Reduced Travel Times

Reducing travel times is simply replacing time on the vertical axis with (time - distance/reduction velocity), where the reduction velocity is chosen to aid the user in determining the best fits to the data. On such a plot, arrivals having an apparent velocity equal to the reduction velocity will lie on a horizontal line. Arrivals with an apparent velocity lower than the reduction velocity will have a positive slope, those with a greater velocity than the reduction velocity will have a negative slope. Because of this, it is possible to make a very confusing looking plot and so this is not usually a good plot to start with in learning how to interpret refraction data. But for a more experienced user, this can improve the quality of interpretation. The option to use reduced travel times is in the Preferences Window, where the reduction velocity is also set.

Consider the plot of perfect data from a structure with a 50 m thick, 4000 m/s layer over a 120m thick, 6000 m/s layer over a 6500 m/s halfspace. With the normal plotting of time against distance, we get a plot like this:

Raw (unreduced) travel time plot

Lines for the reversed third layer and the forward arrivals from the second layer have been extended to show how little the arrivals from these two layers differ. It would be hard to draw lines for these two layers. If we now go into the Preferences Window and choose to plot with a reduction velocity of 6300 m/s, our plot becomes this:

Reduced travel time plot

Note that while in the top plot, time ranges from 0 to 340 ms, here the range is only 29 ms. Now we can clearly separate the arrivals from the second and third layer. Notice that the slopes of the arrivals from the third layer are negative. Clearly it is possible to create a jumble of picks from both forward and reversed shots with this technique, so the user should pay close attention to which arrivals come from which shots. In general, if multiple layers are suspected in a group of arrivals with an apparent velocity about v, chosing a reduction velocity near v will usually bring out any systematic variations related to additional layers.

Adjusitng lines to fit data behaves exactly as usual. The panel in the lower left corner shows the apparent velocity of any line the cursor is near.