Down-hole Seismic Test.
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Down-hole Seismic Test.

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Down-hole Seismic Test.

This article covers the procedures for conduct down-hole Seismic testing for borehole to determine compressional and shear wave velocities in soil and rock as a function of depth, which in turn, can be used with density to derive the dynamic soil parameters (Elastic Modulus (E), Shear Modulus (G) and Poisson's Ratio (ν)). These properties are the key parameters in predicting the response of soils and soil structure systems to dynamic loading. 

The method requires one borehole per test location. The borehole is PVC-cased and grouted.


- Equipment.


The present Down Hole Seismic Test (DHST) has been conducted with Geode Ultra-Light Exploration Seismograph system manufactured by Geometrics, Inc. (San Jose, U.S.A). The system consists of Geode seismic recorder 24 channel seismic records, trigger extension Cable 120 m, hammer switch, strike plate (25x25x3) cm, sledgehammer (12)Kg, in addition to Wall-Lock Borehole Geophones manufactured by Geostuff, Inc. (Grass Valley, CA 95945 USA), the system of borehole geophones consist of geophones (3-component tri-axial sensors) and control unit for borehole geophones model BHGC-4. Figure 1 below shows the equipment used for DHST. 


- Field procedure.


The test consists of lowering a geophone (motion transducer) to a specified depth in the  borehole and clamping it to the casing. An energy source is placed at the surface, near the top  of casing. Generally, the source is a sledgehammer which is struck vertically onto a steel plate  (compressional or P wave source) or, alternatively, horizontally onto a steel plate (shear or S  wave source). The source is struck for P wave transmission as well as for S wave transmission,  and travel time from the moment of source initiation until reception at the geophone is recorded.  The geophone is moved to a new depth and the process is repeated. Interval velocity  (instantaneous velocity over an interval) is determined by comparing successive readings. This  procedure is repeated at a specified depth interval from the bottom of the hole to the top. 



- Data processing.


- First arrival picking.

First arrival picking is the method that is extensively used for velocity calculations in Down Hole  Seismic techniques. Following completion, the field work, the recorded digital records from each  depth were transferred to a personal computer for analysis. Seismic data records were depth-  sorted and spatial information were assigned to each record. Then, first arrival times of the  seismic waves were determined by manual picking using Reflex –Win Version 4 (Sandmeier -  Germany). 

- P-Wave Analysis.

First arrival time are related to the seismic compression P-wave velocity, which is the most  common seismic parameter to describe sediment properties. Travel times of subsequent shots  were loaded, compared to adjacent waveforms and if needed corrected during the travel time  picking procedure. The recorded digital records were analyzed to locate the first minima or first  arrival on the vertical axis records, indicating the first arrival of P – wave energy. The seismic  data showed excellent quality throughout all measurements.



- S-Wave Analysis.

The digital records are studied to establish the presence of clear S-wave pulses, as indicated by  the presence of opposite polarity pulses on each pair of horizontal records. Ideally, the S-wave  signals from the 'normal' and 'reverse' source pulses are very nearly inverted images of each  other.  


The first maxima are picked for the 'normal' signals and the first minima are picked for the  'reverse' signals, although other points on the waveform were used if the first pulse was  distorted.


- Velocities Calculations.


The travel times of the P and S waves are derived from the first arrivals identified on the seismic  trace by manual/ automatic picking using Reflex –Win Version 4, for shear and compression-  waves, for each depth measurements (Z) and used with the known Source – Receiver distances  (R) to calculate the velocities (P and S) for depth by dividing the difference in travel path between two depths by the time difference between the two signals recorded.



The elapsed time between arrivals of the waves at the receivers (Z2-Z1) is used to determine  the average velocity of a 1-meter-high column of soil around the borehole.  


- Calculate Dynamic Soil Parameters.


This process returns five formula logs: Poisson's Ratio (v), Elastic Modulus, E, Shear and  Modulus, G. To calculate these five logs, three well logs are needed: Compressional wave  velocities (VP), Shear Wave velocities (VS), and Bulk density (ρ). If the Bulk density log is not  available, only the Poisson’s ratio can be calculated.  

The dynamic soil parameters (Elastic Modulus, E, Shear Modulus, G and Poisson's Ratio (v)  were derived from data measured in Full Wave Form Sonic test. These parameters were  calculated from the velocity of compressional (VP), shear (VS) waves and Bulk Density (ρ)  using the following relationships.


- Poisson’s ratio (v).


It is fundamental parameter that is difficult to measure and is usually estimated in  engineering calculations. The ratio of horizontal to vertical strain is required to relate the  modulus and strains in a soil body. A typical range of values for Poisson ratio for soil is from  0.3 (for stiff soil) to 0.45 (for soft soils), in general. When both the P-wave and S-wave  velocities are known, Poisson’s ratio (v) can be calculated from the equation.


Where Vp denotes P-wave velocity and Vs denotes shear wave velocity.


Where k is the modulus of incompressibility, µ is the modulus of rigidity or shear modulus and ρ  is the density of the medium through which the wave propagates. 

The minimum and maximum values of Poisson’s ratio are 0 and 0.5. Low values of V imply solid  and hard rock whereas high V represents a weaker and less consolidated rock. If Poisson’s  ratio is 0.33, the S-wave velocity is half the P-wave velocity. In a fluid zone where shear waves  cannot propagate, Poisson’s ratio equals to 0.5. If the lithology is homogeneous the results  from Poisson’s ratio can determine the location of fractures relative to the un-weathered  and un-fractured rock. 


- Shear Modulus (G).


It is the ratio of the applied stress to the distortion (rotation) of a plane originally perpendicular  to the applied shear stress. It is also termed as Modulus of Rigidity. 

The shear modulus is used to perform more advanced soil modeling, and dynamic response  analysis of soil- structure interaction. Shear modulus at low strain levels as measured by  geophysical techniques will provide the elastic parameter that can be used to establish the  variation of modulus degradation versus strain model for the soil behavior. This parameter is  used in defining the stiffness matrices for finite element analysis of earth structures and  foundations on soils. 

When the P-wave, S-wave velocities, and Bulk Density (ρ) are known, Shear Modulus (G) can  be calculated from the equation:    

Where, ρ is the density of the medium through which the wave propagates, Vs: Shear wave  velocity; ρ: Unit weight of the soil, and g: acceleration of gravity. 


- Young Modulus (E).


Stress/strain is the ratio of the applied stress to the fractional extension (or shortening) of  sample length parallel to the tension (or compression). Stress is force / unit area and strain is  the linear change in dimension divided by the original length. 

Also, when the P-wave, S-wave velocities, and Bulk Density (ρ) are known, Young Modulus (E)  can be calculated from the equation:    


In which Vs is the shear wave velocity, and v is the Poisson’s ratio, and ρ is the bulk density. 


- Results and Presentation.


The example results of Down Hole Seismic Test (DHST) are shown in Figure below, which  include velocities of P-wave & S-waves, and the interpreted dynamic parameters (Elastic  Modulus, E, Shear Modulus, G, and Poisson’s Ratio (v), in graphical and tabular forms.  




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