Using refplot.m for first look

The data were picked for first arrivals, and these picks are already in the headers along with the source and receiver geometry. Other sections of this document describe how that is done (see section 6.8.1). We can start by using program refplot2.m to examine the picks and get an idea of velocities, offsets, and a preliminary sense of the data. The program offers two ways to view the data, by offset and by geophone station. Here, we will use the geophone station view. Least squares solutions still use actual distance headers, regardless of our choice here. Begin by starting a Octave session and type the following in the Octave text window:


refplot



You will enter a file name, k008.seg, for example. Next, choose “Stations” for the type of display. I use xfig for the scaling, and make sensible choices for axes limits. When the data appear in a plot, choose DO AN INTERVAL and then use the mouse to click on a starting and ending point for a linear (by offset) fit. One more click positions the computed velocity for the line segment solution. Since we chose Stations rather than Offset, a linear fit by offset may produce a somewhat undulating plot on a Station` display. The default is to use the “vp” label which is not in meters. Also, the line is not straight, and the ground is not flat. The least squares fit uses actual header distances to compute velocities and calculated times. Figure 28 shows what it should look like for a simple single fit for all the stations.

Figure 28: Plots generated with refplot.m.
\includegraphics[scale=.5]{FigureM}

Clearly, we seem to be seeing the granitic bedrock with these high refractor velocities. If we look at the waveforms for the data (see Figure 29) we can form a view about the minimum offset needed for the granitic refraction to arrive before a direct arrival. I picked an offset of 30 meters for this cross-over distance.

Figure 29: Choosing an estimate of the cross-over distance at 30 meters.
\includegraphics[scale=.7]{FigureN}