Seismic Refraction:
Exercise #2: First arrivals & initial interpretation
 

Instructions

Start by reading the Goals and Tasks section. We will explore several of the steps required for processing a complete seismic refraction data set, but we do not have the time to perform all steps on all the data. Please provide an answer for each of the question marked Q#. Be sure to answer questions as they arise in the instructions. Note that a glossary is provided as an appendix.

Some terminology is provided in a Seismic Refraction Glossary.

Goals and Tasks

  1. Survey Setup: Read the description of the survey setup, and make sure you understand what was done.
  2. First break picking: The first task is to pick the first arrivals on each recorded trace. We will do this for one shot into a spread of twenty four geophones.
  3. Build a T-X plot: transfer first break picks to a graph and decide from where the energy has arrived at each geophone. Data gathered for each shot will include direct and refracted arrivals, and deciding which first arrival belongs to which must be done in the T-X (time-distance) plot.
  4. Preliminary interpretation: Use the intercept time method (ITM) to obtain simple interpretations involving dipping planar layers.

1. Survey Setup

A seismic refraction survey was carried out. The survey consisted of a series of shots fired from each end of the line of geophones, that ran North-South. Forward shots were fired from the North end of the line and reverse shots were fired from the South end. A 24-channel seismograph was used in data acquistion. The distance from the first geophone to the last is 70m with equal distance between each geophone (3.04m). The offset of the first geophone from the source is the same as the geophone spacing.

Q1. Sketch the configuration of the survey. Draw the surface and the shots and geophones and label them with distance and names/numbers. Please count the geophone from left to right and place the origin of distance at the leftmost geophone. Please allocate enough drawing space below the surface for Q12.

2. Picking first arrivals

Now lets look at some of the data collected during this survey. Buttons below open images of the shot records associated with the Field Files with ID numbers 1, 6, 13 and 14. Two of these records are reverse shots, one is a forward shot, and one is just noise.

Shot Records:

Before identifying times of first arrivals at each geophone, answer a few questions about the raw data as it is displayed. These are typical issues to consider when carrying out quality assessment, prior to actually interpreting the data.

Q2. Answer the following questions about the shot records

a) Which records are "reverse shots" (with source off the right hand end of the geophone line) and which is the "forward shot"?

b) How much total time in milliseconds was recorded by each of the four shots?

c) Which appears to be a record of ground motion with no shot energy (ie a "noise test")?

d) Which useful record has the most "noisy" traces, and which geophones are noisy in that record?

e) Which of the reverse shots will you use to pick first arrivals?

 

Now we will determine first break times for all geophones, for both the forward shot and one of the reverse shots. Plot the results as a T-X plot.

  1. First open a new blank spreadsheet.
  2. Then follow instructions in the picking window, and mark all 24 first breaks for the forward shot (FFID 13). Once all traces have a first break pick, click the Display Picked Times. An example of picking first breaks is given in GPG section 3.e.4.
  3. From the little window with the list of picks you made for each channel, highlight all the channels and picks, copy, and past into the spreadsheet.
  4. Sort these so channel numbers are in order. Also, add a column next to the channel numbers and enter geophone location.
  5. Make a graph T-X graph of your data using the geophone position for the X-axis
  6. Proceed to pick first breaks on your chosen reverse shot, and add them to your spreadsheet. It is wise to do the copy / paste and sort for the new data separately from the first (forward) data, then to simply copy the first break times from your reverse shot into a column next to the forward shot times, ensuring the numbers are correct for all geophones.
  7. Include the new data on your T-X graph. Note your T-X plot should have symbols for data points but no lines joining them.

Q3. Make sure your graph is neat and tidy (font sizes appropriate etc), that both axis are correctly labelled and include units, and that the two data sets are identified. PRINT THIS ON A SINGLE SHEET. We do NOT want you submitting spreadsheets as email attachments please. Please note the y-axis must intercept the x-axis at the position of shot.

Q4. On your paper copy of the T-X plot draw lines, and label them, for both reverse and forward shots indicating which arrivals are direct arrivals and which are refractions.

 

3. Interpretation using the ITM

The Intercept Time Method (ITM) will yield estimates of depth under each shot point and velocities for the two materials.

Q5. What do we assume about the configuration of the interface between the layers between the end points under our survey line?

Interpretation means finding geophysical parameters about the earth under our survey. ITM interpretation of a simple refraction survey will yield two depths (which means there will be information about the dip of the interface) and two velocities. Equations are given in the notes section 7. These should be enought to help you produce the necessary information using the following questions as a "recipe".

Q6. Identify the two estimates for top layer velocity, V1, then average them.

Q7. Identify estimates for V2u and V2d, the up dip and down dip versions of V2 (up dip velocity should be larger). Note from where these estimates were obtained. (GPG, section 3.e.7)

Q8. Find values for dip angle and critical angle.

Q9. Estimate values for the two intercept times directly from the graphs by drawing the refraction lines back to a point above the corresponding shot location and reading the corresponding time value. REMEMBER intercept times are directly above shot points, regardless of where you place the vertical axis for your graph.

Q10. Use equations for the intercept times (ie for t2 and t2' when x=0) to estimate depths z and z' . Use relations at the end of section 7 of the seismic refraction online notes

Q11. Use these z and z' depths and other information you have just calculated to estimate true depths h and h' .

Q12. Finally, roughly draw or label the items listed below on your figure from Q1:

  • a dipping interface of velocities v1 and v2; indicate the dipping angle and velocities;
  • indicate the depth (z and z') and true depth (h and h') at both the forward and reverse shot locations;
  • draw the forward and reverse ray paths that travel from the source to the last geophone of the spread.

4. Conclusions

Q13. It is worth checking the “ reciprocal times ” for this survey. (See midway down the page of section 7 in online notes.) In a true reciprocal time survey there are shots at both ends of the line and the first shot is coincident with the first receiver and the reciprocal shot is coincident with the last receiver. The data collected in this survey were not acquired exactly in this manner since the shots were off the end of the geophone spread. You may notice some difference in the estimated reciprocal times when you plot your graph. Can you think of what might be happening on your refraction interface that could explain this difference or anything else that could explain the discrepancy.

Q14. Suggest possible materials for our two layers based upon these velocities, and identify your source(s) of information.