Seismic Analysis

Interval Velocities

The Crosscorrelation Technique implements a patented (U.S. Patent #5,177,709 / Canadian Patent # 2,077,387) mathematical algorithm in deriving  DST interval velocity profiles . This technique derives DST interval velocities based upon crosscorrelating the waves recorded at consecutive depth increments (Baziw 1993). The value of the time shift at the maximum crosscorrelation value is assumed to be the relative travel time difference for the wavelet to travel the depth increment. This technique has the following advantages over the standard Reverse Polarity Technique

• Minimizes the human bias associated with visually selecting a crossover point in deriving interval times which is required by the reverse polarity technique.
• Utilizes the full waveform in deriving interval travel times as opposed to a single point.
• The correlation coefficient between the two wavelets can be used as a velocity accuracy estimate. This parameter gives the investigator an indication of the similarity between the two wavelets being correlated and the subsequent accuracy of the velocity estimate.
• Obtains two independent velocity estimate for each depth increment by comparing source wavelets generated on both the right and left side of the seismic probe.

The Batch Job Analysis option facilitates the user to process many seismic traces with similar filter parameters in a batch mode. In addition, Batch Job Analysis allows for the estimation of the arrival times for each depth increment and the implementation of linear least squares regression (LLSR) in deriving interval velocities. The LLSR technique facilitates in the minimization of the variability of the interval velocities. The LLSR utilizes three adjacent crosscorrelation relative arrival times, the corresponding time series depths of acquisition and a reference arrival time and depth to determine the slope of the best fitting line through the three points. The slope of the best fit line is defined to be the interval velocity.

The Forward Modeling / Downhill Simplex Method (FMDSM)  utilizes seismic ray tracing and optimal estimation techniques in deriving DST interval velocities. The standard techniques implemented in deriving DST interval velocities rely upon obtaining reference P and S wave arrival times as the probe is advanced into the soil profile. By assuming a straight ray travel path from source to seismic receiver and calculating relative reference arrival time differences, interval DST velocities are obtained.

The FMDSM offers distinct advantages over conventional DST velocity profile estimation methods. Some of the advantages over conventional techniques provided by the FMDSM are outlined as follows:

• Utilization of Snell’s Law at layer boundaries for ray path refraction.
• Optimization of a non-linear cost function which takes into account more detail of the SCPT testing environment and subsequent seismic data recorded compared to standard techniques.
• Allowance for measurement weights to be specified, the possibility to incorporate unlimited input data (e.g., crossover point arrival times, maximum cross-correlation time shifts, angles of incidence, and P-wave / S-wave time separations) into the interval velocity estimation algorithm.
• The ability to accurately interpolate interval velocities when measurement data is not available.
• Provides meaningful error residuals which indicate the accuracy of the estimated interval velocity.

In the FMDSM the user is provided with user friendly interfaces for specifying seismic wave arrival times and crosscorrelation time shifts information (derived from Depth Profile and Crosscorrelation Technqiue) with corresponding weights within a Windows compatible database.

Once the necessary data has been inputted, the user executes the FMDSM. Upon completion of the FMDSM, SC1-RAV displays the estimated interval velocities graphically with the option to Ray Trace.

The FMDSM derived interval velocities are also stored in the previously described database so that they can be utilized in other applications, report generation or for future reference.

Depth Profile

SC1-RAV's Depth Profiling software allows the user to filter and plot the captured uniaxial seismic traces on a depth vs time plot, specify or automate trend lines for preliminary velocity estimation and display peak particle accelerations, velocities or displacements.

Data Stack

The Data Stack option allows the user to post-stack acquired seismic time series data.

Data Interpolation

This analysis feature allows the investigator to increase the resolution of the captured seismic data for post processing purposes. This facilitates greater data resolution when carrying out analyses such as obtaining relative time shifts utilizing the cross-correlation function, reverse polarity analysis and specifying trend lines in depth profiling. In addition, SC1-RAV’s Data Interpolation software feature allows the investigator to synchronize seismic time series data captured with differing sampling rates (i.e., set data to same sampling rate).

Polarity Change

The Polarity Change option facilitates the investigator in selecting a set of seismic data files and subsequently changing the polarity of the time series.

Signal Decay

The Signal Decay option allows the investigator to minimize the effect of possible source wavelet multiples on the first arriving source wavelet. The technique implemented relies upon the application of an exponential decay applied to the selected time series data after a user specified time.

View

The View menu option consists of the sub-menus Interval Velocities and Seismic Data. These software features allow the user to display derived seismic interval velocities (ie., Interval Velocities) and analyze seismic data on a trace by trace basis (Seismic Data).

Utilities

The Utilities menu option consists of sub-menus Default GUI Settings, Sensor Type, Enable Time Delay and Shear Modulus Calculation. These software features allow the user to specify general interface settings, the type of seismic data recorded (i.e., accelerometer or geophone), enable the ability to implement time delay corrections within Depth Profiling and calculate and store Gmax values.