pillow lavas—a type of lavaflow that is particularlycharacteristic of submarine volcanic eruptions. Pillow lavas can easily beidentified by the naked eye when they outcrop at the surface, for exam-ple in ophiolitic exposures and on the seafloor (Fig. 1A–C). However, bar-ring obvious cases observed in drill cores, the use of geophysical remotesensing or logging data (here represented by a resistivity log as an exam-ple of this type of data acquisition, which characteristically produces 1Ddownhole variations (Fig. 1E)) cannot unequivocally identify pillowlavas in sub-basement formations. If resistivity data can be collected ina 2D manner as microresistivity images, these data can then unequivo-cally identify pillow lavas by enabling the imaging of rounded pillowmargins, radiating fractures from pillow cores towards rims, and the
juxtaposition of individual flows (Fig. 1D). Even though such a tool hasbeen available for a number of years, and the acquired microresistivityimages have been shown to be highly useful during characterization ofdrilled and cored intervals, FMS data have been underused, and theremay be many wider applications still to be developed.
In this review, we outline the potential uses of FMS imagery inoceanic lithosphere studies by summarizing the following: the theo-retical background to rock electrical properties, the development ofdownhole resistivity and FMS tools, configuration of FMS tools, dataacquisition and processing, FMS studies conducted during investigationof the formation and evolution of intermediate–superfast spreadingcenters, and the potential usages of FMS imagery across the marinegeosciences.
2. Background
2.1. Development of downhole “imaging” tools
As is the case for the majority, if not all, downhole tools that havebeen developed to date, the resistivity logging tool was first used bythe hydrocarbon industry to enable more detailed characterizationof reservoirs in sedimentary basins. This characterization is the mostcritical component in determining whether a reservoir is economical-ly feasible during hydrocarbon exploration. In particular, detectingthe location and nature of the in situ distribution of weak zones,such as fractures and cracks, and determining the bulk permeabilityof the formation as accurately as possible are essential to ensure thesuccess of drilling and subsequent oil and gas production. Initially,the Borehole Acoustic Televiewer and the subsequently improved Ultra-sonic Borehole Imager (Fig. 2; Morin et al., 1989), were built andemployed in conjunction with the further development of downhole re-sistivity tools.
Resistivity tools have long been known to be useful in the deter-mination of sub-meter variations within borehole surfaces, as thesetools often have better data resolution compared with other acousticlogging tools and remote sensing. Throughout the history of scientificocean drilling programs, in situ resistivity experiments have beendeveloped in order to determine bulk properties of the oceaniccrust (Coggon and Morrison, 1970; Francis, 1982; Hyndman ad Drury,1976; Kirkpatrick, 1979; VonHerzen et al., 1983). These earlyworks be-came the foundation for using microresistivity imagery in conductingresearch on crustal processes in oceanic basement.
The first notable downhole-resistivity tool with the aim ofdocumenting lateral connectivity and variability of formation structurewas themodern dipmeter, a tool thatwas successfully employed to cap-ture the orientation and extent of fractures in individual formations onland (Pezard and Anderson, 1988; Pezard and Luthi, 1988; Fig. 2). Thistool is currently available as the Dual Laterolog (DLL) shallow- anddeep-penetration tools. Because it only enabled the determination of1D downhole data, in order to characterize and correlate laterallyextending features (e.g., cracks and fractures) intersected by the bore-hole, the dipmeter needed to be deployed over multiple holes (Pezardand Anderson, 1988).
To expand lateral coverage and more accurately capture the archi-tecture of the drilled formation, FMS tools were developed based onthe concept of capturing a 2D image of the in situ strata in a singlehole (Boyeldieu and Jeffreys, 1988; Ekstrom et al., 1987; Lloyd et al.,1986; Pilenko, 1988) and this approach was soon adopted by loggingexperiments in the drilled oceanic basement (see Section 4.1).
Fig. 1. Pillow lavas observed in various types of data: A) pillow lavas recovered from a drilled hole in the oceanic basement (Integrated Ocean Drilling Program (IODP) Expedition324, Shatsky Rise); B) the photo-mosaic of submarine pillow lavas taken from Human Occupied Vehicle (HOV) Alvin (Woods Hole Oceanographic Institution); C) pillow lavaoutcropped within a ophiolite sequence (Cyprus, Greece); D) pillow lavas “imaged” by Formation MicroScanner (FMS) in Ocean Drilling Program (ODP) Hole 1256D, GuatemalaBasin; and E) 1-D resistivity log acquired by Dual Laterolog (LLD: deep penetration, LLS: shallow penetration) of the same interval of the panel D.
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2.2. Electrical properties of in situ rock formations
The electrical properties of oceanic lithosphere vary significantlyaccording to the presence of conductive horizons, including fluid-and/or mineral-filled fractures, cracks, void spaces, and mineral com-positions of host rock formations. Variations in these properties canbe used to determine the in situ fracture porosity, crustal architectureand lithofacies present within the oceanic lithosphere (Walsh and Brace,1984; Becker, 1984; Pezard and Anderson, 1989; Pezard et al., 1989;Pezard, 1990; Revil and Glover, 1997; Revil et al., 1996; Ildefonse andPezard, 2001; Ildefonse et al., 2009).
