A versatile and easily implemented circuit is described that increases the velocity amplitude response of seismometers below their natural frequency in order to extend the useful bandwidth to lower frequencies. The circuit employs a simple design that counteracts the ω2 rolloff of the seismometer by cascading two one-pole active low-pass filters in series. The low-pass stages are each summed with the unfiltered signal to maintain a flat unity amplitude response near and above the natural frequency. The design, testing, and field implementation are described for a version of the circuit used with Mark Products L4-C 1.0 Hz seismometers. The design produced a nominally flat response extending down to 0.1 Hz with tolerable stability over a wide range of ambient temperatures. Design procedures are described so that the circuit may be easily modified for any arbitrary natural frequency and range of desired equalization. The circuit can be installed very easily in nearly any recording equipment with minimal modification of existing circuitry. An example is shown of the measured and theoretical amplitude and phase responses for an existing seismic event recorder with and without the equalization circuit installed. The agreement between the measured and theoretical responses is excellent for both of these instruments configurations.

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