 The selection of specific downhole logging tools for a particular leg is an ongoing procedure that starts with the proponents and (usually) ends when the Program Plan is approved. The standard toolstrings (Triple Combo and FMS/Sonic) are always on the ship, often accompanied by one or more of the specialty toolstrings (GHMT, ARI, etc.). Specialty tool use is dictated by the scientific objectives of the cruise leg, and (inevitably) by the size of the year's budget for specialty tools. The operational plan for logging is determined at the pre-cruise meeting, usually held 6 to 8 months before the cruise, and is included in the cruise prospectus. Confirmation of the logging plan and time estimates are made onboard before each site at the "pre-site" meetings.
The Triple Combo is always run first, because it collects most of the basic petrophysical and lithological logs. It also measures borehole width, an important indicator of borehole and log quality. The FMS/Sonic is usually run next: the FMS resistivity image reveals the fine details of the formation, and the sonic velocity completes the basic logs. Then the specialty tools are run, usually in order of scientific importance. The WST is usually the final tool to be run, because the fact that it is clamped against the borehole wall means it can destabilize the hole, and it is perhaps the one tool most prone to getting stuck.
When time is short, or when there are adverse logging conditions, the Logging Staff Scientist has the responsibility of preserving the integrity of the logging plan and making appropriate changes to it if necessary. He will keep in regular contact with the Operations Superintendent and the Co-Chiefs, so as to be up to date with the latest operational developments. Shipboard scientists should understand that it is the Logging Staff Scientist’s job to act as an advocate for the logs, based on their scientific merit -- not just because they are part of the logging plan in the prospectus.
The principles to keep in mind when prioritizing toolstrings are:
- To get the logs most relevant to the leg's scientific objectives.
- To run the toolstrings most likely to get good results.
- To minimize the risk of harming the tools or getting them stuck down the hole.
The scientific, environmental, and technical issues relevant to toolstring selection are described briefly below.
Lithology
The natural gamma, magnetic susceptibility, and PEF logs yield information on aspects of the chemical and mineralogical composition of the formation, which can be used to infer lithology (see individual tool summaries). This information can then be used to fill gaps in the core record, to pinpoint boundaries, etc. The absence of gaps in the logs makes them particularly useful for studies of sediment cyclicity, where a complete record is essential.
Petrophysics
The porosity, density, resistivity, and sonic velocity logs collect petrophysical and geotechnical information about the penetrated formations. In sediments, the general trend in these logs is of consolidation with depth. Deviations from this trend are caused by lithological change, lithification (cementation), under-consolidation (due, for example, to high fluid pressure, or a framework provided by microfossils), or the presence of gas hydrates. (Hydrate in the pore space increases resistivity and sonic velocity.) The principal advantage of these logs over the equivalent core measurements is that the logs record the in-situ property, whereas the cores are expanded and depressurized, and can suffer from end-effects and biscuiting.
Relation to the seismic section, synthetic seismograms
The Well Seismic Tool (WST) and air gun are used for checkshot surveys (to obtain a depth-traveltime relation) and zero-offset VSP experiments (to obtain seismograms at the site). The depth-traveltime relation can also be derived from the sonic velocity log, which together with the density log and seismic source wavelet combine to make a synthetic seismogram. Thus, reflectors on the seismic section can be identified with lithological or petrophysical changes in the borehole.
For almost every leg there is an extensive (and extensively interpreted) set of site survey seismic sections, and so it is of great importance that the borehole information can be associated with seismic reflectors and mapped along the seismic lines.
Structure and fabric
FMS data provide resistivity images of the borehole wall, showing detailed structural (faults, fractures), sedimentological (turbidites, beds, bioturbation, concretions, clasts), and igneous (veins, alteration, and basalt pillows, breccias, and flows) features. Moreover, the orientation of these features can be analyzed, since the GPIT is on the same toolstring. Under favorable circumstances, a borehole televiewer (BHTV) or azimuthal resistivity imager (ARI) can provide images of the same features.
Crustal stress and anisotropy
Borehole televiewers measure the shape of the borehole, which can be interpreted in terms of crustal stress (the borehole is deformed according to the maximum horizontal stress direction). The FMS caliper arms will tend to follow the major and minor axes of the borehole if it is elliptical, and thus can also be used to infer stress orientation.
The Dipole Sonic Imager (DSI-2) can reveal sonic S-wave anisotropy, which may be due to crustal stress or a preferential rock fabric.
