The nodal seismometer(ZLAND) with a sampling frequency of 1000 Hz is buried in a shallow hole and the upper of ZLAND is exposed to the air. The seismic signal is excited by manual hammering at a distance of 20 m. Can both air waves and seismic waves be collected by Zland?
Dear Tao Xie,
The clue is the velocity in each king of records that you can identify in the shot-gathers for example.
Speed for the sound wave in the air is average 335 m/s, 1524 m/s in water and 3000 m/s – 6000 m/s in sedimentary rocks. Also, when you ask: “… will the ZLAND receive the air waves signals and the seismic signals transmitted on the ground at the same time? The answer is NO! Fortunately, NO!!!!!! due that not only the differences between air and rocks velocities and arrival time, but also because the former is noise, and the latter is signal (reflections).
Also, please, let me explain two more things that call my attention of your query.
The objective of the installation of the seismometers (geophone) array below the surface (“...buried in a shallow hole...”) is to significantly reduce or cancel the noise level.
For near-surface geological applications, 1 ms is more than sufficient (Nyquist frequency = 500 Hz) in the practice 1000 Hz sampling frequency too much.
Thinking about the noise that you must cancel or attenuate: for example, an airwave with velocity of 335 m/s and two geophones separated by 5 m, maximum attenuation is achieved when the waves recorded at the two geophones differ in phase by one-half wavelength. Therefore, lambda divide by two is five meters, and frequency is 335 m/s divide by ten, also the frequency is 33 Hz for air wave attenuation.
To summarize, seismic field records contain:
(1) reflections, (2) coherent noise, and (3) random noise.
Reflections on shot records are recognized by their hyperbolic travel times. If the reflecting interface is horizontal, then the apex of the reflection hyperbola is situated at zero offsets. On the other hand, if it is a dipping interface, then the reflection hyperbola is skewed in the up-dip direction.
Coherent noise I will mention:
1. Ground roll is recognized by low frequency, strong amplitude, and low group velocity.
2. Guided waves are persistent in shallow marine records in areas with hard water bottom.
3. Side-scattered noise occurs at the water bottom, where there is no flat, smooth topography.
4. Cable noise is linear and low in amplitude and frequency. It primarily appears on shot records as late arrivals.
5. The air wave with a 335 m/s velocity could be a serious noise.
6. Power lines also cause noisy traces in the form of a mono frequency wave.
7. Multiples are secondary reflections with interbed or intra-bed ray paths.
Random noise has various sources. A poorly planted geophone, wind motion, transient movements in the vicinity of the recording cable, wave motion in the water that causes the cable to vibrate, and electrical noise from the recording instrument all can cause ambient noise.
Please, let me know if you are looking for any other thing,
Best regards,
Mario E. Sigismondi
- Mario Emilio Sigismondi. Dr. Sigismondi, thank you for your careful answer. I'm sorry for the incomplete description of the question, but your answer has solved my question well.
- In the experiment, we focused on the surface wave. However, the Zland signals contained some high-frequency components up to 400Hz. The source of the high-frequency components is uncertain because there is no significant difference in arrival times over the short distance. This was confusing to us. After reading your answer, we are now almost certain that the source is air waves.
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