Description
The BHC sonde measured the time required for a compressional sound wave to travel through one foot of formation. The BHC consisted of an upper and lower transmitter arranged symmetrically on either side of two pair of receivers. The spacings T₁-R₂ and T₁-R₄ were 3 and 5 feet apart, as well as the spacings T₂-R₃ and T₂-R₁. The transmitters were pulsed alternately and the transit time of the compressional wave in the formation, measured in microseconds per foot, was given by: Δt=1/2 (T₁R₄-T₁R₂+T₂R₁-T₂R₃)
The BHC was used during the Deep Sea Drilling Project from 1975-1982 and the Ocean Drilling Program from 1987-1998. It was also deployed during IODP Expedition 302.

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Applications

Porosity and “pseudo density”: The sonic transit time can be used to compute porosity by using the appropriate transform, and to estimate fracture porosity in carbonatic rocks. In addition, it can be used to compute a “pseudodensity” log over sections where this log has not been recorded or the response was not satisfactory.

Seismic impedance: The product of compressional velocity and density can be used to compute synthetic seismograms for time-depth ties of seismic reflectors.

Sonic waveform analysis: If a refracted shear arrival is present, its velocity can be computed from the full waveforms, and the frequency content and energy of both compressional and shear arrivals can also be determined.

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Environmental Effects One common problem is cycle skipping: a low signal level, such as that occurring in large holes and soft formations, can cause the far detectors to trigger on the second or later arrivals, causing the recorded Δt to be too high. This problem can also be related to the presence of fractures.
Transit time stretching appears when the detection at the further detector occurs later because of a weak signal. Finally, noise peaks are caused by triggering of detectors by mechanically induced noise, which causes the Δt to be too low.

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Log Presentation
Delay times (µsec/ft) were usually displayed along with gamma ray and resistivity data.

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Tool Specifications

Temperature rating:	350° F (175° C)
Pressure rating:	20 psi (138 MPa
Diameter:	3.625 in (9.21 cm)

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Main Outputs

DT:	Delay time (5 ft) (µsec/ft)
TT1:	Transit time 1 (5 ft) (µsec)
TT2:	Transit time 2 (5 ft) (µsec)
TT3:	Transit time 3 (5 ft) (µsec)
TT4:	Transit time 4 (5 ft) (µsec)

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Deployment Notes
The BHC could be run alone or in combination with resistivity and gamma ray tools.

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Description
The APS sonde is the key module in the  Integrated Porosity Lithology system components. The electronic neutron source (minitron) allows epithermal neutron measurements and detector shielding, resulting in porosity values that are less influenced by environmental conditions. The near-array ratio epithermal porosity is the primary porosity measurement. Its source-to-detector spacing is optimized to yield a formation hydrogen index measurement that is essentially free of formation matrix density effects. Five detectors provide information for porosity, gas detection, clay evaluation, improved vertical resolution and borehole corrections.

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Applications

Porosity: In reservoir engineering the importance of porosity measurements is quite evident. In the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is critical to the correct understanding of the crustal structure, for two reasons: first, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and the large-scale fractures not sampled by drilling; and second, because other properties such as density, seismic velocity, and permeability depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.

Lithology: Because the hydrogen measured by the tool is present not only as free water but also as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals, or minerals such as gypsum, which has a high hydrogen index due to its water of crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings respectively).

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Environmental Effects
Eccentralization of the tool by a bow spring is the most important requirement to obtain reliable porosity measurements. The triple combo string utilizes an in-line eccentralizer to maintain consistent contact with the borehole wall. The eccentralizer is vital in preventing poor contact of the tool with the borehole wall, which can lead to attenuation of the formation signal by the borehole fluid and, in turn, the overestimation of the true porosity of the formation.
Hole size also affects the neutron log response; the formation signal, particularly for the epithermal count rates, tends to be masked by the borehole signal with increasing hole size.
In liquid-filled holes the influence of the borehole fluid depends on its salinity — chlorine is a strong neutron absorber — and density: the addition of weighting additives such as barite will yield a lower porosity reading. When the neutron tool is run through the drilling pipe and bottom hole assembly, the recordings should be used with caution. Because iron is a strong neutron absorber, an increased porosity reading will result, its degree depending on the thickness of the pipe.

