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METHODST8

DETERMINATION OF THE NOMINAL DEPTH OF LAYERS, BY MEANS OF SHORT-RANGE SEISMIC WAVES,

FOR SUBSURFACE INVESTIGATIONS

SCOPE

This method covers the determination 01 the nominal depth of layers of rock or soil by measuring the velocity of compression waves through the ground The shock waves are produced by a hammer blow or other suitable means at varying distances from a geophone (receiver) which activates the electronic counters to record the time intervai. The velocity is roughly proportional to the density or hardness of the rock or soil.

2 APPARATUS

2.1 An engineering refraction seismograph. accurate to 0,5 milliseconds.

2.2 A geophone. (With some seismographs it is possible to operate wIth two or more geophones simultaneously: see 5.1 )

2.3 A sledge-hammer, approximately 6 kg, fitted wIth an Inertia switch, a cable and a connector plug (see 5.1)

2.4 A suitable length (say 30 to 50 m) of insulated cable to connect the hammer to the seismograph.

2.5 A striking plate, approximately 150 x 150 x 25 mm (see 5.1)

2.6 A measuring tape, 30 m long.

2.7 A sponge cushion. 100 mm thick.

3 METHOD

3.1 Preparation of apparatus

Survey the area where the test is reqUIred and select a suitable site which is as uniform and level as possible. On a slope the test should be done aiong the contour.

Connect the hammer cable to the hammer (see 5.1) and to the seismograph, and also connect the geophone to the seismograph. Place the seismograph on the sponge cushion to eliminate vibration eHects as tar as possible.

lay the tape along the line of test and mark the hammer statIons at intervals of 3 m, except that the first station must be 1.5 m from both the geophone and the second station.

Anchor the geophone firmly in the ground at one end of the traverse. If loose soil is present, the geophone is inserted 50 to 100 mm deep in the earth. If grass is present, this is first removed.

Switch the instrument on and allow a few seconds for it to stabilize. Set the gain control as high as possible without outside noise causing any disturbance (see 5.2). Depress the "Reset" switch. (See 5.4 for instruments with recording de­vices.)

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3.2

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Taking of ITMNlsurement. Place the striking plate at the first hammer station, 1,5 m from the geophone, ensuring that the plate is firmly seated on the ground. Ensure that the area im­mediatelY 'surrounding the geophone is cleared of all personnel and that no one is walking around.

Hold the hammer with the switch on the handle facing upwards, and strike the plate firmly (see 5.1).

Read the time, in milliseconds, on the instrument and record it (see 5.3).

Repeat the operation at least twice to confirm that tne reading bemg obtained is correct. False readings may be obtained where there is noise interference (small reading) or where the second cycle wave IS recorded (large reading) (see 5.2).

Move the plate and the hammer to the next station and repeat the procedure until the traverse is completed.

Switch off the instrument.

Record the distance of each station in metres from the geophone and the corre­sponding time intervals in mi lliseconds on appropnate seismic forms (see also 5.3).

A return traverse should be carried out in difficult terrain to confirm results.

CALCULATIONS

Eliminate false readings and calculate the average time interval for each sta­tion.

On graph paper, plot the time in mitliseconds against the distance in metres.

Assuming that a return traverse is obligatory over the same distance, it is im­portant that the total time for the forward traverse IS the same as that for the reverse traverse. If not, there could be an error, which should be checked and corrected.

Straight lines afe drawn through points having approximately the same lu:eanty to pass through zero. The velocity of these lines is calculated from their slope in metres per second. Le.

distance velocity (v) =

time

Where two lines intersect, this represents the contact between two layers of dif­ferent density and the depth of this contact is calculated from the formula:

Xc ) V2 - Vi 01=-.

2 V2 + Vi

Xc jV3-V2 02=-.

2 V3 + V2

where

+ 0,833 01 + 1,5

01 = depth of 1 st contact in metres

02 == Depth of 2nd contact in metres

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Xc = distance of intersection point 011 graph from zero in metr&s

V1, V2. V3, V4, etc. ;::;; velocities of different layers (see Figure STB/I),

4.5 Report the velocity and thickness of each layer of material at the site.

5 NOTES 5.1

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5.3

In order to determine the depth of deeper layers of rock where traverses of longer than 30 m are required, the hammer may be replaced by a falling mass or explosive charges. Use may also be made of a signal-enhancement recorder (see 5.2) and multi-channel equipment.

When the hammer strikes the plate, shock waves are produced which are transmitted through the ground, · picked up by the geophone. amplified by the apparatus and used to switch off the timer which has been started by the ham­mer striking the plate,

If the sensitivity control (or gain control) is not correctly adjusted a wrong time reading could be caused by:

(a) the second cycle of the shock wave. or

(b) extraneous noise due to background vibrations from wind, traffiC. etc,

The first cycle readings are the ones required for interpretation and any second cycle readings should be identified and rejected.

These second cycle readings can be recognized because they produce dis­tinctly longer time readings that the first cycle readings. They will also deviate markedly from the pattern established by previous hammer stations when the seismic graph is plotted.

The gain control should therefore be adjusted such that both noise and second cycle readings will not activate the seismograph.

To overcome this problem, use can also be made of a signal-enhancement re­coroer which, with each hammer blow. enhances only the wave with the same arrival time as previously recorded. thus eliminating noise and making It easier to differentiate between first and second cycle readings.

Instruments with automatic and multi-channel recorders (employing two or more geophones) may also be used and they simplify the operation .

5.4 Results often appear 10 be inconsistent because of non-horizontal layers. dis­continuity in layers, the presence of boulders or water, etc. Expert opinion should be sought in such cases.

5.5 it is advisable to use the seismograph to complement information obtained by drilling.

Special methods

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w ~ j: 30

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0 0 2 ., 6

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D\ " 4,1 m

°2 " 12 ,5m

X!

12 14 \8

01 STANCE ! 1'11)

X;a= 23,4m I I

20 22 24 26

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~ m ~

3 II'

30-

20

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0 28 30

FIGURE STall PLOT OF SEISMIC DATA

Special methods

Draft TMH6. Pretoria, South Africa, 1984