 The Dipole Shear Sonic (DSI-2) tool combines high-speed telemetry with simultaneous, 12-bit dynamic range digitization of an eight-receiver array. The sonde incorporates both monopole and crossed-dipole transmitters with an eight-station array of electronically configurable hydrophones for monopole and dipole reception. The MAXIS wellsite unit acquires and processes these data.
The DSI-2 tool combines new dipole-based technology with the latest monopole developments into one system, providing the best method available today for obtaining borehole compressional, shear and Stoneley slownesses. (Slowness is the reciprocal of velocity and corresponds to the interval transit time measured by standard sonic tools.)
Dipole technology allows borehole shear measurements to be made in "soft" rock as well as "hard" rock formations. Limited by borehole physics, monopole tools can detect only shear velocities that are faster than the borehole fluid velocity -- or in hard rocks only. Dipole tools overcome this fluid velocity barrier.
The DSI-2 is a multireceiver tool with a linear array of eight receiver stations, a monopole transmitter and two dipole transmitters. The receiver array provides more spatial samples of the propagating wavefield for full waveform analysis. The arrangement of the transmitters and receivers allows measurement of wave components propagating deeper into the formation.
The DSI-2 tool is distinguished from the DSI by an upgraded receiver section. The upgrade improves the shear measurements in slow formations. The unimproved DSI is no longer available on board the JOIDES Resolution.
The DSI-2 can be combined with most ODP tools.
The DSI-2 tool has several data acquisition operating modes, any of which may be combined to acquire digitized waveforms over each 6-in. logging interval. For waveforms, eight channels are digitized simultaneously with a 12-bit dynamic range.
1. Upper and lower dipole modes
Eight dipole waveforms from firings of either of the dipole transmitters -- 40 sec per sample, 512 samples/waveform.
2. Crossed dipole mode
Standard acquisition of 32 total waveforms, in-line and cross-line from both transmitters.
3. Stoneley mode
Eight monopole waveforms from firings of the monopole transmitter driven with a low-frequency pulse -- 40 sec per sample, 512 samples/wave form.
4. P and S mode
Eight monopole waveforms from firings of the monopole transmitter driven with a high-frequency pulse -- 10 sec per sample, 512 samples/waveform.
5. First-motion mode
Eight sets of monopole threshold-crossing data from firings of the monopole transmitter driven with a high-frequency pulse -- primarily for compressional first-arrival applications.
New fast tool bus and data reduction techniques have allowed double the maximum logging speed in most instances.
A switchable power regulator has enabled a one-third reduction in power needs, resulting in broader combinability with other tools.
Additional human-interface engineering has improved field acquisition quality and efficiency.
A new low-frequency transmitter driver improves signal-to-noise ratio and allows successful logging of extremely slow formations and greatly enlarged holes.
Improved waveform processing techniques have greatly improved vertical resolution.
New answer products utilize Stoneley slowness to evaluate fractures and indicate permeability.
In addition to the new dipole features, acquisition of the Stoneley wave velocity utilizes a low-frequency monopole energy pulse for highest-quality Stoneley measurements. Stoneley-derived permeability is useful for evaluating fractures as well as investigating deeply into the formation.
A new technique for detecting compressional wave arrival--digital first-motion detection (DFMD)--provides measurements that are compatible with previous sonic logs, in addition to a 6-in. vertical resolution compressional sonic.
Processing with the MAXIS wellsite unit displays a full wave and its component characteristics. Its high-speed array processor uses the slowness-time-coherence (STC) method to determine compressional, shear and Stoneley slowness values. A choice of band-pass filters permits utilization of the optimum frequency range within a mode. The process reliably provides unambiguous transit times even in difficult borehole conditions. The resulting values are useful inputs for mechanical properties, formation evaluation and seismic applications.
1) Transmitter section
The transmitter section contains three transmitter elements: one omnidirectional monopole ceramic transducer and two unidirectional wide-band electrodynamic dipole transducers oriented perpendicular to each other. Wide-band transducers are preferable to a single narrow-band source because they allow examination of the entire frequency spectrum without phase-matching problems at their resonant frequencies and are not subject to reduced output because of aging. A low-frequency pulse drives the monopole transducer for Stoneley wave excitation, and a high-frequency pulse drives it for compressional and shear measurements. A low-frequency pulse drives each dipole transducer for the creation of shear waves. In addition, a new low-frequency source option provides excitation below 1 kHz for extremely large holes and for very slow formations and shear waves.
2) Isolation joint
The isolation joint is a mechanical filter that keeps the transmitter signals from traveling up the tool.
