Shallow Seismics in Earthquake Engineering Interim Report submitted to the Indian Institute of Technology, Kharagpur By

CHINTHA NARESH Roll No: 07CL6004 MASTER OF TECHNOLOGY IN EARTH SYSTEM SCIENCE & TECHNOLOGY Under the supervision of Dr. A.N.V.Satyanarayana Supervisor and Dr. B.K Rastogi, Co-Supervisor Director, Institute of Seismological Research, Gandhinagar 

Centre for Oceans, Rivers, Atmosphere and Land Sciences Indian Institute of Technology Kharagpur – 721302

November, 2008

Abstract The Indian sub-continent is highly prone to multiple natural disasters including earthquakes, which is one of the most destructive natural hazards with the potentiality of inflicting huge loss to lives and property. Earthquakes pose a real threat to India with 59% of its geographical area vulnerable to seismic disturbance of varying intensities including the capital city of the country. In the span of last 15 years, India has experienced six earthquakes of moderate intensity. Although moderate in intensity, these earthquakes caused considerably high degree of losses to human life and property, which highlights the vulnerability of the population and infrastructure to earthquakes and the inadequacy of preparedness measures in the country. During the historical earthquakes and recent earthquakes it is seen that most of the losses and deaths occurred due to the man made structures hence in order to minimize the damage and the death, it is highly essential to design our structure according to their geological conditions. Earthquakes radiate seismic energy as both body and surface waves. These properties are tapped to develop seismic techniques to estimate soil stiffness characteristics for the strong structural foundation as well as for evaluating the exploring in the oil and natural gas. In this present work an attempt has been made to utilize three techniques namely Multi-channel Analysis of Surface Waves (MASW), Refraction and Reflection based on the movement (or propagation) of surface and body waves. In the MASW technique the frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface to prepare the microzonation map Gandhinagar/Kutchh areas of Gujarat. Reflection and Refraction methods would be used to estimate the structure of the soil layer up to nearly 30 meter depth over the region. Seismic reflection and refraction is the principal seismic method by which the petroleum industry explores hydrocarbon-trapping structures in sedimentary basins. In this study the utility of these two methods are limited to delineate the geological structure of the Gandhinagar region. Key words: Earthquake, seismic waves, MASW, refraction, reflection.

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1. Introduction In most surface seismic surveys when a compressional wave source is used, more than twothirds of total seismic energy generated is imparted into Rayleigh waves (Richart et al., 1970), the principal component of ground roll. Assuming vertical velocity variation, each frequency component of a surface wave has a different propagation velocity (called phase velocity, Cf at each unique frequency, f component. This unique characteristic results in a different wavelength (λf ) for each frequency propagated. This property is called dispersion. Although ground roll is considered noise on body wave surveys (i.e., reflection or refraction profiling), its dispersive properties can be utilized to infer near-surface elastic properties (Nazarian et al., 1983; Stokoe et al., 1994; Park et al., 1998a). Construction of a shear (S)-wave velocity (vs) profile through the analysis of plane-wave, fundamental-mode Rayleigh waves is one of the most common ways to use the dispersive properties of surface waves (Bullen, 1963). This type of analysis provides key parameters commonly used to evaluate near-surface stiffness—a critical property for many geotechnical studies (Stokoe et al., 1994). As well, the near-surface vs field can provide useful information about statics during body wave data processing (Mari, 1984). The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source ('shot') located on the surface. Energy radiates out from (as form of body waves) the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor. Seismic refraction

technique is routinely

used in many applications such as engineering, environmental, groundwater, hydrocarbon, and

industrial-mineral exploration (Lankston, 1989; Hodgkinson and Brown, 2005; Bridle,

2006; Yilmaz et al., 2006). Seismic reflection profiling involves the measurement of the twoway travel time of seismic waves transmitted from surface and reflected back to the surface at the interfaces between contrasting geological layers. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these layers. The strength of the contrast in the acoustic impedance of the two layers determines the amplitude of the reflected signal. The reflected signal is 2

detected on surface using an array of high frequency geophones. As with seismic refraction, the seismic energy is provided by a 'shot' on surface. For shallow applications this will normally comprise a hammer and plate, weight drop or explosive charge.

