THE APPLICATION OF BEAM-FORMING TECHNIQUE TO ENHANCE LOW QUALITY SEISMIC REFRACTION DATA
THE APPLICATION OF BEAM-FORMING TECHNIQUE TO ENHANCE LOW QUALITY SEISMIC REFRACTION DATA
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Date
1990-03
Authors
OGAH, Arewa James
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Abstract
The beam-forming (or delay and sum) method applied
to refraction data involves steering of seismic arrivals from
the same refractor for coherency, summing them and taking their
average to obtain a 'master trace' called the beam. Based on
the basic time-distance equations of refraction semismology, the
technique achieves signal-to-noise ratio improvement in the
process of forming the beam. Automatic determination of apparent
layerivelocities was achieved by using ranges of trial velocities,
and finding the velocity which maximizes the beam energy.
Corresponding intercept times were determined by statistical
estimation.
A computer programme written to perform the beam-forming
operation was tested on data consisting of noiseless synthetic
data, noisy synthetic data, and real refraction data from the
Kubanni Drainage Basin, Results with the syntetic data showed
that errors in velocity determination varied from 0.67 - 4.20%
for clean data to 1.00 - 6.00% for very noisy data; while for
intercept times, corresponding errors were 0.01 - 3.00% and
5.00 - 24.00% respectively.
Results from real data collected in the Kubanni basin
revealed existence of two distinct geologic interfaces separating
different rock units. P-wave velocity ranges of 527 - 700 m/s and
713 - 802 m/s correspond to different mixtures of silt, sand and
savannah loam, and a mixture of weathered laterite and savannah
loam respectively, which form the top layers at different locations.
A range of 937 - 1870 m/s corresponds to a mixture of wet sand and
clay that wholly or partly form the middle layer, while 2055 -
3663 m/s and 3810 - 4325 m/s correspond to weathered and fresh
basements respectively. Correlation of the refraction results with
water depths in some wells suggest that the Qquifers in the basin
have P-wave velocity range of 1667 - 2417 m/s. The results also
revealed that the basement depths vary between 12 m in some locations
to 24m in others, and that the general direction of dip of the
layers is toward the Kubanni river, although a local depression
exists in the southern part of the project area.
Description
Thesis submitted to the Postgraduate School,
Ahmadu Bello University, in partial fulfillment
of the requirements for the award of the degree
of MASTER OF SCIENCE IN APPLIED GEOPHYSICS.
Department of Physics
Faculty of Science
Ahmadu Bello University
Zaria, Nigeria.
March, 1990
Keywords
APPLICATION,, BEAM-FORMING,, TECHNIQUE,, ENHANCE,, LOW QUALITY,, SEISMIC,, REFRACTION,, DATA.