A receiver in seismic survey senses ground vibration and converts it into electric current. The most common type of receiver traditionally used in exploration seismic surveys is the geophone, which senses the ground vibration through a mass-spring system enclosed in a magnet as illustrated in a figure on the right. The frequency- amplitude relationship of a geophone is called "response curve," and is determined mainly by the size of hanging mass and stiffness of the spring.
The low-frequency side of the curve is more specifically defined than the high- frequency side, which usually is open with a gradual attenuation in response. In this sense, a geophone acts as a low-cut filter from input-output perspective. Geophone frequency represents this “cut frequency” (e.g., 4.5-Hz), which is not sharply defined but tapers down within a relatively narrow frequency band near this frequency as shown in two response curves in the figure.
The lowest (longest) frequency (wavelength) of surface wave a geophone can detect, therefore, is directly related to this "geophone frequency" as explained in the the figure below.
There are "vertical" and "horizontal" geophones depending on the sensing orientation of the mass-spring system. Horizontal geophones are commonly used for shear-wave (reflection and refraction) surveys with either transverse or longitudinal orientation (see figure below).