Archie's law (Archie, 1942) andmodifiedmodels that account for thetotal conductivity of the porous media with respect to pore space geom-etry and conductivity caused by the presence of fluids (i.e., seawater) inthe pore spaces have been extensively used in investigating electricalproperties of oceanic lithosphere (Hyndman and Drury, 1976; Reviland Glover, 1997, 1998; Revil et al., 1998; Waxman and Smits, 1968). Adetailed understanding of formation conductivity is crucial when com-paring the results of in situ logging and hand-specimen-scale data fromrock formations because of the different electrical conduction paths(Bruce et al., 1965), alteration products (Becker, 1984), mineralogy andgrain size distributions in in situ oceanic lithosphere (Eunaudi et al.,2005; Ildefonse and Pezard, 2001; Ildefonse et al., 2009).
Revil and Glover (1997) and Revil et al. (1998) defined the macro-scopic electrical conductivity σ in a porous medium as:
where σf denotes pore fluid electrical conductivity, t+f is the fractionof electrical current carried by cations in the free electrolyte, and F
is the porosity-related electrical formation factor (Archie, 1942). Inaddition, the dimensionless parameter ξ was defined by Sen and Kan(1987) as:
ξ≡σ S
σ f≈2
3ϕ
1−ϕ
� �BSQv
σ fð2Þ
where σs denotes surface conductivity, ϕ is porosity, Bs is counterionsurface mobility, and Qv is excess surface charge per unit pore volumerelated to the cation exchange capacity (CEC; Pezard, 1990; Waxmanand Smits, 1968). Eq. (1) can be modified for high- or low-salinitypore fluids, with low-salinity fluids having electrical conduction domi-nated by σs, whereas high-salinity fluids have electrical conductiongoverned by F-related pore volume topology (e.g., Revil and Glover,1998). Eqs. (1)–(2) indicate that resistivity logs can be sensitive tomicro-scale lithological and architecture changes, both of which havedifferent mineral and water content cation-exchange capacities relatedto conductive pore-space fluids in porous media (e.g., Becker, 1984;Eunaudi et al., 2005; Ildefonse and Pezard, 2001; Pezard, 1990). Thephysics behind the resistivity tool responses during downhole mea-surements, which translates the origin and state of the formation con-ductivity described above, are explained in detail, e.g., in Ellis andSinger (2007).
3. Formation MicroScanner tools and imagery
3.1. Tool configuration
The Schlumberger FMS tool used in scientific ocean-drilling pro-grams is 7.68 m long and is made specifically for “slim holes” to en-able the tool string to fit in a drill pipe with a diameter of 4.125 in.It should be noted that the Formation MicroImager (FMI) tool, which
Fig. 2. A) A cartoon redrawn after Pezard and Anderson (1988a) as an example of theusage of dipmeter tools for characterizing in situ faults across multiple drill holes. Blacktick marks show the locations of potential fracture zones detected by dipmeter tool mea-surement, and blue sinusoid lines show interpreted faults based on the measurement;B) FMS imagery that includes two in ODP Hole 1256D. Sinusoids indicate the subparallelfractures seen in the borehole, interpreted as dike margins in the sheeted dike complex.Note the difference in scale between A and B; and C) a three-dimensional lava morphol-ogy imagery reconstructed by Borehole Acoustic Televiewer data (Morin et al., 1989).
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enables 360° lateral coverage of borehole walls, cannot be used in these“slimholes” (Ekstromet al., 1987; Gaillot et al., 2007; Lovell et al., 2005).
The FMS tool is part of a Schlumberger wireline tool string thatuses a hepta cable (7 conductor wireline) connected to the head ofthe logging tool string (Fig. 3A). This cable enables a data uplink ofup to 1000 bps, and the head of the tool string accommodates thetension and weight of the tool during logging operations. An unusedcable can accommodate a maximum tension of ~19,700 lb, with therecommended safe working load being half of this value. The tool ar-chitecture concentrates any load to a single weak-point within theupper part of the head, with the other end of the head including agold-plated connector that transmits signals from the tools to onboarddata-receiving hardware during data acquisition and processing (Fig. 3D).
The FMS tool is pad-based, with pads containing electrode buttons(Fig. 3E–H), located orthogonal to the tool string, that are closed dur-ing deployment. First-generation FMS tools were developed with27-electrode buttons, but current FMS tools typically accommodate16 buttons. The FMS tool sends a passively focused electrical currentinto the rock formation, with electrode buttons receiving return currents(Ellis and Singer, 2007; Jackson et al., 1990; Pezard and Anderson, 1989);tools can deal with a large range of dynamic responses between 0.1 andN10,000 Ω·m (Serra, 1989).
