THE REFRACTION SEISMIC
The Procedures and Calculations of Refraction Seismic
The method of refraction seismic
The refraction seismic method uses a source of sound (a hammer or a black powder charge) and a recording unit (a seismograph) to calculate the section between the surface and the bedrock.
A digital seismograph manufactured by ABEM (an Atlas Copco company), a Swedish geophysical instruments maker.
"MINILOC" is a highly portable digital seismic recorder with 36 virtual channels.
The number of samples: 2048
The A/D converters: 16 bit
The frequency range: 2 - 1000 Hz
The recording time: 200, 400, 800 msec
The notch filters: 50, 60 Hz
The low cut filters: off, 35, 70, 105, 140, 175, 210, 245, 280 Hz
The Input impedance: 650 Ohm
The line of geophones
For a standard refraction record for up to 30 m. depth, a low frequency (14 +/- 0.7 Hz) geophones are disposed along 108 meters long linear array. The separation of recording points is 3 meters, and the sound source to geophone offset is 1.5 meter. The signals are sampled every 3 meters, which offers a resolution of 3 meters. The sound source, used on a solid surface, is a 15 pounds hammer with a rubber pad. A seismic gun, loaded with black powder, is used on a soft surface.
The preparation of terrain for recording a seismic section
The surface, along the planned sections, has to be cleared from the brush. The required width is 1.5 m.. A one point of topographic reference must be provided to tie the sections to the local topographic grid.
The data processing and mathematical transformations
1/. Initially, the record is processed to make sure that it is usable (without errors).
2/. Later, the software algorithm, approved by the seismic instrument manufacturer, calculates the section of distribution of sound velocities corresponding to the record. This process is independent of the number of layers, and maps vertical / lateral changes of sound velocities.
The final section is the best mathematical model, calculated by the optimization algorithm, fitting the data record. The result is a section of distribution of sound velocities.
To check the quality of a velocity section, a confidence chart is calculated. It represents a sampling rate per unit a surface of section. A high sampling rate increases the confidence level. The geophone array is adapted to obtain the high sampling rate where necessary. (For example the contact with bedrock)
The sound propagation velocity
Each type of fill material has a specific sound propagation velocity. The overburden is composed of three types of fill material: the dry topsoil layer on the top, the water saturated alluvium or eluvium layer in the middle, and the bedrock at the bottom.
The sound propagation velocities versus the average depth error
Due to lateral non-conformities of sound propagation in the topsoil layer an error affects all depth estimates. The profile of bedrock contact is not affected or modified; only the depth to it can have an error. A layer-independent optimization algorithm reduces this error to less than 5%, but this involves a time consuming data processing.
The quality of first arrivals
In refraction seismic, the values of first arrivals are used. The environmental noise generated by a rain, a wind, a water flow, a human / animal activity, or a 50, 60 Hz electric lines obscures the first arrivals generated by a low energy sound sources.
The seismic studies near a river, or near a frequently used road, are slower to do. In addition, of using digital filters, the operator edits first arrivals and any affected record is rerecorded.
Firing a seismic gun
Refraction seismic field examples
The example of a single record (alluvial terraces of Napo River)
The example of two records profile (the same area)
Locations of previous seismic contracts:
Rafal Swiecki, geological engineer email contact
This document is in the public domain.