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Tool Specifications

Temperature rating:	350° F (175° C)
Pressure rating:	20 kpsi (138 MPa)
Length:	13 ft (3.96 m)
Weight:	222 lbs (100.8 kg)
Diameter:	3.625 in (9.2 cm)
Sampling interval:	6 in (15.24 cm); 2 in (5.08 cm)
Maximum logging speed:	1,800 ft/hr (548 m/hr)

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Measurement Specifications

Vertical resolution:	14 in (35.56 cm)
Depth of investigation:	7 in (17.78 cm)
Accuracy:
0-7 pu:	±0.5 pu
7-30 pu:	±7%
30-60 pu:	±10%
Sigma:	±0.1/m

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Main Outputs

APLC:	Near/array limestone porosity corrected (%)
STOF:	Computed standoff (in)
SIGF:	Formation capture cross section (cu)
FEC:	Far detector count rate (cps)
ANEC:	Near detector count rate (cps)

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Deployment Notes
The APS was typically run with the HRLA/DIT, HLDS, and HNGS tools.

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Description
The Array Seismic Imager (ASI) consisted of an array of five seismic shuttles linked by a bridle to a signal-conditioning cartridge. Each shuttle sensor package contained three mutually orthogonal geophones fixed relative to the sensor package geometry. One geophone lay along the axis of the package (z-axis); the other two geophones (x- and y-axes) formed a 45° angle relative to the clamping direction. This design allowed the ASI tool to operate in wells with a 90°deviation while not exceeding the 45° limitation of the X and Y geophones. For the study of anisotropy and analysis of split shear, these features made the ASI tool reliable in both vertical and deviated wells, with consistent X and Y component response.
The ASI tool was unique in that it ensured consistent, lengthy coupling periods during downhole seismic acquisition, both in vertical and deviated wells. This feature made the ASI tool ideal for 2D and 3D time-lapse borehole seismic surveys, reservoir monitoring applications and amplitude variation with offset (AVO) calibration walkaways.
The ASI was deployed during ODP Leg 156 (North Barbados Ridge).

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Applications
The ASI acquired three-dimension walkaway vertical seismic profile (VSP) surveys in both vertical and deviated wells. One of the primary benefits was its low deployment time, since multiple geophones were deployed simultaneously.

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Tool Specifications

Temperature rating:	350° F (175° C)
Pressure rating:	20 kpsi (13.8 MPa)
Diameter:	3.375 in (8.57 cm)
Minimum tool length:	280 ft (85 m)
Sampling interval:	1, 2 and 4 msec
Maximum Logging Speed:	Stationary

Temperature rating:	350° F (175° C)
Pressure rating:	20 kpsi (13.8 MPa)
Diameter:	3.375 in (8.57 cm)
Minimum tool length:	280 ft (85 m)
Sampling interval:	1, 2 and 4 msec
Maximum Logging Speed:	Stationary

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Deployment Notes
The ASI tool could be used in cased holes without employing special equipment. Adding a bowspring assembly, however, allowed surveying of open holes from 8 1/2 to 13 in (21.5 to 33 cm).

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* ®trademark of Schlumberger

Description

The Borehole Geometry Tool Kit (Schlumberger Seismos Prakla VSP Tool) was a three-component VSP tool that was used for many years in the petroleum exploration industry. At the time of its deployment in ODP, it was the slimmest available Schlumberger 3-axis VSP tool.
Each of the BGKT’s three components contained eight 10 Hz geophones. The preamplifier was 20 or 40 dB, selected at the surface.
The BGKT was deployed during ODP Leg 176.

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Tool Specifications

Temperature rating:	500° F (260&deg& C)
Pressure rating:	23 psi (158.5 MPa)
Diameter:	3.75 in (95 mm)
Length:	10.5 ft (3.2 m)
Weight:	253 lb (115 kg)
Max. borehole diameter	16 in (40 cm)
Signal conditioning:	Eight 10 Hz geophones/axis with preamplifier (20/40 dB, selected at surface)
Other:	Monolithic tool, no stand-offs

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*®trademark of Schlumberger

Description
The CNT-G tool employed a chemical source (Am-Be) to bombard the borehole and the formation with fast neutrons (4.5 MeV) and two pairs of sensors to detect the number of neutrons (count rates) in the epithermal (100 eV – 0.1 eV) and thermal energy range (<0.025 eV). In the scattering process the neutrons interacted elastically with the atoms in the formation, were slowed down, and lost part of their kinetic energy with each collision; upon reaching the thermal energy level, they were absorbed by the surrounding nuclei. The amount of energy lost by the neutrons depended on the relative mass of the nuclei with which they interacted Because the greatest energy loss occurs during the collision with hydrogen atoms – which have a mass almost equal to that of neutrons – the slowing down and capture processes were mainly controlled by the hydrogen concentration in the formation. By taking the ratio of count rates at each pair of detector, a measurement of the porosity of the formation — compensated for the borehole parameters — was provided.
The response at the thermal detectors could be greatly affected by elements with a large thermal neutron capture cross section, such as chlorine, boron, gadolinium, and samarium; these are usually present in very small quantities in the borehole fluid or in clay or alteration minerals, yet they can cause the porosity of the formation to be overestimated. The epithermal detectors, instead, were less sensitive to these neutron absorbers and provided a more reliable measurement of the true porosity of the formation over clay-rich intervals. Because the epithermal neutron count rate is about one order of magnitude less than that for the thermal neutrons, the detectors were placed closer to the source in order to improve statistical variations.
The CNT-G was deployed extensively during the Ocean Drilling Program.