3.2. FMS data acquisition
The success of FMS imagery study is dependent on coherent dataquality and lateral coverage of the borehole wall. During loggingoperations, four orthogonal pads are pressed against the borehole
wall so that the tool can scan the formation exposed in the boreholewall. Sampling speeds can vary and are dependent on operationalconditions; speeds are commonly around 500–1000 m/h with a 2.5 mmsampling interval. A four-pad tool can cover up to 40% of the lateral bore-hole in a single run (Serra, 1989). During multiple runs, pad scanningpaths often deviate from one run to the next, providing a wider lateralcoverage of the borehole wall.
The FMS tool string is run together with a General Purpose Inclina-tion Tool (GPIT), which consists of an accelerator and a fluxgatemagne-tometer that provides the downhole location and rotation of the padswith respect to geomagnetic north. With GPIT data, FMS data are fullyoriented, enabling FMS imagery to be presented in two dimensions.
The quality of FMS responses during operation is a vital factor inobtaining reliable images. The tension or cable elasticity, depth, pres-sure, pad frictional forces, and the temperature of the wire-line are allmonitored in real-time, as non-constant or intermittent tool motionscan cause changes in tool velocity, thereby potentially negativelyinfluencing data quality. In particular, the following need special atten-tion during operations: borehole condition (can be measured with cal-iper tools), tool rotation (excessive rotation could lead to pad standoff),pad pressure, tool sticking and slipping, dead buttons, thickness of themud cake (N1.25 cm) and smear, hole deviation (can cause differencesin pad contacts), and pipe wallow (shallow borehole wall scars;Frisinger and Gyllensten, 1986; Serra, 1989).
In particular, operating the FMS tool in highly resistive formationscan cause significant problems. If the resistivity contrast between theformation and conductive borehole fluid is N10,000 Ω·m, only a verysmall amount of current can penetrate the formation from the FMStool, producing data with extremely low signal-to-noise ratios. Oneeffective method that can avoid this situation is to flush the boreholewith resistive fluid, for example by using freshwater instead of sea-water, in order to mitigate any resistivity contrast (e.g., http://www.ldeo.columbia.edu/BGR/ODP/ODP/LEG_SUMM/176/leg176.html).
3.3. FMS data processing
Systematic data processing can be conducted using the properprocessing software, with typical data processing including conversionfrom measured current intensity to variable-intensity color images,and electrode speed correction by shifting nominal depth between indi-vidual rows of buttons (cf. “normal moveout” in multichannel seismicreflection data processing; Fig. 4). This processing enables the calcula-tion of offset and current intensity corrections, and the determinationof signal quality.
Image processing can be performed in both static and dynamicmodes, where static image production uses a histogram equalizationtechnique to partition the whole-hole recorded interval into 17 clas-ses, with each class having the same data count (Serra, 1989). This re-sults in a normalized static image of the downhole FMS data, and isuseful when comparing variations over the entire depth-range.
Dynamic or enhanced image processing is used to emphasize localresistivity contrasts across a particular borehole depth interval andinvolves the application of a linear transform to the input data toensure a constant mean and standard deviation within a sliding win-dow. Mean and standard deviation values are computed inside a win-dow for each pad, and, in combination with GPIT-derived data, lineartransformation parameters are computed for the central line of eachwindow, with results dependent on window size (Serra, 1989). Win-dow sizes should be greater than the largest object in the interval, anddynamic images produced in this way are useful in determininglithofacies and to conduct core–log integrations (e.g. Serra, 1989;Tominaga et al., 2009).
Fully processed FMS images can be presented in either 3D or 2D,with 2D images that provide a horizontal view of the borehole from0° (geomagnetic north) to 360° often used during electrofacies in-terpretation (Fig. 1D). Images derived from FMS data are thought
Fig. 3. Wireline FMS tool configuration: A) The “head” of the wireline tool string; B) Hepta cable in the top part of the “head”; C) the “wireline” and hepta cable that can beconnected to the cable in the “head”; D) gold-plated connector to the tool strings at the bottom of the “head”; E) the top part of the FMS tool string; F) a cartoon showing theorthogonal pads of the FMS; G) 16-electrode buttons lined up at 2-rows at each pad; H) the dimension of an electrode button.
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to correspond to features at a distance of ~2 cm from the boreholesurface.
4. Determination of oceanic crustal architecture by logging dataduring scientific ocean drilling
4.1. Use of logging data in sub-basement drilling
Accurately determining the architecture and volcanic and mag-matic lithology of sub-basement formations during scientific ocean
Fig. 4. Data shifting (e.g. speed correction) between the data acquired between Row-1and Row-2 electrodes on a FMS pad.
drilling has been the foundation of drilling-based studies of the oceaniclithosphere. Although sub-basement drilling has provided a unique andpowerful tool to obtain this information, one of the major challengesduring these operations is the typically low rate of core recovery(b30%) and the biased recovery of certain rock types (e.g., less fracturedrocks),meaning that the recovered cores only provide piece-meal infor-mation on the nature of the oceanic crust.
