Elements of seismic surveying - An Introduction to Geophysical Exploration

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In-Depth Information

and potentially reveals more information about the

physical properties of each subsurface layer.

ber of channels of data rises into the hundreds. Modern

systems distribute the task of amplification, digitization

and recording of data from groups of detectors to indi-

vidual computer units left unattended in the field.These

are connected together to make a field computer net-

work using lightweight fibre-optic cables or telemetry

links. The separate units can then be controlled by a

central recording station, and upload their digital seis-

mograms to it on command.

Distributed systems

In seismic surveying the outputs of several detectors are

fed to a multichannel recording system mounted in a

recording vehicle. The individual detector outputs may

be fed along a multicore cable.The weight and complex-

ity of multicore cables becomes prohibitive as the num-

Problems

1. How does the progressive loss of higher fre-

quencies in a propagating seismic pulse lead to

an increase in pulse length?

2. A 10 Hz seismic wave travelling at 5 km s - 1

propagates for 1000 m through a medium with

an absorption coefficient of 0.2 dB l - 1 . What is

the wave attenuation in decibels due solely to

absorption?

3. A wave component with a wavelength of

100 m propagates through a homogeneous

medium from a seismic source at the bottom of a

borehole. Between two detectors, located in

boreholes at radial distances of 1 km and 2 km

from the source, the wave amplitude is found to

be attenuated by 10 dB. Calculate the contribu-

tion of geometrical spreading to this value of at-

tenuation and, thus, determine the absorption

coefficient of the medium.

4. What is the crossover distance for direct and

critically refracted rays in the case of a horizontal

interface at a depth of 200 m separating a top

layer of velocity 3.0 km s - 1 from a lower layer of

velocity 5.0 km s - 1 ?

5. A seismic pulse generated by a surface source

is returned to the surface after reflection at the

tenth of a series of horizontal interfaces, each of

which has a reflection coefficient R of 0.1. What

is the attenuation in amplitude of the pulse

caused by energy partitioning at all interfaces

encountered along its path?

6. At what frequency would a 150 Hz signal be

recorded by a digital recording system with a

sampling rate of 100 Hz?

Lavergne, M. (1989) Seismic Methods . Editions Technip, Paris.

SEG (1997) Digital Tape Standards (SEG-A, SEG-B, SEG-C,

SEG-Y and SEG-D formats plus SEG-D rev 1&2). Compiled by

SEG Technical Standards Committee. Society of Exploration

Geophysicists,Tulsa, USA.

Sheriff, R.E. & Geldart. L.P. (1982) Exploration Seismology Vol 1:

History.Theory and Data Acquisition . Cambridge University Press,

Cambridge.

Sheriff, R.E. & Geldart, L.P. (1983) Exploration Seismology Vol 2:

Data-Processing and Interpretation . Cambridge University Press,

Cambridge.

Waters, K.H. (1978) Reflection Seismology — A Tool For Energy Re-

source Exploration .Wiley, NewYork.

Zoeppritz, K, 1919. Uber reflexion und durchgang seismischer

wellen durch Unstetigkerlsflaschen. Berlin, Uber Erdbeben-

wellen VII B, Nachrichten der Koniglichen Gesellschaft der

Wissensschaften zu Gottingen, math-phys. Kl. pp. 57-84.