ALBERT

Notes: Previously ignored characteristics of the seismic recording instrument are presently experienced as limitations as more sophisticated interpretive methods using wider frequency ranges are developed to extract stratigraphic information from seismic land data for hydrocarbon and mineral exploration. Most of these limitations arise from inadequate characteristics of the first element of the seismic instrument: the geophone. A geophone does not faithfully follow the motion of the earth for higher frequencies due to poor geophone-earth coupling. This filtering effect brings about time shifts that are dependent on the frequency and the soil type. A geophone can also produce spurious outputs, brought about by the motion of the suspended part of the geophone, with a magnitude comparable to that of the desired output. The suspension is made very compliant to obtain the required sensitivity. A compliant suspension, however, gives a large sag. The geophone can therefore only be used in one position, tolerating little tilt. A compliant suspension also widens the traveling range of the movable part. Minor sensitivity changes with travel are then noticeable as nonlinearity, since the surface wave is large with respect to the reflected wave. A compliant suspension is usually realized in the form of thin, spirally shaped spring-spiders. Such suspensions exhibit transverse or rotational resonances that are in or close to the seismic frequency band. Excited by ground roll, they can produce considerable undesirable output.The novel geophone we describe is a light-weight (17 g) acceleration-sensitive transducer which gives good ground coupling and partial correction for the increasing damping in the earth with increasing frequencies. It employs internal hybrid electronics for a magnetodynamic velocity-nulling feedback system. Velocity nulling makes the movable part of the geophone virtually rigid with respect to the housing. This makes the geophone characteristics independent of the suspension. The springs used are stiff in a transverse and rotational direction so that the suspension resonances are well outside the useful frequency band. This suspension also allows the geophone to be used in any orientation while being only sensitive to the vibration component along the main axis. The feedback system makes the sensitivity flat within 1 dB from 2 Hz to 500 Hz, with a phase tolerance smaller than 5°. The geophone is robust, has no moving internal wires, employs a current output [sensitivity 1 mA/(m s−2)] and internal gain so that the signal-to-cable-noise ratio is improved. This type of output allows parallel connection without any interaction between the geophones.