Q & A - Borehole Receiver Array

Q1: What is the most significant difference between P/GSI's downhole seismic array and conventional downhole seismic arrays?

A1: P/GSI can deploy from 80 to 400 levels of three component geophones in one borehole without any practical bandwidth limitation. Conventional technology can only deploy a maximum of 16 three component levels while recording seismic data over 100 Hz. If you need to record data between 200 Hz and 400 Hz you can only record a maximum of 8 three component levels using a conventional wireline. If you need to record data between 400 Hz and 800 Hz you can only deploy a maximum of 4 three component levels using conventional wirelines. For recording high frequency borehole seismic data the choice is thus between 4 levels using conventional wireline technology or 400 levels using P/GSI's tubing deployed borehole seismic system.

Q2: What is the reason for the difference in the number of receivers that can be deployed by P/GSI's borehole arrays as compared to conventional technology?

A2: P/GSI's downhole seismic array's are deployed using production tubing or drill pipe conveyed seismic cables. Conventional arrays use 7-conductor wirelines. The largest array that can be deployed using a 7-conductor wireline digitizing the data with 2 ms sampling rate is 16 levels. The reason for the 16-level, 48 channel, limitation of a 7-conductor wireline is the 0.5 MB/second maximum data transmission rate of this wireline technology.

Q3: Any other significant differences between P/GSI's downhole seismic array and conventional downhole seismic arrays?

A3: The strength of the tubing used for the P/GSI array makes it possible to deploy many hundreds of receiver levels. The number of receivers that can be deployed using a 7-conductor wireline is limited by the strength of the wireline to a few tenths of receiver levels. In addition, a 7-conductor wireline based receiver array cannot be deployed into a highly deviated or horizontal well without being pushed by tubing or pulled by well tractors.

Q4: What are the main benefits of a long borehole seismic array?

A4: The main benefits of a long borehole seismic array are:

  1. Continuous sampling of the downgoing wave field, which records how the wavefield is changing with depth. The downgoing wavefield is used to deconvolve the upgoing wavefield, eliminating or dramatically reducing problems with multiples.
  2. A long array eliminates reshooting of source points, so changes of source related properties are eliminated.
  3. The survey will be recorded faster with a long array, so a long array survey will get your well back online faster.
  4. A 3D image from a long array has better lateral and vertical resolution, which leads to better targeting of the drilling location.

Q5: How does P/GSI's downhole array transmit the data to the surface?

A5: The P/GSI array is using seismic cables utilizing twisted pair data transmission technology with a dedicated twisted pair for each geophone.

Q6: Where is the data digitized using the P/GSI Array?

A6: The data is digitized on the surface using standard 24 bit seismic recording systems such a Sercel or Geometrics recording system.

Q7: What type of clamping mechanism is used to couple the geophone pods to the inside of the borehole?

A7: P/GSI's patented clamping mechanism is using fluid pressure conveyed by the production tubing to inflate bladders that clamp the geophones to the borehole.