3) Receiver section
The receiver section contains eight receiver stations spaced 6 in. apart and spanning 3.5 ft. Each station contains two hydrophone pairs: one oriented in line with the upper dipole transmitter and the other in line with the lower dipole transmitter. The outputs from each pair are differenced for dipole reception and summed for monopole reception. Receivers are carefully matched during manufacture.
4) Acquisition cartridge
The acquisition cartridge contains the circuitry to perform automatic gain control, digitize eight separate waveforms simultaneously, stack these waveforms from more than one firing and then transmit the signals uphole. Threshold detectors for recording amplitude threshold crossing times for each waveform are also present. These are for compressional first-motion detection and allow derivation of compressional slowness in a manner similar to the analog threshold detection scheme used in conventional sonic tools.
1. Monopole compressional and shear
Compressional and shear waves (sometimes referred to as p- and s-waves) are excited in the formation, along with various modes in the borehole, by a monopole source operating at high frequencies (typically 10-20 kHz). They propagate as body waves in the formation and along the borehole. As they do so, they leak energy (refract) back into the borehole, creating headwaves in the borehole fluid. Compressional waves propagate along the borehole in the direction of the borehole axis with minute vibrations (or displacements) of the formation in the same direction. Shear waves propagate in the direction of the borehole axis with minute radial vibrations of the formation.
Monopole shear waves have a lower velocity (higher t), generally a larger amplitude, and a slightly lower frequency than the compressional waves. Shear waves have a larger refraction angle than the compressional waves. The mud speed is usually nearly constant, so that the refraction angle depends on the phase velocity of the body wave in the formation. As the shear t becomes large (soft formations), less shear energy is refracted back into the hole. If the shear t surpasses the mud slowness (typically 190 sec/ft), none of the shear waves will be detected by the receivers.
2. Monopole Stoneley
At low frequencies, perhaps a few kHz, where typical wavelengths in the mud are greater than the borehole size, monopole signals are dominated by the Stoneley wave, a dispersive mode of the borehole. Stoneley waves are guided waves associated with the solid-fluid boundary at the borehole wall, and their amplitude decays exponentially away from the boundary in both the fluid and formation. At extremely low frequencies, the slowness of this mode approaches that of the tube wave, while at higher frequencies, it approaches that of the Scholte (planar interface) wave. It is most easily excited using a low-frequency monopole source. For all frequencies, the Stoneley slowness is determined predominantly by the mud and to a lesser extent by the formation compressional and shear slownesses, formation permeability, and other variables.
3. Dipole shear
In a dipole shear sonic tool, a directional (dipole) source and directional receivers are employed. The source is operated at low frequencies, usually below 4 kHz. Compressional and shear waves are excited along with a dispersive flexural mode of the borehole. The slowness of this mode has the same high-frequency limit as the Stoneley wave, but at low frequencies it approaches the formation shear slowness rather than the tube wave slowness.
The amplitudes of both the flexural and the shear wave are peaked in frequency, the flexural generally peaking higher. They fall off very rapidly toward low frequencies and more gradually toward high frequencies. The flexural mode dominates the response down to very low frequencies where the shear wavelength is several times the borehole diameter. At such low frequencies, the direct shear wave is the only appreciable feature on the waveform. However, the amplitude of the waves at these frequencies (below 1 kHz for a typical slow formation) is very low and noise is likely to be a problem. A practical frequency range is 1-4 kHz. In this range, the flexural mode dominates the signals, but travels at nearly the shear slowness. A continuous shear log then is obtained by measuring the flexural slowness at as low a frequency as is practical and applying a small correction.
In very fast formations, the dipole compressional signal is usually very weak and may not be visible. The flexural mode is very dispersive in fast formations, there being as much as a factor of two difference in slowness between low frequencies (shear slowness) and high frequencies (Scholte slowness, approximately the mud slowness). The flexural arrival is therefore quite long in duration and spreads rapidly as the transmitter receiver spacing is increased. Low-frequency components traveling near the shear slowness become well separated from the slower higher frequency components. Often the (nondispersive) shear headwave is detectable in fast formations.
In slow formations, the flexural mode is again dispersive, but to a much lesser extent. Typically, the ratio between the high- and low-frequency limiting values of the flexural slownesses is about 1.2 or less. The flexural arrival is shorter in time duration and the spectral content is concentrated at lower frequencies. As in the figure, a higher frequency compressional arrival is often visible in slow formations, and in large boreholes and very slow formations can become the largest amplitude event. A distinct shear headwave arrival cannot be detected in slow formations.
Slowness-Time Coherence examines each waveform set for coherent arrivals across the array. It does this by stepping a time window of fixed duration through a range of times across the waveforms and a range of slowness across the array. For each time and slowness step, the waveforms within the window are added or stacked and the corresponding stacked or coherent energy is computed. When the window moveout or slowness aligns with a particular component moveout across the array, the waveforms within the window add in phase, maximizing the coherent energy. Coherent arrivals are thus identified by maxima in the coherent energy.