The advantage of using physical property logging during drilling-based investigations into sub-basement formations is exemplifiedby the continuity of downhole data (e.g., ODP logging manual byConsortium For Oceanleadership, Inc., 2007; http://www.odplegacy.org/PDF/Operations/Science/Lab_Procedures/Cookbooks/Downhole/Logging_Manual.pdf). This approach means that data from drilledsequences, including unrecovered intervals, can be complementedby examining responses from a variety of wire-line logging tools(e.g. Kirkpatrick, 1979). As described in Section 2, resistivity loggingtools can identify changes in the physical and chemical state of rocktypes, such as variations in lithology, petrology, mineralogy, alteration,formation architecture, and the presence of cracks, voids, fractures, andveins. The way in which 1D logging of physical properties, includingshallow- and deep-penetration DLL logging, can indicate changes inthese variables has been extensively investigated using a variety of rockand alteration types, including studies by Becker (1984; ODP Hole 504B,bulk porosity from apparent resistivity), Lovell and Pezard (1990; ODPHole 504B, electrical properties on basalt with pore space morphology),Eunaudi et al. (2005; ODP Hole 1105A, electrical properties of gabbros),Bartetzko (2005), Bartetzko et al. (2001, 2002; log responses of mafic
rocks), Pechnig et al. (2005; log responses of acid and intermediate rocks),Delius et al. (2003; Leg 183 basalt/alteration), Bartetzko (2005; globalsummary of logging responses to hydrothermal properties in basalticformation), and Ildefonse et al. (2009; IODPHoleU1309D, electrical prop-erties of gabbros and troctolite).
However, the 1D nature of wire-line physical property loggingdata means that high-resolution and accurate determination of the li-thology and architecture of drilled formations is difficult. The “wiggles”produced by 1D wireline logging mean that interpretations are basedon classifying the amplitude and shapes of curves for different rockand alteration types. In addition, the difficulty in accurately locating in-dividual data points in the borehole wall, due to a lack of orientationtools, inevitably leads to an averaging of lateral changes in the forma-tion properties and architecture (e.g., Francis, 1982). In comparison,FMS logging imagery using 2D microresistivity data provides an effec-tive alternative that can overcome these weaknesses and can providebetter and more continuous lateral and vertical coverage of the bore-hole wall.
The first FMS data obtained during a scientific ocean drilling programwere acquired duringHole 792E of ODP Leg 126 in the Izu–Bonin Forearcin 1989. The objective of this logging operation was to map the locationof downhole borehole breakouts in order to study the in situ sub-seafloordistribution of stress (Taylor et al., 1990). Between 1989 and 2010, andbetween ODP Leg 126 and IODP Expedition 324, a total of 33 basementholes have been logged with the FMS tool (Fig. 5). These data are re-leased to the public domain after a 1-year IODP moratorium and areavailable through the Lamont–Doherty Earth Observatory Borehole Re-search Group data archive (http://brg.ship.iodp.tamu.edu/logdb/holes.php). In these basement drillholes, particularly holes with low rates ofcore recovery, the lateral data coverage offered by FMS logging hasprovided important information that enabled the mapping of downholelithologies and crustal architecture. In the majority of cases, interpreta-tion of FMS imageswas augmented by the acquisition of Ultrasonic Bore-hole Imager and other physical property data, as well as data obtainedfrom the recovered core.
FMS imagery has been used to determine and enable the detailedinterpretation of lithological and volcanic features such as lava flowtypes, flow folds, vesicles, veins, and breccias (e.g., Bartetzko et al.,2003, 2006), the presence of Fe-oxides that cause conductivity con-trasts (e.g., Haggas et al., 2005), and the continuity of these featureswithin drilled sequences (Fig. 6). The term “electrofacies” refers toan individual qualitatively determined lithofacies identified usingFMS imagery; this type of analysis is particularly useful in identifyingunrecovered, potentially fragile, or difficult to drill formations duringreconstruction of downhole lithostratigraphies. The locations of indi-vidual electrofacies are used to evaluate drilled sequences, and canoften provide new and useful information on the thickness, juxtapo-sition, and fracturing of lithologies that are typically only modeledusing recovered core intervals (Bartetzko et al., 2003, 2006; Bückeret al., 1998; Haggas et al., 2001, 2002; Itturino et al., 2005; Luthiand Souhaite, 1990; Tartarotti et al., 1998). This capability of FMSimagery analysis was first fully utilized during investigation of thePACMANUS basin hydrothermal systems in drillholes that containedcomplex interlayering of volcanic and hydrothermal facies (Bartetzkoet al., 2001, 2003; Brewer et al., 1998). Similarly detailed electrofaciesanalysis was also conducted during drilling in an intra-plate magmaticenvironment at IODP Hole 1347A (Shatsky Rise; Sager et al., 2010).Here, the juxtaposition of vesicles, pipe vesicles, and lava flow packetsidentified using FMS imagingwas used to determine changes in eruptionstyles and volumes over time during the formation of the uppermost sec-tion of a Large Igneous Province (Tominaga et al., in preparation).
