Recording Equipment

Typical exploration surveys lines record from 48 to over 1000 channels of data per shot, where there might be anywhere from 50 feet to over 300 feet between takeouts or geophone stations. To avoid spatial undersampling or aliasing, the shot intervals and the takeout distances are usually no more than 1/2 the wavelength of your important reflection events. However, economic considerations prevent this ideal shot or receiver interval from being fully realized in most 3-D surveys. See Figure 1.7 for views of the recording box, geophones and the source used in a recent Moab, Utah experiment that searched for clues to a large meteorite impact.

To increase the signal-to-noise, at any one takeout station there may be many geophones (from 6 to over 48 in a group) connected in series to one another so that the summed signal is linked to one channel in the multi-channel recording cable. Similarly, there may be several sources spaced out so as to cancel the surface waves or ground roll (see top right picture in Figure 1.6). If the group length (i.e., maximum separation between any two geophones in a group) is about the same as the wavelength of the surface wave, then the serial geophones in a group tend to cancel the short wavelength surface waves while passing the long wavelength reflected waves.

  

Figure 1.7: Clockwise starting from top left: Receiver cables and geophones, cable spool, weight drop source, and 48-channel portable Bison recorder in Moab, Utah experiment.