What is Spurious Frequency?

How Does Spurious Frequency Affect Seismic Data?

In seismic research, geophones are critical devices used to capture minute ground movements. However, these devices are not perfect, and one important parameter is “spurious frequency.” Spurious frequency is usually specified by manufacturers, but users often know little about it. So, what is spurious frequency, and how does it affect seismic data?

What is Spurious Frequency?

Spurious frequency refers to the resonance phenomenon occurring in a geophone in directions perpendicular to its normal working axis. This resonance is caused by a combination of multiple motion modes, typically appearing outside the geophone’s working axis. Geophones must respond to very small ground displacements.

For example, a particle velocity of 0.1 mm/s produces an amplitude of 3 mV in a geophone, which is caused by a ground displacement of only 160 nm at 100 Hz. This indicates that any mechanical micro-movement within the geophone can generate a voltage signal comparable to these tiny ground displacements.

Fig 1 from GEOPHONE SPURIOUS FREQUENCY: WHAT IS IT AND HOW DOES IT AFFECT SEISMIC DATA QUALITY? KEES FABER AND PETER W. MAXWELL

Traditional geophones use flat diaphragm springs designed to move linearly along the working axis while also being able to move freely perpendicular to this axis. Although this lateral movement is kept minimal, it is crucial for allowing the geophone coil to move freely along the primary axis.

The natural frequency is well understood as the resonance frequency of the spring-mass system along the working axis of the geophone. All other resonance peaks in the frequency response of the geophone above the natural frequency are referred to as “parasitic resonances.”

Impact of Spurious Frequency

Spurious frequency sets the upper limit of the usable recording bandwidth for geophones. At this resonance frequency, the phase and sensitivity of the geophone output change significantly. These changes, due to the combination of multiple modes, make the variations in amplitude and phase very complex and difficult to model.

In seismic data, spurious frequency often manifests as strong ringing noise throughout the recordings. Large amplitude initial signals can trigger spurious resonances. Since spurious frequency is easily induced by lateral or rotational movements of the geophone, it typically appears after the onset of ground roll.

The elliptical particle motion of ground roll can easily excite spurious frequencies in the geophone. In such cases, the spurious frequency of the geophone (typically around 180 Hz) does not match the anti-aliasing filters in the recording system, leading to noise contamination.

Mitigating Spurious Frequency

To reduce the impact of spurious frequency on seismic data, it is better to select geophones with spurious frequencies higher than the anti-aliasing frequency of the recording system. Strong high-cut filters can effectively attenuate signals caused by mechanical resonances within the geophone components. However, this choice has not always been adequately emphasized in the past, resulting in more noise in seismic data.

By carefully selecting geophones and using appropriate filters, noise interference caused by spurious frequencies can be significantly reduced, thereby improving the accuracy of seismic research.

Seis Tech offers a variety of specialized geophones, covering a wide range of applications from low to high frequencies. These geophones are meticulously designed and rigorously tested to provide high sensitivity and low noise signal capture in complex geological environments, ensuring the reliability and accuracy of the data.

Additionally, Seis Tech’s filter technology further optimizes signal processing, minimizing the impact of parasitic resonances and other interference factors on seismic data. This professional equipment and technical support form a solid foundation for high-quality seismic research.

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