4.2.2. Reorienting cores and relocating in situ structures by core–logintegration
Geomagnetically oriented FMS imagery is a powerful tool thatallows the determination of the in situ coordinates of recoverednon-oriented rock cores. Electrofacies analysis can assist in the relo-cation and reorientation of recovered core pieces by comparing andmatching features in a core piecewith FMS imagery. This core–log inte-gration is a highly useful technique that allows accurate downholestructural and paleomagnetic studies. The features used during core–log integration are identical to those identified during electrofaciesanalysis, although with slightly more emphasis on determining me-chanical signatures at moderate angles to the borehole that are pre-served in both cores and FMS images. It should be noted that core–logintegration is only possible if core recovery rates are sufficiently high;if recoveries are low, integration may also be possible by the juxtaposi-tion of key features in both recovered core pieces and FMS imagery. Inaddition, caution must be taken during core–log integration when fea-tures are oriented at a high angle to the dip of the drillhole, primarily be-cause the in situ locations of features identified in FMS imaging can bemeters distant from where the same features are observed in corepieces.
Investigating reoriented core features allows an increased under-standing of the link between drilled formations and regional tectonics(Fontana et al., 2010; Haggas et al., 2001, 2005;Morris et al., 2009). Usingre-oriented cores,MacLeod et al. (1992, 1994, 1996) reconstructed struc-tural features identified downhole during investigation of crustal rota-tions within the IntraRift Ridge of the Hess Deep rift valley (Célérier etal., 1996; MacLeod and Pratt, 1994; MacLeod et al., 1995). In addition,crustal fracturing of the incoming plate at the Tonga Trenchwas detectedby core–log integration (MacLeod et al., 1994), and integration of FMSimagery and paleomagnetic data provided new insights into the evolu-tion of an oceanic core complex at the Atlantis Massif (Morris et al.,2009; Pressling et al., 2012).
4.2.3. Crustal formation processes inferred from FMS imagesUnderstanding the crustal formation and evolution processes that
occur atmid-ocean ridges (MOR) is one of the keystones of geodynamics.Here, we discuss a number of drilled basement holes in intermediate to(super-) fast spreading MOR crust that have adequate FMS data toallow the combination of FMS imagery with marine geophysical remotesensing and surface imagery. This combination provides both verticaland lateral cross-sections of the drilled sub-basement that allow thelinking of horizontal and vertical changes in oceanic crustal architectureand lithofacies.
4.2.3.1. DSDP/ODP Hole 504B. The first drillhole through basement in anormal oceanic crust setting where a wide range of physical propertylogging data were used to augment recovered core observations wasODP Hole 504B, located in the Costa Rica Rift (Cann et al., 1983). Thebasement intercepted by this drillhole has been considered as the typeexample of oceanic crust formed in an intermediate-spreading-rate envi-ronment, analogous to the Penrose-style oceanic crust model (PenroseConference Participants, 1972). A total of seven Deep Sea Drilling Pro-gram (DSDP) and ODP drilling expeditions have been dedicated to deep-ening of this drillhole during 1979–1993, drilling through an intactsection of extrusives to the middle of the sheeted dike complex (Alt etal., 1993; Anderson et al., 1985; Becker et al., 1988; Cann et al., 1983;Dick et al., 1992; Honnorez et al., 1983; Leinen et al., 1986). The dataobtained fromODPHole 504B basement havemade significant contribu-tions to our knowledge of crust formation andhydrothermal processes inmid-ocean ridge environments (Alt, 1995, 2004). Average core recoveryrates in ODP Hole 504Bwere 29.8, 25.3, and 4.8% in the extrusive, transi-tion and intrusive sections, respectively (Alt et al., 1993).
Early logging operations were conducted in this drillhole (Beckeret al., 1982; Becker, 1984; Francis, 1982), and Pezard (1990) proposeda downhole lithostratigraphy based on 1D resistivity logging
Fig. 5. A) A summary of logged basement holes during Ocean Drilling Program and Integrated Ocean Drilling Project from 1989 to 2010. “Basement” here includes oceanic basementof fore- and backarc basins, seamounts, guyots, Large Igneous Provinces, rifts, Mid Ocean Ridges (both igneous and plutonic), and volcanoclastics; and B) location map of holes withmore than 200 m logged interval. Note that the distance between ODP Holes 735B and 1105A is only 1.3 km and these holes are indicated with a single star-symbol on the map.
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responses; however, as discussed previously, the combination of1D resistivity data with low rates of core recovery means that inter-pretation of detailed lava morphology is equivocal and the defineddownhole volcanic stratigraphy may be significantly inaccurate(e.g., Fig. 1E). Borehole Acoustic Televiewer imagery acquired duringODP Leg 111 was used to detect stress distribution downhole(Morin et al., 1989). Three-dimensional acoustic televiewer imagerycaptured pillow basalt texture in the middle of the transition zone(1475 mbsf); and from their detailed interpretation of the imagery,Morin et al. (1989) pointed out that the structure of upper oceaniccrust can be “…a complex interlayering of lithologies that extendsover a thicker vertical interval than previously thought”.
