# Filter effects on waveshape

Series | Geophysical References Series |
---|---|

Title | Problems in Exploration Seismology and their Solutions |

Author | Lloyd P. Geldart and Robert E. Sheriff |

Chapter | 7 |

Pages | 221 - 252 |

DOI | http://dx.doi.org/10.1190/1.9781560801733 |

ISBN | ISBN 9781560801153 |

Store | SEG Online Store |

## Problem

Figure 7.12a illustrates filter effects. Evaluate the characteristics of (a) low-frequency cut, (b) high-frequency cut, (c) bandwidth, and (d) filter slope on (i) time delay to a point that could be timed reliably, (ii) apparent polarity, and (iii) ringing. The conclusions can be generalized for filters of other design types.

### Background

Minimum phase is discussed in Sheriff and Geldart, 1995, Sections 9.4 and 15.5.6.

### Solution

The first three columns in Figure 7.12a illustrate the effect of increasing the low-frequency cut and the last three columns do the same for the high-frequency cut. Broadly speaking, (i) as low frequencies are increasingly removed, the wavelet becomes more ringy (oscillatory) and energy shifts later in the wavelet; (ii) as high frequencies are removed, the pulse is lengthened without adding more cycles; (iii) as the filter slope increases, the energy is pushed later in the wavelet, resulting in pulse shape changes as evidenced by decreased ratios of first trough to first peak amplitudes.

- Importance of
**low-frequency cut**on- time-delay: the backward drift of energy means that the first pickable point may be shifted to later time;
- apparent polarity: the backward energy drift also means a weaker first half-cycle, increasing the chance of missing it and picking the next half-cycle with reversed polarity;
- ringing: increases ringing.

- Importance of
**high-frequency**cut on- time delay: lengthening the pulse delays the first pickable point;
- apparent polarity: no appreciable effect;
- ringing: no appreciable effect.

- Importance of
**bandwidth**: the 6–60 Hz passband is roughly 3.5 octaves, the 6–30 Hz is 2.5 octaves, the 18–60 Hz is 1.5 octaves;- time delay: with narrower bandwidth, energy shifts backward within the wavelet, making it more difficult to pick early cycles, thus increasing time delays;
- apparent polarity: the same effect increases the likelihood of mistaking wavelet polarity;
- ringing: ringing increases with narrower bandwidth.

- Importance of
**filter slope**on- time delay: increasing slope increases the delay;
- polarity: since the ratio of amplitudes of the first peak to the first trough changes, apparent polarity can be affected;
- ringing: increases ringing.

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Filtering effect of geophones and amplifiers | Effect of filtering on event picking |

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Characteristics of seismic events | Reflection field methods |

## Also in this chapter

- Radiolocation errors because of velocity variations
- Effect of station angle on location errors
- Transit satellite navigation
- Effective penetration of profiler sources
- Directivity of linear sources
- Sosie method
- Energy from an air-gun array
- Dominant frequencies of marine sources
- Effect of coil inductance on geophone equation
- Streamer feathering due to cross-currents
- Filtering effect of geophones and amplifiers
- Filter effects on waveshape
- Effect of filtering on event picking
- Binary numbers