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Applications

Porosity
In reservoir engineering its importance is quite evident; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is most important to the correct understanding of the crustal structure: first, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and the large-scale fractures not sampled by drilling; and second, because other properties such as density, seismic velocity, and permeability, depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.

Lithology
Because the hydrogen measured by the tool is present not only as free water but as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals or minerals such as gypsum, which has a high hydrogen index due to its water crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings, respectively).

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Environmental Effects
Eccentralization of the tool by a bow spring could be very helpful in obtaining reliable porosity measurements. The lack of contact of the tool with the borehole wall during the recording results in the attenuation of the formation signal by the borehole fluid and, in turn, the likely overestimation of the true porosity of the formation. In the majority of ODP holes, however, the CNT-G was run without an eccentralizer.
Hole size also affects the neutron log response: the formation signal, particularly for the epithermal count rates, tends to be masked by the borehole signal with increasing hole size.
In liquid-filled holes the influence of the borehole fluid depends on its salinity – chlorine is a strong absorber – and density: the addition of weighting additives such as barite will yield a lower porosity reading.
In the Ocean Drilling Program, the neutron tool was sometimes recorded through the drilling pipe and the bottom hole assembly. Because iron is a strong neutron absorber, the effect was an increased porosity reading, depending on the thickness of the pipes.

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Log Presentation
The CNT-G provided an epithermal (ENPH) and thermal neutron porosity (NPHI) measurement. The porosity curves were presented either in decimal units or in percents along with the bulk density.

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Tool Specifications

Temperature rating:	400° F (200° C)
Pressure rating:	20 kpsi (138 MPa)
Diameter rating:	3.375 in (8.6 cm; without bow spring eccentralizer)
Length:	7.25 ft (2.21 m)
Weight:	120 lbs (54 kg)
Sampling interval:	6 in (15.24 cm)

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Measurement Specifications

Vertical resolution:	12 in (30.48 cm)
Depth of investigation:	~ 9 in (22.9 cm)
Accuracy:
0-20 pu:	±1 pu
30 pu:	±2 pu
45 pu:	±6 pu

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Main Outputs

ENPH:	Epithermal Neutron Porosity (pu)
TNPH (or NPHI):	Thermal Neutron Porosity (pu)
CFEC:	Corrected Far Epithermal Counts (cps)
CFTC:	Corrected Far Thermal Counts (cps)
CNEC:	Corrected Near Epithermal Counts (cps)
CNTC:	Corrected Near Thermal Counts (cps)
ENRA:	Epithermal Neutron Ratio
TNRA:	Thermal Neutron Ratio

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Deployment Notes The CNT-G was typically run in combination with the density and gamma ray tools.

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Description
The compensated neutron log tool (CNL) contained a 16-curie Am/Be radioactive source that bombarded the formation with fast neutrons and two detectors at different spacings from the source. The neutrons were slowed down through elastic collision with the formation nucleii and then captured, primarily by hydrogen atoms in the formation. The slowed neutrons deflected back to the tool were counted by detectors. The tool responded primarily to the hydrogen content of the formation because hydrogen has a mass close to that of the neutrons and thus is most efficient in the slowing process.
The measurements (ratios of far to near detector counts) were transformed to porosity units on the basis of laboratory calibrations. The 16-curie source produced neutrons at four times the rate of a standard nuclear source, greatly reducing statistical variation. The use of longer source-to-detector spacings increased the depth of the investigation.
The CNL was used during the Deep Sea Drilling Project.

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Applications

Porosity
In reservoir engineering its importance is quite evident; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is most important to the correct understanding of the crustal structure: first, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and the large-scale fractures not sampled by drilling; and second, because other properties such as density, seismic velocity, and permeability, depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.

Lithology
Because the hydrogen measured by the tool is present not only as free water but as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals or minerals such as gypsum, which has a high hydrogen index due to its water crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings, respectively).

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Environmental Effects
The tool was designed to minimize the effect of hole size. When run in combination with the FDC (formation Density Compensated Tool), the caliper measurement was used to apply an automatic correction for hole diameter. Other factors that might affect the porosity reading were temperature, salinity, lack of eccentralization (standoff) and type of drilling fluid.