FMS imagerywas obtained fromHole 504B during Leg 148, althoughunfortunately the lateral coverage of this FMS data is 15% or less, pri-marily because one of the four pads failed at the beginning of one ofthe FMS logging operations (Alt et al., 1993). As a consequence, these
logging results only provided low-resolution images of the detailedcrustal architecture intercepted in this drillhole. Nevertheless, Ayadi etal. (1996, 1998a, 1998b) undertook coherent structural mapping inthe upper crust and sheeted dyke complex intersected by Hole 504B(2080–1900 m below the seafloor) using FMS imagery. In addition,Brewer et al. (1998) and Pezard et al. (1997) utilized FMS imagery tocharacterize the physical and electrical properties of formations en-countered in this drillhole.
4.2.3.2. ODP Hole 801C. The first extensive mapping of lava morpholo-gy in investigating the MOR crustal formation processes was under-taken in the fast-spreading crust intersected by ODP Hole 801C,located in Jurassic seafloor of the western Pacific (Barr et al., 2002;Révillon et al., 2002), with an average basement core-recovery rateof 47%. Barr et al. (2002) used core observation-based lithologies tointerpret logging tool responses, including FMS images, in an attempt
Fig. 6. Examples of volcanic electrofacies deciphered from FMS imagery: A) Sheeted dike complex (from ODP Hole 1256D); B) massive sheet flow (from ODP Hole 1256D); C) brec-cias (from ODP Hole 1256D); D) pillow lavas (from ODP Hole 1256D); and E) Vesicles and pipe vesicles that indicate the degassing paths within a massive flow (from IODP Hole1347A).
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to reconstruct the downhole lithostratigraphy. Their core–log inte-gration and electrofacies analysis indicated that the abundance ofbrecciated sections increased by a factor of six downhole from thoseidentified during shipboard core descriptions. Furthermore, the pro-portion of pillow lavas was double that predicted from core observa-tions, allowing a more accurate estimation of the magnitude of thechemical cycle at the Marianas Trench associated with subduction ofthe Pacific Plate (Barr et al., 2002; Jarrard, 2003).
In addition to lithostratigraphic studies, comprehensive structuraland magnetic analyses using FMS imagery were undertaken in ODPHole 801C (Pockalny and Larson, 2003). Tivey et al. (2005) reconstructedlava loading processes at an ancient mid-ocean ridge by integrating FMSimagery-based mapping of downhole fracture orientations andGPIT-based magnetic forward modeling using a similar scheme thatwas employed during a lava loading study at the East Pacific Rise (EPR)(Schouten et al., 1999) and the CY-1A onland drillhole in the TroodosOphiolite of Cyprus (Schouten and Denham, 2000).
4.2.3.3. ODP Hole 1256D. The best FMS logging imagery of the oceanicbasement to date is from ODP Hole 1256D, drilled through crust of thesuperfast-spreading EPR (Teagle et al., 2006, 2012; Wilson et al.,2003). Hole 1256D is the first drillhole to penetrate an intact section ofupper oceanic crust and into underlying gabbro (Wilson et al., 2006).The volcanic section in Hole 1256D has an average recovery rate of
37%, and a comprehensive wire-line logging program was conductedduring Leg 206 of Expeditions 309 and 312 (Tominaga et al., 2009).One notable difference compared with any previous FMS logging datais the greater lateral coverage of the borehole wall by FMS imageryobtained from this drillhole, with coverage as high as 216° or across40–60% of the borehole wall. This high lateral coverage was achievedfor over 65% of the logged interval, primarily because multiple FMS log-ging operations were undertaken in Hole 1256D throughout the threedrilling legs that deepened this hole. Tominaga et al. (2009) determinedthe downhole lava morphology and architecture of the upper crust atSite 1256 using electrofacies and other physical property logs, providinga comprehensive electrofacies catalog that describes 10 individual lavaflow types, including previously unidentified breccias within the hole(Barr et al., 2002; Bartetzko et al., 2003). The downhole stratigraphicmodel derived from FMS electrofacies analysis provided amore accuratearchitecture of the igneous formations in Hole 1256D with a spatial res-olution of 0.1 m. Tominaga and Umino (2010) used the Hole 1256Delectrofacies-based downhole stratigraphy to reconstruct the relation-ship between lava flow types and ridge axis–ridge slope morphology toprovide the first spatial and temporal reference frame for modeling thehistory of lava deposition. Using these reference frames and assuming aconstant paleo-spreading rate, they calculated the construction rateof the upper crust at a lava-flow scale that virtually ground-truthed apredicted crustal construction process model for the EPR derived from
Fig. 7. A comparison among FMS imagery (left), recovered core (middle), and RABimagery (right). Modified from Bartetzko et al. (2006).
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seismic interpretation (e.g., Hooft et al., 1996). This study manifests thatwhen combined with the well-studied mid-ocean ridge surface geology,the flow stratigraphy model determined from electrofacies analysis canprovide the sub-meter scale cross-sectional realization of in situ oceaniccrust construction processes as previously predicted in ophiolites or tec-tonic windows (e.g., Gillis and Sapp, 1997; Karson et al., 2002;Yamagishi, 1994) and at the present-day EPR ridge axis (e.g., Auzendeet al., 1996; Ballard and Moores, 1977; Batiza and White, 2000;Embley et al., 1998; Haymon et al., 1993; Kurras et al., 2000; Perfit etal., 2003; Sinton et al., 2002; Soule et al., 2005, 2007; Umino et al.,2002; White et al., 2000, 2002).