2. Objectives The objectives of the study are 1. Estimate the soil stiffness using different seismic techniques at Gandhinagar/Kutchh area 2. To evaluate the structure of the soil by estimating shear wave velocity using MASW technique 3. To delineate the different layers of the soil structures by using Refraction and Reflection techniques

3. Methodology In this study three different techniques namely a) Multi-channel Analysis of Surface Waves (MASW), b) Refraction and c) Reflection. The details of these three techniques are given here under:

3.1 MASW technique There are three different types in MASW technique; they are (i) Active, (ii) Passive remote and (iii) Passive roadside. In the present study we have used Active technique only. The data acquisition procedure for MASW is same as standard CMP (Center mid point method), body wave reflection surveys employed in oil exploration. Surface waves are best generated over flat ground at least within one receiver spread length (D) there in small tomographic variations do not matter. A heavy sledge hammer (5-10kg) is a good for surge generation .In MASW low frequency i.e. 4.5 Hz vertical geophones are always recommended (usually 24 channels) acquisition will be optimal. Length of the receiver spread (D) (Figure 1) is directly related to the longest wavelength ( λ max ) that can be analyzed, which in turn determines the maximum depth of investigation (Zmax):

D ≈ λ

max

≈ z

max

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On the other hand, (minimum if uneven) receiver spacing (dx) is related to the shortest wavelength ( λ min ) and therefore the shallowest resolvable depth of investigation (zmin).

D

≈ λ

min

≈ z

min

The source offset (x1) controls the degree of contamination by the near-field effects. Its optimum value has been a debatable subject. We, however, suggest a value of about 20% of D (e.g., x1=5 m when D=25 m). A large value of x1 (e.g., > 10 m) and a large D (e.g., > 100 m) will increase the risk of higher-mode domination and reduce S/N for the fundamental mode. The most recommended values of x1 is 5 m and dx is 1 to 2 m. Vertical stacking with multiple impacts can suppress the ambient noise and increase the signal to noise ratio(S/N). Active MASW technique is the most common type of MASW survey that can produce a 2-D Vs profile (Park et al., 1999). The maximum depth of investigation (Zmax) that can be achieved is usually in 10-30 m range, but this can vary with sites and types of active sources used. Field procedures and data processing steps are briefly explained below. Some of the field parameters, for example, source offset (x1) and receiver spacing (dx), are described based on the most recent research results and therefore may be different from those previously reported.

Figure 1: Setup of Active MASW Technique

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3.1.1 Determination of Harmonic Mean shear Wave velocity Determination of Harmonic Mean shear Wave velocity is an important parameter for characterizing the given site. The classification of sites (table -2) based on Determination of Harmonic Mean shear Wave velocity is given by the Federal Emergency Management Agency (FEMA, 1997) and Uniform Building code (UBC-ICBO, 1997). Harmonic Mean shear Wave velocity to depth 30 m (Vs30) is determined by the formula given below n

Vs30

=

∑ i =1 n

di di

∑ Vsi i =1

Where di is the thickness of ith layer (in meters) and Vsi is the Shear wave velocity in ith layer (in m/sec).

3.2 Refraction Technique Refraction experiments are based on the times of arrival of the initial ground movement generated by a source recorded at a variety of distances. Later arriving complications in the recorded ground motion are discarded. Thus, the data set derived from refraction experiments consists of a series of times versus distances. These are then interpreted in terms of the depths to subsurface interfaces and the speeds at which motion travels through the subsurface within each layer. These speeds are controlled by a set of physical constants, called elastic parameters that describe the material. In refraction seismology they are two methods are used for determining velocity at depth from arrival times treats the earth as a) Flat layer (or) horizontal refractor method and b) Dipping layer method. In the present study Flat layer or Horizontal Refractor method is used and is explained below. The simplest situation, shown in Figure (2) , is a layer of thickness, h0 ,with velocity

v0 ,overlying a half space with a higher velocity v1.we write velocities as “v” to indicate analysis applies P or S waves. There are three rays basic ray paths from a source on the 5

surface at the origin to a surface receiver at x. The travel times for these paths can be found using Snell’s law.

Figure 2: Three basic ray paths for a flat layer method

Figure 3: Travel time versus source-to-receiver distance for ray paths. The first ray path corresponds to a direct wave that travels through the layer with travel time

x v0

TD(x) =

This travel time curve is a linear function of distance, with slope origin. The second ray path is for a wave reflected from the interface.

(

TR ( x ) = 2 x 2 / 4 + h0

)

2 1/ 2

/ v0

.

This curve is a hyperbola.

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1 , that goes through the v0

For x=0 the reflected wave goes straight up and down, with a travel time of

TR (0) = 2h0 / v0 . The third type of wave is head wave often referred as a refracted wave

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x 1 ⎞2 ⎛ 1 TH ( x) = + 2 h 0⎜ 2 − 2 ⎟ v1 v1 ⎠ ⎝ v0 x = +τ1 v1 Thus head wave’s travel time curve is a line with a slope of 1/v1 time axis intercept at τ1.