Magnetic polarity
The GHMT total field and magnetic susceptibility measurements are processed to find the magnetic polarity of the remanent magnetization of the sediment, which can then be used for magnetostratigraphic dating. Note that the GHMT can log when descending the hole, as well as while going up the hole, unlike the other toolstrings.
Heat flow, fluid flow
The Temperature/Acceleration/Pressure (TAP) tool records the temperature of the borehole fluid, which increases downhole. The borehole fluid temperature equilibrates towards the actual formation temperature over the course of the logging run, and thus gives a lower limit to the actual formation temperature. Where formation fluids locally enter the borehole, they will cause an anomaly in the temperature log.
The state of the hole for logging can be assessed from the conditions experienced during coring. Before logging, the Logging Staff Scientist will confer with the Operations Superintendent and drillers about the general condition of the hole, and whether there are any "tight spots" or likely washouts. The Schlumberger Engineer, the Operations Superintendent, the Drilling Superintendent, the drillers, and the core-techs all have a wealth of experience in dealing with adverse hole conditions, and should be able to advise on specific matters such as how long to spend trying to break through bridges, what the risk to tools might be, how to retrieve stuck tools, and so on.
 Logging-while-drilling (LWD) tools may be assigned to legs where hole conditions are anticipated to be unsuitable for conventional logging. Available LWD tools include the Azimuthal Density Neutron Tool (ADN), the Compensated Dual Resistivity Tool (CDR), and the Resistivity-at-the-Bit (RAB) Tool. In cases where real time acquisition of downhole data are required, Measurement-while-drilling (MWD) tools may be utilized.
Time-limited logging
Although adequate time for logging is usually allocated in the leg prospectus, it is not uncommon for unforeseen events (bad weather, difficult formations slowing the pace of coring, etc.) to reduce the actual time available for the logging program at a given hole. In this case, the Logging Staff Scientist will discuss with the Co-Chiefs the relative merits of allocating extra time to carry out the original program, cutting back on repeat runs or even forgoing a toolstring entirely. The Triple Combo will still be run first, but the others should be prioritized according to the leg objectives.
Bridged holes
Some holes may contain constrictions (bridges) that slow the toolstring’s descent into the hole. The heavier toolstrings (Triple Combo and FMS/Sonic) have a better chance of passing through a bridge than the lighter toolstrings (GHMT, WST); therefore, these are run first. One cause of bridges is swelling clays; this phenomenon can be combated by adding KCl to the drilling mud, although this will degrade the natural gamma potassium log. The capillary suction test equipment should be employed when swelling clays are suspected.
Blocked holes
There are various options if the toolstring cannot penetrate beyond a certain depth in the hole. If the blockage is near the base of the hole, it is probably best to just log the open interval above the blockage. If the blockage is midway down the hole, several options exist: 1) log only above the blockage; 2) dismantle the logging cable and lower the BHA to tag the blockage, then raise the BHA back to the original position; or 3) tag the blockage and only log below it. If the blockage is near the top of the hole, it is likely that there will be similar blockages further down and the hole is unloggable, but dismantling the wireline cable and re-reaming the hole is always an option.
Wide holes
Wide holes can result in poor contact between the tool sensor and the borehole wall, and hence degraded logs. Affected tools are the HLDS and APS (max caliper extension 18"), the FMS (max 16"), and the WST (max ~18"). The borehole width is measured by caliper during the first (Triple Combo) run. Depending on the scientific objectives, it is sometimes preferable to run the GHMT (which is relatively insensitive to borehole width) before the FMS/Sonic.
High heave conditions
The wireline heave compensator (WHC) reduces the effect of ship heave on tool motion, but higher heave conditions lead to increased uncertainty in the downhole tool depth, particularly if the heave is too great (more than 6m) for the WHC to be used. Increased tool motion (up-down oscillation) poses a risk to those tools with caliper arms (e.g., the HLDT and FMS), as there may be downward tool movement even when logging upwards; higher logging speeds will help. Additionally, high heave makes the process of bringing the tools back into the pipe from the open hole after logging more difficult.
There is an increased risk of the wireline cable slipping on the cable reel when lowering the tools down though the pipe, especially at the start of the descent, because initially there is only a small weight to provide tension in the cable. Tools must be lowered slowly, adding to the logging time particularly in deep waters. The risk of cable slip is worse with the lighter toolstrings (GHMT, WST).