5. The leading edge of physical property logging andmicroresistivityimagery analysis
Among a number of possible scientific studies that can utilizemicroresistivity imagery analysis, we here introduce two possiblecutting-edge research foci that demonstrate the full capabilities ofmicroresistivity imagery: (1) analysis of the lower oceanic crust andupper mantle in normal ocean crust, and (2) the location of waterin the oceanic lithosphere.
5.1. Characterizing the logging properties of the oceanic lower crust andmantle
Scientific ocean drilling was initiated to sample intact oceaniccrust and pristine mantle rocks from archetypical sections of normaloceanic lithosphere (“Project Mohole”, 1958–1966). This goal hasnot as yet been reached, primarily because of the numerous chal-lenges encountered during oceanic basement drilling (Teagle et al.,2004). The new science plan for the Integrated Ocean DiscoveryProgram has defined mantle drilling as one of the six major goals tobe pursued in the next stages of scientific ocean drilling (The SciencePlan for the International Ocean Discovery Program 2013–2023,http://www.iodp.org/initial-science-plan). Since 2006, a series ofworkshops has been held to prepare the drilling and coring technol-ogy necessary for this task, to select a suitable site, and to broaden thecommunity involved in this effort (Ildefonse et al., 2007; Programcommittee, 2011; Teagle et al., 2009). Drilling and coring strategieshave already been extensively discussed (Fig. 8 in “TheMohole A CrustalJourney and Mantle Quest” Workshop Report, 2010). The sampling ofmantlematerial will involve drilling to unprecedented depths in the oce-anic lithosphere; as such, it is important to use the most efficient coringstrategy to achieve this goal. If partial-coring proves to be the best ap-proach, alternative methods are needed to interpolate the lithology, ar-chitecture, and chemical state of unrecovered sections of drillcore. Evenif full-coring is undertaken, although drilling in the lower oceanic crusthas historically had better core recovery rates (e.g., 75% in 1309D, 87%in 735B) than volcanic sections, it is unclear whether such core recoveryrates will be achievable in formations deeper than a few kilometersbelow the seafloor. To obtain coherent data for the formations encoun-tered during drilling, in particular to construct a magmatic stratigraphyfor the lower crust and uppermantle, downhole FMS logging data shouldprove to be highly useful.
A number of drilling expeditions that focused on the lower crustand upper mantle have been conducted in various magmatic environ-ments. During these expeditions, the physical properties of gabbro andserpentinized peridotite samples from formations drilled and coredhave been measured (e.g., Eunaudi et al., 2005; Hyndman and Drury,1976; Ildefonse and Pezard, 2001). Although some of these drilledholes provided FMS and physical property logging data (e.g., Haggaset al., 2001; Morris et al., 2009), logging responses have rarely beenlinked to changes in petrology, mineralogy, and alteration in core sam-ples. Haggas et al. (2005) were the first to use the Fe-oxide content ofgabbros as conductive zone markers to enable the detailed decipheringof FMS imagery. To fully utilize FMS imagery for the determination of
magmatic stratigraphies, to complement recovered core data, and tofully characterize the nature of the lower crust and upper mantle, theapproach employed by Haggas et al. (2005) can be expanded to corre-late changes in the petrology, mineralogy, and alteration of drilled for-mations to variations in FMS and physical property logging responses.
A number of previous studies can also be considered; for example,short lengths of FMS logged sections from ODP Hole 735B (Haggas etal., 2001) can be integrated with detailed physical property studies ofHole 735B gabbro core samples (Ildefonse and Pezard, 2001). Similarly,the electrofacies analysis of gabbro encountered within Hole 1105A(Haggas et al., 2005; Miller et al., 2003) can be integrated with the de-tailed petrological and geochemical investigations of gabbro core sam-ples undertaken by Eunaudi et al. (2005). Furthermore, the detailedelectrofacies analysis and reorientation of core pieces from IODP HoleU1309D (Morris et al., 2009; Pressling et al., 2012) can easily be inte-grated with petrological and geochemical studies. Any future drillinginto the lower crust and upper mantle, in combination with compre-hensive physical properties and FMS logging, will contribute to this en-deavor, regardless of the magmatic origin of these samples.