The STC module is used to find and extract slowness (Dt) and other information about various coherent arrivals in the sonic waveforms. Then the STC computation performs a sequence of operations on a set of waveforms aimed at identifying coherent arrivals in the set and extracting their slownesses. The following steps taken are: Waveform filtering, Waveform stacking, Peak searching, and Labeling.
An additional step is needed to identify and separate the desired arrivals (flexural, compressional, shear, or Stoneley) from any others. This is done by the labeling algorithm part of the STC computation. The slowness, arrival time, and coherence of each arrival are examined and compared with the propagation characteristics expected of the compressional, shear or Stoneley waves for the given physical conditions. Classifying the arrivals in this manner gives a continuous log of wave-component slowness versus depth.
STC processing of high-frequency monopole waveforms generally results in compressional and shear slowness estimates in fast formations. Narrow band filtering is applied to low-frequency monopole (Stoneley) waveforms, since this mode is dispersive and we want to estimate slowness within a consistent band of frequencies. In slow formations, no shear slowness estimate is available from monopole waveforms.
1. Dipole labeling bias correction
In STC processing of dipole waveforms, a coherence peak corresponding to the dispersive flexural mode occurs at a slowness near that of the frequency of peak excitation after filtering. The estimate is therefore biased slower than the true shear, and must be corrected. The bias depends on the time signature of the source excitation, the filter characteristics, the borehole size and shear slowness. In slow formations, the correction is less than 10%, and usually much less. In fast formations, where the dispersion of the flexural mode is greater, a large correction is required only in large (>17 in.) boreholes. In a fast formation with a moderate hole size (<12 in.), very little or no bias is found.
2. Depth-derived borehole compensation
One way to obtain borehole compensation is to derive slowness (delta t) measurements from both upward and downward propagating waves. The effects of borehole size changes tend to have an opposite effect on the slownesses derived from each. The standard BHC tool accomplishes this by having a transmitter above and below the receivers. The Long Spaced Sonic (LSS) tool, though, simulates this with depth-derived borehole compensation. The DSI-2 employs the same depth-derived technique. Instead of having transmitters above and below the array, it constructs a pseudo-transmitter array from several tool positions as it moves up the hole. The pseudo-transmitter array looks like an array of transmitters with one receiver above. This approximates a single transmitter on top with a receiver array below.
Depths of investigation for sonic devices depend on the formation type, shear and compressional slowness, the transmitter-to-receiver spacing, wavelength of the wave considered and whether it is a head wave or a guided wave, the source frequency and signal types.
Frequency determines the wavelength that drives the depth of investigation of the measurement.
Typical sonic wavelengths at different frequencies and slownesses are shown in the "Additional Specifications" table. Low frequency penetrates deeper into the formation and helps read beyond altered zones.
Numerical simulations verified by measurements from scale models show that when eccentering is small compared to the borehole radius, there is little change in the character of the dipole waveforms or in the STC-processed slowness values. Large eccentering, on the order of 2 to 4 in. in a 12-in. borehole, increases the flexural wave amplitude relative to the compressional. For the DSI-2 tool, the variation in the shear slowness estimate is ± 2 percent over the normal slowness range.
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Temperature Rating:
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350° F (175° C) |
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Pressure Rating:
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20 kpsi (13.8 kPa) |
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Tool Diameter:
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3.375 in (8.57 cm) |
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Minimum Tool Length:
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280 ft (85 m) |
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Sampling Interval:
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1, 2 and 4 msec |
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Max. Logging Speed:
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Stationary |
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Vertical Resolution:
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N/A |
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Minimum Hole Size:
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5.5 in (13.9 cm) |
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Maximum Hole Size:
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21 in (53.3 cm) |
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Tool Length:
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51 ft (15.5 m) |
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Maximum Logging Speed:
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One eight-waveform set (single mode) |
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3600 ft/hr |
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All six modes simultaneously, without 6-in delta t |
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1000 ft/hr |
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All six modes simultaneously, with 6-in delta t |
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900 ft/hr |
| Digitizer Precision: |
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12 bits |
| Digitizer Sampling Interval Limits: |
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Variable from 10 to 32,700 µsec per sample |
| Digitized Waveform Duration Limits: |
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Up to 15,000 samples / all waveforms |
| Acoustic Bandwidth: |
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Dipole and Stoneley |
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80 Hz to 5 kHz |
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High-frequency Monopole |
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8 to 30 kHz |
| Combinability: |
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All MAXIS tools, any resistivity tool |
* ®trademark of Schlumberger |