High temperature conditions
When in a high temperature environment (such as a hydrothermal ridge system), careful attention is paid to the temperature channels on (for example) the DIT-E. It is important not to exceed the tool temperature ratings. Circulating water in the hole immediately prior to logging will cool the hole for a period of time. Some measurements are temperature dependent (e.g., resistivity).
Logging tool limitations
The logging operation is limited to downhole tools with a diameter of 3.75 inches or less. All tools listed in the tool section of this document can be deployed in a standard bottom hole assembly (BHA). The absolute maximum tool diameter which can be run in a standard BHA is 3.81 inches, but this is pushing the tolerances to unsafe limits. To run tools up to 4.0 inches in diameter, the BHA can be modified by removing the Kinley crimper landing sub. This is strongly discouraged and formal approval would be necessary since this action would severely limit stuck tool recovery efforts.
Stuck/lost tools
This issue is discussed in more detail on the stuck/lost tools page. Needless to say, every effort should be made to avoid getting any of the tools stuck in the hole. The loggers are required to fish for any tool that is stuck or lost. It is particularly undesirable to lose the HLDT, as that tool contains a radioactive source; losing it would require cementing of the hole, a process that would take days and sour the mood on the ship considerably. Don't lose the GHMT either, as there are only two of them in existence.
In summation, losing a tool is awkward and unpleasant. Try very hard not to do it.
Conical side-entry sub (CSES)
The CSES makes it less risky to log under unstable hole conditions, however, it can increase the logging time by 50% or more, and cannot be used in shallow water depths. A more detailed discussion can be found in the CSES section on the Other Equipment page.
Dedicated logging holes
The more time spent coring a hole, the wider and more unstable it will become. For this reason, a fresh hole should provide better logs. However, the time involved is usually prohibitive.
Logging APC/XCB vs. RCB holes
The logging tools can pass through the APC/XCB bit, whereas the RCB bit has to be "dropped" at the bottom of the hole before logging tools can pass through. The hole cannot be deepened or bridges tagged after the RCB bit has been dropped. The RCB bit is about 2 inches narrower than the APC/XCB bit, so the RCB hole is less likely to be wide, and consequently better for the FMS.
The go-devil
It is important to understand the principles behind the deployment and operation of the go-devil. For details, see the go-devil section on the Other Equipment page.
ODP Logging Services provides support for broad aspects of third-party downhole tool deployment. Third party tools are designed and developed by investigators at other institutions involved with ODP and are reviewed by the JOIDES Scientific Committee (SCICOM) and the Scientific Measurements Panel (SCIMP) for deployment on the JOIDES Resolution. ODP Logging Services provides support to third party investigators in the areas of data acquisition systems and software, tool design and manufacturing assistance, and tool testing. Recently, successful deployments of the Lamont Shear Sonic tool (SST) and the WHOI Vertical Seismic Profile tool have been completed.
The need for custom-designed surface instrumentation, acquisition systems, and specialized power supplies has been addressed through the development of a multipurpose data acquisition system installed in the Downhole Measurements Lab (DHML). This system offers numerous benefits, including a standard computer platform from which to launch acquisition software, several power supplies, and a work space in the acquisition area devoted to third party equipment. Data telemetry software currently available includes modules utilizing a Windows 3.11/LabView4.0 graphic environment for acquisition of the following data types:
- Temperature
- Acoustic
- Depth and Heave
- Acceleration
Third-party tool support also includes the design and production of a cablehead crossover that allows third party tools to connect to the Schlumberger cablehead via an inexpensive, modified off-the-shelf connector. Hardware components currently available for third party tool support at LDEO include:
- PC-based data acquisition system at LDEO and on the JOIDES Resolution
- Multiple power supplies in a wide variety of voltage and amperage outputs
- Crossover for connecting third party tools to a Schlumberger-style cablehead
- Telemetry connection to a depth measurement system
- Access to pressure test vessel capable of 10,000 psi
- Access to 740 foot test hole at LDEO
- 22,000 feet of 7-46 wireline with terminations
Assistance during the development of third party tools is provided through ODP Logging Services personnel and the facilities available at LDEO. On-site facilities are available to assist in manufacturing, assembly, and pressure and field testing. Interested investigators should contact ODP Logging Services' Technical Services Manager, Greg Myers, at gmyers@ldeo.columbia.edu.
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