Downhole logging tools
Borehole Seismics Tool String
Versatile Seismic Imager (VSI*)
Description
The Versatile Seismic Imager (VSI) represents the latest available technology in the acquisition of seismic waves generated by a seismic source. The VSI employs three-axis single sensor seismic hardware and software and advanced telemetry for efficient transmission of the data from the borehole to the surface. It consists of three parts (a power cartridge, a control cartridge, and the measurement sonde) and takes its measurements by means of a three-axis gimbaled accelerometer package in the sonde. Each sensor package delivers high-quality wavefields by using three-axis geophone omnitilt accelerometers, which are acoustically isolated from the main body of the tool and provide a flat response from 3 to 200 Hz. The configuration of the tool (number of sensor packages, sensor spacing, and type of connection (stiff or flexible) varies to provide the maximum versatility of the array. A maximum of 20 shuttles can be used, though only one has been used so far in ODP and IODP.
Regardless of configuration, the tool is used to collect seismic data by anchoring in the hole at the desired depth using a caliper arm. When anchored, the accelerometer package is pressed firmly against the formation while remaining decoupled acoustically from the body of the shuttle. Air guns deployed from the rig by crane then provide the necessary source pulse, and the resulting acoustic wave is recorded downhole on all three axes. On the JOIDES Resolution, the guns are typically held in one location relative to the borehole while the tool is moved to each of the desired depth stations within the hole. The anchoring, size, and acoustic isolation of the sensors allow for suppression of the tool harmonic noise and removal of tube waves from the borehole-seismic band. Furthermore, digitization close to the sensor package helps reduce signal distortion.
Among the benefits of using the VSI tool are the increased operating efficiency (rapid mechanic deployment and reduced time between stations), the short shot-cycle time during remote source surveys, and the real-time quality control and data processing.
The VSI tool can be combined with a gamma ray tool for accurate depth control and an inclinometry tool for spatial orientation. For the sake of avoiding excess noise in the data and damage to the tool due to excessive weight in case of high heave, it is standard practice to run the VSI with only a basic gamma ray sensor and telemetry, normally both provided by the EDTC-B.
The VSI was first successfully deployed during ODP Leg 204.
Applications
- Integrated processing for interpretation of borehole and surface seismic data
- Planning for well placement
- Simultaneous surface and borehole seismic recording for high definition images
- Shear wave processing and analysis
Tool Specifications
| Temperature rating: | 350° F (175° C) | |
| Pressure rating: | 20 kpsi (13.8 kPa) | |
| Diameter: | 3.625 in (9.21 cm) | |
| Cartridge length: | 20.9 ft (6.37 m) | |
| Cartridge weight: | 190.8 lbs (86.5 kg) | |
| Shuttle length: | 6.4 ft (1.96 m) | |
| Shuttle weight: | 70.6 lbs (32 kg) | |
| Sensor length: | 11.4 in (29 cm) | |
| Sensor weight: | 6.4 lbs (2.9 kg) | |
| Sensor natural frequency: | 25 Hz | |
| Flat bandwidth in acceleration: | 2-200 Hz | |
| Length (multiple shuttles): | Up to 1,040 ft (317 m) for 20 shuttles | |
| Weight (multiple shuttles): | Up to 2,200 lbs (998 kg) | |
| Maximum number of shuttles: | 20 | |
| Stationary | ||
| Sampling rate : | Seismic waveform recording: 1, 2, 4 msec | |
| Combinability: | Bottom only | |
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