Effects of weathered layer (LVL) and permafrost

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	5
Pages	141 - 180
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

Problem 5.10a

Assume that raypaths have angles of approach of $10^{\circ }$, $20^{\circ }$, $30^{\circ }$, $40^{\circ }$, and $60^{\circ }$ in the subweathering where the velocity is 2400 m/s. For a weathered layer 10 m thick with velocity 500 m/s, how do travel-times through the weathered layer compare with that for a vertically traveling ray? What are the horizontal components of the raypaths in the LVL?

Solution

Referring to Figure 5.10a,

${\begin{aligned}\sin \theta _{1}=\left(V_{1}/V_{2}\right)\sin \theta _{2}=0.208\sin \theta _{2},\\\Delta t=0.010/\left(0.500\cos \theta _{1}\right){\rm {s,}}\\\Delta x=\left(10\tan \theta _{1}\right){\rm {m.}}\end{aligned}}$

Table 5.10a. Traveltimes and $\Delta x$ for raypaths in the LVL and permafrost.

Incident	In low-velocity layer			In permafrost
$\theta _{2}$	$\theta _{1}$	$\Delta x$ (m)	$\Delta t$ (ms)	$\theta _{1}$	$\Delta x$ (m)	$\Delta t$ (ms)
$0^{\circ }$	$0.0^{\circ }$	0.0	20.0	$0.0^{\circ }$	0	27.8
$10^{\circ }$	$2.1^{\circ }$	0.4	20.0	$15.1^{\circ }$	27	28.8
$20^{\circ }$	$4.1^{\circ }$	0.7	20.1	$30.9^{\circ }$	60	32.4
$30^{\circ }$	$6.0^{\circ }$	1.1	20.1	$48.6^{\circ }$	113	42.0
$40^{\circ }$	$7.7^{\circ }$	1.4	20.1	$74.6^{\circ }$	363	104.6
$42^{\circ }$	$8.0^{\circ }$	1.4	20.1	$90.0^{\circ }$
$60^{\circ }$	$10.4^{\circ }$	1.8	20.3

Substituting the values of $\theta _{2}$, we get the results in Table 5.10a. The traveltimes in the LVL vary by only 0.5% over most of the range of $\theta _{2}$, and, even for $\theta _{2}=60^{\circ }$, change by only 1.5%.

Problem 5.10b

For permafrost 100 m thick with a velocity of 3600 m/s, answer the questions in part (a).

Solution

We repeat the calculations of part (a) changing $V_{1}$ to 3.60 km/s and layer thickness to 100 m. The results are also shown in Table 5.10a. Because rays now have large horizontal components, the changes in $\Delta x$ and $\Delta t$ are considerable. This large ray bending makes corrections for permafrost very difficult. If $\theta _{2}>42^{\circ }$, upcoming waves are totally reflected.

Figure 5.10a.  Raypath bending at LV base.

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Table of Contents (book)

Also in this chapter

• Maximum porosity versus depth
• Relation between lithology and seismic velocities
• Porosities, velocities, and densities of rocks
• Velocities in limestone and sandstone
• Dependence of velocity-depth curves on geology
• Effect of burial history on velocity
• Determining lithology from well-velocity surveys
• Reflectivity versus water saturation
• Effect of overpressure
• Effects of weathered layer (LVL) and permafrost
• Horizontal component of head waves
• Stacking velocity versus rms and average velocities
• Quick-look velocity analysis and effects of errors
• Well-velocity survey
• Interval velocities
• Finding velocity
• Effect of timing errors on stacking velocity, depth, and dip
• Estimating lithology from stacking velocity
• Velocity versus depth from sonobuoy data
• Influence of direction on velocity analyses
• Effect of time picks, NMO stretch, and datum choice on stacking velocity

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