Downhole Seismic Velocity Logging Test
Objective
In this experiment, Velocity Logging Test (Downhole method) is done to find the physical properties of thesoil ground from site investigation. Velocity logging test includes the measurement of travel time, i.e., thetime from generating the elastic wave which propagates through the ground till it is observed by the receiverand to predict the propagating velocities required by elastic waves generated; P-wave (primary wave) and Swave (shear wave) to propagate through the soil ground concerned.
Testing Method and Procedure
1. A wooden board or a metal plate which is fixed on theground surface is hit to generate elastic waves (P-wavesand S-waves). Observation of both waves is done by thereceivers set up inside the borehole.
2. From the observed waveform data, readings for the initialpoint at which the waves were generated are made andthen the time required by each of P-wave and S-wave toreach the position at which receivers are placed isdetermined. Travel time required by the waves for thedifferent changed positions (depths) of receivers ismeasured.
3. Travel time data thus obtained is then arranged in the formof receiver’s position (depth) and travel time relationship.
Measured travel time data is then corrected for the casewhere the generation of wave is done within the borehole
itself. Plot a graph taking vertical axis for depth and horizontal axis for the corrected travel time.
4. Considering the measurement error, “travel~time curve” is plotted from the relationship of depth andtravel time. From the slope (gradient) of the travel~time curve, P-wave and S-wave velocity profiles arepredicted.
Report
The report should contain the following items:
1. The objective, principle, method and procedure of the downhole seismic velocity logging test.
2. Analyze the test data following the procedure and obtain the velocity profiles for S-wave.
Note the followings:
1.
Soil ground is assumed to be three horizontally stratified layers. For the depths of these soil
layer boundaries, refer to the data sheet/s distributed on the experimental day.
2.
Consider the offset as 50 cm and ignore the effect of wave refraction.
3. Comment on the relationship between N-value, results of velocity profile and hardness of the ground, etc.,by referring to descriptions such as soil classification (rough classification of soil), report items(description of soil condition), N-value, etc. given on the data sheet provided.
4. Note your impressions about the test.
***Submit the reports of Standard Cone Penetration test, Centrifuge Modeling test, and Velocity Logging
test, separately.
Record of horizontal movement (after Reverse Stacking)Recording for 3m, 5m and 7m are done on the experimental day. Data of each group will be uploaded on thesite given below.
http://wwwcatfish.dpri.kyoto-u.ac.jp/~goto/lecture.html

12Practice on Velocity Logging Test
Objective:
From the view point of velocity logging practices and problems, following items are learnt:
1. Method of knowing the internal condition of the ground.
2. One can understand meaning of “dynamic behavior of the ground”.
3. Importance of knowing the behavior of ground.
From the velocity logging test, following things are predicted:
1. Thickness of the ground layer.
2. P-wave and S-wave velocity of ground layer.
What is Velocity logging?
Assume the ground is made up following layers (stratification).

In this condition, it is difficult to know the thickness as well as behavior of the each ground layers.

Behavior ??

Layer thickness ?

Behavior ??

Layer thickness ?

Behavior ??

Layer thickness ?

Behavior ??
3

By saying behavior of the ground, there are different things to consider. Here, consider thebehavior of the ground as how fast the wave can propagate through that ground.
P-waveS-waveLet’s try to find the thickness, S-wave and P-wave of each layer.

P-wave velocity?

S-wave velocity?

Layer thickness ?

P-wave velocity?

S-wave velocity?

Layer thickness ?

P-wave velocity?

S-wave velocity?

Layer thickness ?

P-wave velocity?

S-wave velocity?

Structure of Velocity Logging
How about using the borehole?

Borehole4Measure the time required for propagating the waves.

Measurement is made at different depths.

Stacking
Methods of selecting the signal structure by piling up the waveforms recorded.

NoiseSignal

NoiseSignal

Changes every timeRandom summation
Random summation

Diminish5

Increment range

Records of vertical movement

Reverse Stacking
Records obtained by hitting from different directions are piled up
Method of selecting the components of S-wave

Hitting from side

Component of
P-waveOriginates similarly

Hitting from side

Negates each other

Records of horizontal moment

6

Component of
S-waveReversesComponent of
P-waveDiminishComponent of
S-waveIncrement range

Reading of arrival time of wave
Reading of arrival time of wave for each depth is read.
P-wave is obtained from the record of vertical movement
S-wave is obtained from the record of horizontal movement

Records of vertical movement

Records of horizontal movement

Way of finding velocity of ground layer
Relationship between the propagation of time and depth can be obtained.

Time

Depth

7

If the arrival time calculated is for the case as shown below (the top of the borehole), thencorrection for the time is necessary to make.
Corrected time

Depth

An appropriate travel~time curve is plotted.

Corrected time
Travel~time curve

Depth

8

Slope (gradient) of the travel-time curve represents the velocity.

Thickness

Slope=Velocity

Thickness

Corrected time

Slope=Velocity

Slope=Velocity
Depth

A model of velocity structure is made.

Velocity
Thickness

Thickness

Depth9