1 ⎞ ⎛ 1 − 2⎟ 2 ⎝ v 0 v1 ⎠

τ 1 = 2 h 0⎜

1 2

The intercept is found by projecting the travel time curve back to x=0, although the head wave appears only beyond the critical distance, xc = 2h0 tan ic , where critical incidence first occurs Crossover distance setting

⎛v +v ⎞ xd = 2h0 ⎜⎜ 1 0 ⎟⎟ ⎝ v1 − v0 ⎠

1

(TD ( x) = TH ( x))

2

Hence the crossover distance depends on the velocities of the layer and the half space and the thickness of the layer.

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S1

S4

S3

S2

1

12

13

… 100m

10m

5m 5 m

14 25 … …

S5

26 36 …

S6

S7

37 48 …

Profile length 235m

10m

Total field length 535m

Figure 4: The Field Experiment using Refraction method carried out in the field.

3.3 Reflection Technique Shallow seismic-reflection methods are used to obtain information on subsurface structure. Signals generated by a small acoustic source are transmitted into the ground, reflected at subsurface boundaries where there is a change in acoustic impedance (the product of material density and seismic velocity), and recorded as a function of time by a series of receivers geophones) on the ground surface. Contrasts in acoustic impedance are generally associated with lithological boundaries, such as the overburden-bedrock interface, so shallow seismicreflection techniques provide an effective means of mapping bedrock topography. In this present interim report work, the Reflection Technique was not used. Hence, the detailed methodology is not presented.

4. Field Experiment I have participated in a field experiment in Kutchh region which was conducted by scientists of Institute of Seismological Research, Gandhinagar in the month of August, 2008. The experiment is mainly to study the surface stiffness and geological structure of the place. In the present work MASW and Refraction techniques were used.

4.1 Experimental details for MASW survey At the field station Kutchh a horizontal plane land of 100 meters was selected. Geophones are arranged linearly at equal distance. These geophones are connected to a Engineering

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Seismograph which was connected to a Laptop installed suitable softwares for the data acquisition and archival. In the present field experiment, the interest is to study the first 30 meters of the soil column in order to estimate the soil stiffness. For this purpose, we have to use a triggering mechanism, through which the vibrations in the soil will be generated. As mentioned, we need to study very shallow depth; we have used a hammer for triggering purpose. The details of the setup was given in Table 1 and explained in the previous section in MASW methodology. Table 1: Parameters to active source MASW survey Parameter

Setting

Spread configuration

Linear

Spread Length

94 m

Geophone interval

2m

Total number of geophone

24

Source equipment

Hammer 5-10 kg

Sample interval

0.25 ms

Record length

2 sec

Shot location

2m offset

4.2 Experimental details for Refraction survey Experiment carried out in the region of Kutchh area field parameters is described below. 1) The source Offset (Distance between source to first geophone) is 100m . 2) Total number of geophones used in field 48. 3) Survey length is 535m 4) The profile length (geophone arranged in a horizontal array) is 235m 5) Each Geophone spacing (or) interval is 5m. 6) The experiment conducted by 7 shot gathers

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5. Data Acquisition In this section, I have used the following steps to collect the data using both MASW and Refraction techniques. Even though the main equipment including geophones is same, but the set up is different. These details had already given above. First the experiment with MASW was conducted and the relevant data was collected. Then the setup is changed for the Refraction technique and the experiment was attempted. I have used the software provided by the Institute of Seismological Research, Gandhinagar at their laboratory for the Data Processing. For the sake of completeness, the data processing software techniques were given briefly in the following section.

5.1 MASW survey The entire procedure for MASW usually consists of three steps: 1. Acquisition of ground roll in the field or (shot gathers), 2. Extracting of dispersion curve (one curve from each record), and 3. Inversion of extracted dispersion curve for shear wave velocity profiles (change of shear wave velocity with depth i.e. 1-D profile) Two dimensional (2-D) shear wave velocity map can be constructed by repeating 1-D shear wave velocity profiling in a consecutive manner and using SurfSeis software.

5.2 Refraction survey Several shots were conducted during this survey as explained in the experimental details in the above section. Each shot gather one travel time-distance (the interval between the shot instant and the arrival of the energy at the geophone group is known as arrival time or travel time) curve plotted. Using Software tool (SeisImager/2DTM) data processing using Refraction method is done in the present study.

