I. INTRODUCTION
1.1. General Statement
Applied seismic on engineering structure divided into 2 (two) advantages, that are:
Firstly, refraction seismic exploration is to get lateral distribution of velocity layers underneath ground level. The velocity layers related to bearing capacity of soil or rocks. Beside that, the velocity layers could be known unconsolidated material and solid rock, depth of soil or base rock. Others, usefully of seismic exploration are to reduce core drilling in feasibility stage engineering study.
Secondly, down hole seismic is to get bearing capacity parameter in relation to engineering structure when the earthquake happened. The method to do down hole seismic is to install geophone on the bore hole, than making artificial trigger on the surface. The impulse or artificial trigger can be rise compression wave and shear wave. The result of preliminary wave and the secondary wave can be use to get the dynamic characteristic of the soil or rocks.
The result of the applied seismic given to engineer for designing the resistance of earthquake on engineering structure.
1.2. Basic Theory
In the seismic refraction method an explosive charge, weight drop or hammer blow is used to generate an elastic pulse (shot) at the earth’s surface. Some of the radiating energy which travels by several paths in the medium is refracted along subsurface boundaries and returns to surface to be recorded by a line of detectors (seismometer or geophone). The time lapse between the shot and the first arrival of the refracted energy at each of geophones is plotted on the time-distance curve (Fig.1) and this provides information on the depths to the refracting horizons and the seismic velocities of underlying layer. Fortunately, refracting horizon normally corresponds to distinct geological horizons and thus the depths to the geological interfaces may be computed. Modern interpretation techniques permit the measurement of depth to an irregular refracting interface at each seismometer position along the profiles.
1.3. The Down Hole Seismic
The purpose of down hole seismic is to be able to predict with accuracy behavior earthquakes of structure and the ground on which its rest, and used for effective a seismic design. It is necessary to know the dynamic characteristics of the ground.
The dynamic characteristics of soil that must be known in order to analyze deformation and stress resulting from dynamic loads are the Poison’s ratio (α), shear modulus (G), Young’s modulus (E) and kinetic bulk density (K).
Those parameters can be calculated using the formula as shown bellows:
Kinetic Poisson ratio’s α = {1 – 2 (Vs/Vp) 2} / {2-2(Vs/Vp) 2}
Kinetic Rigidity Modulus G = 1/g.r.Vs2 (in kg/cm2)
Kinetic Deformation Coefficient E = 2(1+r) G (in kg/cm2)
Kinetic Bulk Modulus K = E/3(1-2r) (in kg/cm2)
Where g = acceleration from gravitation (9.75 m/sec2)
r = bulk density of the ground (tonf/m3)
Vs and Vp = seismic velocity (m/sec)
The primary wave (Vp) and shear wave (Vs) is known from the down hole seismic record.
CHAPTER 2. PROCEDURE OF DOWN HOLE SEISMIC
The three of geophones is inserting to the bore hole as shown in Fig.2. The length of prove is 1.00 m, where on the tip of prove contained three geophones perpendicular each others.
The P wave and S wave propagation determine by using 3 geophones, 1 geophone placed vertical and two geophones horizontal. The two geophones placed on the right angles to each others. The shot point is toward the hole about 1.50 m – 2.00 from the hole. The wooden plate hammering is method to generate shear wave referred to every depth of hole.
The wooden plate is 1.50 m – 2.00 m long, 50 cm wide and 10 cm thickness is firmly fixed on the ground. The data will be checked for be sure measurement of S waves. The record will be every 1 m interval and the record similar up to the depth of hole. The V wave and S wave determined from the record and plotted on the paper to determine of velocity layers.
Figure 2. Schematic Down Hole Seismic using the Oyo Mc Seis 160.
2.1. Calculation of Dynamic Parameter
The down hole seismic method is applied to find the velocity distribution of P wave and S wave in bore hole. The first step of finding velocity distribution is to read the first arrival time on the seismic record. Afterward, data is plotted on the millimeter paper and drawing the time - travel curve. On the curve find the best fit of velocity distribution (Vp or V s= distance / time = m/sec.)
The velocity distribution of P wave and S wave are used to calculate the dynamic elastic constant as shown on the flow chart on Table 1.
Table 1. Processing to produce dynamic elastic constants from S wave and P wave (OYO, 1978, TN 18).
P wave velocity is chiefly a function of volume elasticity and rigidity of the layer, become smaller in rigidity in proportion as the layer is soft with result that volume elasticity comes to have a larger influence.
S wave velocity, which is function of only rigidity, is a volume which serves s direct standard of hardness of layers. The values of material density (r) is obtained from Table 2.
Table 2. Density values (r), angle of friction (φ0), compaction (C) according to JIS Manual.
| Material types | Rock types | Unit Weight (r) (tf/m3) | Angle of friction (φ0) | Compaction ( C ) =kgf/cm2 | Soil Classification |
| Banking Material | | 1,4 – 2,0 | 150 - 400 | 0,1 – 0,5 | GW,GP SW,SP SM,SC ML,CL VH |
| | Gravel | 2,0 1,8 | 40o 35 | 0 | GW,GP |
| | Sand with gravel | 2,1 1,9 | 40 35 | 0 | GW,GP |
| | Sand | 2,0 1,9 | 35 30 | 0 | SW,SP |
| Natural | Sandy soil | 1,9 1,7 | 30 25 | 0 | SM,SC |
| Material | Clayey soil | 1,5 1,6 | 25 20 | 0 | ML.CL |
| | Clay and silt | 1,6 – 1,7 1,4 – 1,5 | 20 15 | 0 | ML.CL MH |
| | Volcanic ash | 1,4 | 5 | 0,3 | VH |
CHAPTER 3. THE RESULTS OF DOWN HOLE SEISMIC
The time travel curves and elastic modulus shown on Table 4 and Table 5. The elastic modulus related to the depth of ground bed design and the weight of proposed engineering structure. The summary result of down hole seismic shown on Table 3.
CHAPTER 4. CONCLUSIONS
Elastic modulus taken from the data of AR 1 on the depth of 15 m is as follows :
α = 0.33 – 0. 50
G = (0.2 x 103 – 189.9 x 103) kg/cm2
E = (1.34 x 103 – 1025.34 x 103) kg/cm2
K = (41.22 x 103 – 483.50 x 103) kg/cm2
r = 1, 7 – 1, 8
REFERENCES
1. Masuda, H., 1975, Seismic Refraction Analysis for Engineering Study. OYO Technical Note TN 10.
2. Imai, T., 1975, An Introduction to the geophysical prospecting for civil engineering purposes. OYO Technical Note TN 11.
3. Hawkins, L.V., 1961. The reciprocal method of routine shallow seismic refractions lines. Geophysical Prospecting, 6, 285 -182.
4. Hawkins, L.V., 1961. Seismic Refraction Surveys for Civil Engineering. Geophysical Memorandum 2/69, ABEM Printed Matter No.90091.
5. Angela M. Davis, 1977, A Technique for Insitu Measurement of Shear Wave Velocity, Marine Science Laboratories, University Collage of North Wales U.K,, Abem Case History, ABEM Printed Matter – No.90180
6. Takeshi Okubo, Akhiro Satake, Masaki Ishoguro, Minuro Nakagawa and Ken Ito, 1978, Seismic Survey for Civic Engineering by Handy Seismograph, OYO Technical Note TN – 18, OYO Corporation.
Satoru Ohya, Tsuneaki Takeuchi, Tsuneo Imai and Ken Ito, 1978, Geophysical Investigation for Civil Engineering purposes in Japan. OYO Technical Note TN – 33 OYO Corporations.
