aglo

Advanced techniques

 

There are several other, more involved methods of interpretation, and modern instruments often include one or more of these methods coded into the computers that drive the instruments. One such method is the GRM or Generalized Reciprocal Method, which expands upon the ideas behind the Plus-Minus method introduced above.

The figure to the right illustrates one program that uses the GRM approach. On top is a time distance plot of the data, with the sympols and colours indicating which layer each arrival is interpreted to have refracted from. The bottom panel shows the cross section resulting from careful interpretation using the GRM method. Purple is the interface below the top layer, yellow is the second interface, and the third interface (between the third layer, and bedrock) is outlined with red arcs. The arcs help identify a more accurate horizon when the dip is interpreted to be steeper than around 10o. Use of arcs like this is a processing step known as "migration", a topic more commonly covered in relation to seismic reflection.

There are also wave-propagation methods implemented by some commercial programs. These are inversion techniques. They start with a "guess" or estimated first model, calculate how wavefronts will propagate through a the model from the sources to the geophones of the survey, then compare the resulting calculations to measured first arrivals. The process is repeated after adjusting the model so as to generate a data set that more closely matches the measurements. The process is "finished" when there is a model that can cause data that look close to measurements according to pre-defined statistical criteria - ie when "misfit" is within pre-defined specifications.

Examples of two interpretation methods at one site are given to the right. They come from the paper "2-D velocity structure of the buried ancient canal of Xerxes: an application of seismic methods in archaeology", 2001, V.K. Karastathis, S. Papamarinopoulos, and R.E. Jones; Journal of Applied Geophysics 47 2001 29–43.

TOP: The refraction first arrival T-X plot (ie, raw data) for line 96-12.

MIDDLE: A GRM solution derived from the data shown. There are clearly "steeply" dipping sections of the interface between oveburden and bedrock. The 34m wide channel represents the target for this survey, which was trying to determine whether a canal referenced in greek legends was real or not.

BOTTOM: A ray-tracing inversion solution derived from the same data set. The outcome is "smoothly" varying velocities, but there is an apparent interface represenetd by tightly spaced contours. This corresponds to the interface derived from the GRM solution. It illustrates how different methods will produce different results, but that both are equally interpretable for geological or geotechnical information.


TOP: Raw T-X plot from first arrivals.


MIDDLE: model produced using GRM interpretation method.


BOTTOM: model produced using rat-tracing inversion.


Ray tracing ray plot, and data matching graphs. The ray tracing inversion algorithm can also produce an image showing the pathways that energy would have travelled from sources to each geophone, using the model that was determined at the end of the inversion process. The bottom portion of the image shows how well the calculated data (based on the model) corresponds to real measurements. The small vertical bars are first arrivals with error bars on them. The lines show "T-X" graphs of calculated data, and they should be within the error bars assigned for measurements. They are sometimes a little off, but the outcome is considered quite good given this challenging situation.