In addition to utilizing FMS imagery analysis, an alternative downholeimaging method for normal oceanic lithosphere lower crust and uppermantle lithologies will be needed, primarily because wire-line loggingoperations are difficult and risky in ultra-deep drill holes. Resistivity-at-the-Bit (RAB) tools can also be used during Logging While Drilling(LWD) operations and are currently available as non-wireline boreholeimaging logging devices, where resistivity scanning electrodes arehoused above the drill bit. These tools obtain 2D borehole resistivity im-ages similar to FMS imagery while drilling (Bartetzko et al., 2003; www.ldeo.columbia.edu/BRG/ODP/LEGACY/PDF/LWD-RAB.pdf). RAB has beenused during drilling through sediments and enabled the imaging ofunrecovered and/or difficult to recover sedimentary sequences (NankaiTrough, Cascadia Margin, Gulf of Mexico); however, RAB has only beenused in a few basement drill holes to date (e.g., Leg 193 PACMANUSbasin logging summary (http://www.ldeo.columbia.edu/BRG/ODP/ODP/LEG_SUMM/193/leg193.html); Leg 209 — Hole 1275C abandoned), andthe intense tool rotation involved in basement drilling meant that theRAB imagery was more smeared than FMS imagery, even after speedand azimuth corrections were applied, resulting in lower-resolutionelectrofacies (Fig. 7). Despite the resolution issues, the images obtainedusing RAB were still continuous, and, by combining with other physicalproperty logging data acquired from the LWD, RAB imagerymay becomea useful addition to FMS imagery in identifying lithofacies of the lowercrust and upper mantle, and in characterizing the architecture.
5.2. Water in the oceanic crust
Seafloor covers nearly 70% of the Earth's surface, and provides akey interface for chemical and heat fluxes among the atmosphere,lithosphere, hydrosphere, and biosphere. The upper oceanic crust con-tains void spaces in a variety of forms and at various scales; e.g., mineral
Fig. 8. The downhole plots showing correlations between different physical property measurements and observations in ODP Hole 1256D. A) electrofacies-based lithostratigraphy(color convention is same as Figs. 6 and 7); B) Vertical Seismic Profile (VSP) (red line, 0.5 m station interval, see Swift et al., 2008) and wire-line sonic compressional (P-wave)velocity log (green line); C) wire-line natural gamma ray (total counts of U, Th, and K) log; D) Apparent conductive area (%) calculated from the image analysis from each lithofaciesdownhole; and E) example of FMS images from three end-member lava facies used for the image processing—note that 1). the threshold of the gray-scale was set for 60%, and thisscale adjustment could be arbitrary, and 2). the electric current sent by the FMS tool string penetrates to a depth of ~2 cm into the formation.
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matrix, pore space, cracks, and fractures (cf. Schultz, 1995). The presenceof these features means that the architecture of upper oceanic crust isheterogeneous. The connectivity of void spaces in the oceanic crust is amajor control on the permeability of the crust (Fisher, 1998), which inturn controls the location and duration of high- and low-temperaturehydrothermal circulation at and distal frommagmatic centers—processesthat enable chemical, mineral, and nutrition circulations (Fisher andHarris, 2010). Knowledge of void spaces within the crust is important inorder to investigate hydrogeological, thermal, chemical, and biologicalrelationships.
One potential use of FMS imagery of the upper oceanic crust is toevaluate vacant spaces during hydrogeological studies. Surficial voidspaces in a formation can be evaluated as the proportion of the totalarea of conductive zones detected by FMS imagery. Although thesedata provide no information on the penetration of void spaces intothe formation, extrapolating the void spaces into volumes per drilledhole can provide new insights into the heterogeneity of the oceaniccrust.
As an example, Fig. 8 shows an apparent conductive area on a 2-cm(FMS current depth) thick sheet around the borehole wall from Hole1256D. The distribution and area of surface voids indicate that apparentconductive spaces in the 1256D crust which make up 10%–60% of therock mass vary depending on lithofacies (Fig. 8D). Void characteristicsalso vary between massive flow, breccia, and pillow lava sequence
lithologies in Hole 1256D, with massive flows containing steeply dip-ping fractures that contribute to crustal-scale permeability; in contrast,other lithologies contain few conductive zones (Fig. 8). In comparison,the pillow sequence containsmany interconnected void spaces, indicat-ing that pillow margins are easily altered by fluid circulation, as com-monly observed in ophiolites (Fig. 1). Within breccia sequences, thedistribution and size of void spaces are seemingly random throughout,and it is not clear whether these void spaces are connected. This sug-gests that the permeability of breccias within the ocean crust can below, particularly when these spaces are compacted, despite the appar-ent void spacewithin these units. Nevertheless, it is important to recog-nize that a large void space detected by the FMS and this sub-meterscale void space, which is typically difficult to be preserved in cores,may be filled with pore fluid thereby possibly fostering deep biosphere.
Acknowledgment
I would like to greatly thank all the Schlumberger engineers aswell as logging scientists who sailed on Ocean Drilling Program andIntegrated Ocean Drilling Program for their efforts in establishingthe history of basement logging, which has not necessarily been“standard” in hydrocarbon industry and hence always challenging. Iamparticularly humbled by all the efforts done by scientists at the Bore-hole Research Group of Lamont-Doherty Earth Observatory and
generous offer by Sanny Saito to use the GeoFrame atJAMSTEC-Yokosuka. I also would like to thank Timothy Horscroft andtwo reviewers whose constructive comments significantly improvedthis manuscript.
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