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Log Presentation
The CNL was recorded in linear porosity units (or %) for a particular matrix lithology (limestone, sandstone, dolomite, usually limestone). When a CNL was run in combination with another porosity log, both curves were recorded on the same porosity scale. This overlay-type presentation permitted visual qualitative interpretation of porosity and lithology or the presence of gas.

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Main Outputs

NPHI:	Neutron porosity (%)
NCNL:	Near detector counts (cps)
FCNL:	Far detector counts (cps)

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Tool Specifications

Diameter:	3.375 in (8.6 cm)
Sampling interval:	6 in (15.24 cm)

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Deployment Notes
The CNL was usually run in combination with the FDC or LDT tools.

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*®trademark of Schlumberger

Description
The WST-3 was a Schlumberger three axis check shot tool for both check shot and vertical seismic profiles (VSP). It consisted of three geophones that pressed against the borehole wall and recorded the acoustic waves generated by an air gun located near the sea surface. The tool was designed specifically for use in an offset VSP experiment, where a remote seismic source would be fired from a second ship.
The WST-3 was used on several ODP legs in 2002 and 2003 and on IODP expedition 305.

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Applications
VSP data acquired by the WST-3 are useful for:

• Providing seismic interval velocities which can be compared to the rock sequence intersected by the borehole
• Placing the borehole results in their proper setting with respect to the seismically defined structure of the oceanic crust and mantle
• Correlating borehole lithology with the up-going seismic reflected wavefield
•  Predicting structure and lithology changes below the drill hole
• Estimating physical properties of rock on seismic scales by studying particle motion and downhole seismic attenuation. In check shot mode, the WST-3 data can be used to produce a depth-travel time tie and to calibrate the sonic logs and determine accurate drilling depths and their relative position with respect to targets on the seismic reflection profiles.

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Tool Specifications
Mechanical:

Temperature rating:	350° F (175° C)
Pressure rating:	20 kpsi (138 MPa)
Diameter:	3.625 in (9.21 cm)
Length:	19.9 ft (6.07 m)
Weight:	310 lbs (141 kg)
Sampling interval:	1, 2 or 4 msec (selectable)
Minimum hole diameter:	5 in (12.7 cm) with “short” arms
Maximum hole diameter:	19 in (48.3 cm) with “long” arms
Maximum logging speed:	Stationary

Sensors:

Axis:	3 axis
Geophone:	3 axis
Geophone type:	SM4 (3ea gimbaled)
Geophone frequency:	10 Hz
Damping:	60 dB
Sensitivity per axis:	83 V/m/sec or 0.8 V/in/sec at 25° C
Low-cut frequency:	0.2 Hz
Low-cut slope:	18 dB per octave
High-cut frequency :	250 Hz for 1 msec or 125 Hz for 2 and 4 msec sampling
High-cut slope:	36 dB per octave
Digitization:	Downhole
Sampling rate:	1, 2 or 4 msec (selectable)
ADC resolution:	11 bit + sign
Autoranger steps:	Five 6 dB steps
Preamplifier gain:	40 – 160 dB by 6 dB steps for each axis
Dynamic range per waveform (shot):	90 dB
Total dynamic range:	156 dB
Input noise level:	2 μV
Anti-aliasing filters:	330 Hz / 24 dB per octave

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Deployment Notes The WST-3 could be used in both checkshot and vertical seismic profile experiments. A remote seismic source was required for an offset survey, while a traditional check shot survey or zero-offset VSP could be completed with existing equipment on the JOIDES Resolution.
For each type of experiment, the deployment routine for the WST-3 was approximately the same. The main difference was simply the location of the source and the handling of the trigger pulse. For a check shot, a 120 in³ air gun was suspended by buoys at a depth of 3 mbsl, offset 48.5 m from the hole on the portside. The WST-3 was clamped against the borehole wall at intervals of approximately 50m, and the air gun fired five to seven times. The resulting waveforms were stacked and a travel time was determined from the median of the first breaks in each trace. These check shot experiments attempted to reproduce the seismic reflection profiling by simulating a similar geometry and source frequency.

The WST-3 was typically the last tool run and was always run alone. At each selected station, a seismic shot was produced at the sea surface using either air or water guns. Schlumberger provided a blast hydrophone for synchronizing the gun pulse with the system timer.

The WST-3 was sensitive to pipe noise and ringing of pipe following a shot. Efforts were made to reduce pipe noise at each station. If time and resources permitted, a drill string packer was deployed to dampen the banging motion of the pipe against the borehole. In addition, a 50 to 75 m distance was left between the tool and the bottom of pipe.

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