6 Preliminary Results Few of the records collected during the field experiment are processed. It requires lot of time for the data archival, quality control and digitization. Lot of data sets is needs to be processed and analyzed. In this present interim report, few results obtained using MASW and Refraction techniques are presented.

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6.1 MASW Technique Using MASW technique the shear wave velocity of the surface waves and its variation with the depth at Gandhinagar location are calculated and presented in Figure 5. From the figure one can see the variation of shear velocity with the depth. We can interpret the difference in the shear velocity can be attributed to the layered structures. Based on the previous experiments, the identification of the type of the soil layer i.e. whether the soil is sandy, lime structure, or any other can be estimated. The general shear velocity versus the type of the soil was given in Table 2.

Figure 5. Variation of Shear Velocity with Depth at location in Gandhinagar

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Table 2. The classification of sites based on Determination of Harmonic Mean shear Wave velocity is given by the Federal Emergency Management Agency (FEMA, 1997) and Uniform Building code (UBC-ICBO, 1997) Class A B C D E F

Vs30 (m/sec) average Shear wave velocity in the upper 30 m at the site > 1500 760 – 1500 360 – 760 180 – 360 < 180 Special soil requiring

Description Hard rock Rock Very dense soil and soft rock Stiff soil Soft soil Site specific evaluation

6.2 Refraction Technique In this technique results consists three inversion methods explain below.

6.2.1 Two-layer Time-term Interpretation In this method data file was created in pickwin module and imported to poltrefa module, then assigning the layer of two arrivals and doing time–term inversion, we get the two layer time term interpretation (Elevation versus Distance) as shown Figure 8. In this method delay times are calculated automatically using a linear least-squares inversion technique. It is noticed that S-wave velocity varying different for different layers.

Figure 6. Travel time Vs Distance curves

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Figure 7.Assigning two different layers

Figure 8 The variation of shear wave velocity within two layers using Time-term interpretation method. From the Figure 8 one can that the shear velocity in the first layer is 1.7 km/sec and the second layer shear wave velocity if 2.4 km/s .

6.2.2 Two-layer Reciprocal Method Interpretation In this method the delay times are calculated manually and create the velocity model it is shown in Figure 9.

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Figure 9 The velocity model for delay time two layer reciprocal method.

Figure 9: The velocity model

6.2.3 Tomographic Interpretation In this method travel time-distance curves were ploted and the velocity model had been created then by tomographic inversion we get variation of elevation with distance as shown in fig10.

Figure10.Tomographic interpretation

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7. Summary The present study was conducted at Institute of Seismological Research, Gandhinagar. During course of interim report, I have completed the literature survey connected to my proposed work. I have participated in field experiments and utilized different seismological techniques. I have got used to apply different software tools that are useful in data processing of the seismological data. The preliminary work in estimating the shear velocities using MASW and Refraction techniques right from the data archival, processing and analyses at Gandhinagar is completed and are presented in this report.

8. Work remaining 1. To compute the soil stiffness using MASW technique at Gandhinagar 2. Estimation of layer structure using details data set using Refraction technique 3. Data analysis using Reflection technique

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References Park, C. B., Xia, J., and Miller, R. D., 1998a, Ground roll as a tool to Image near-surface anomaly: 68th Ann. Intern at. Mtg., Soc. Cordier J.P. (1985). Velocities in Reflection Seismology. D. Reidel Publishing Company, Holland, 197 pp. Dobrin, M.B. (1984). Introduction to geophysical prospecting. McGraw-Hill Book Co., Tokyo, 630 pp. Fowler C.M.R. (2001). The Solid Earth-An Introduction to Global Geophysics. 5) Cambridge university press, London, 472 pp. Hill, M.M. (1963). The Sea. Vol. 3, Interscience Publishers, New York, 963 pp. 222 Parasnis, D.S. (1979). Principles of applied geophysics. Chapman and Hall Ltd London, 275 pp. Sheriff, R.E. and Geldart, L.P. (1986). History, theory and data acquisition. Cambridge University Press, Cambridge, 253 pp. Telford, W.M., Geldart, L.P., Sheriff, R.E. and Keys, D.A. (1988). Applied geophysics, Oxford and IBH Publishing Co., New Delhi, 860 pp. Jones, E.J.W. (1999). Marine

Geophysics. John Wiley and Sons ltd., England. 466 pp.

Bullen, K. E., 1963, An introduction to the theory of seismology: Cambridge Univ. Press.

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