erth3021 seismic refraction prac

7/29/2019 Erth3021 Seismic Refraction Prac

1/3

ERTH3021 Exploration and Mining Geophysics

Practical 3: Seismic Refraction Interpretation

Aims

Gain experience at picking arrival times from a simple refraction record.

Carry out manual interpretation of simple 12-channel data using both slope-intercept and reciprocal methods.

Implement Python code to perform reciprocal refraction analysis.

Perform reciprocal analysis on a 120-channel reversed refraction recording.

Make geological and engineering deductions based on the seismic refraction data.

1. Slope-intercept analysis of 12-channel refraction record.

You will be provided with forward and reverse recordings from a 12-channel refractionspread acquired near the UQ lakes. The geophone interval was 5m, and the shots

were 2.5m from the nearest geophone. For convenience, we will do our interpretationdirectly on these field records.

(i) Estimate the arrival time on each channe, and mark on the records.

(ii) Mark in the time axis at each shotpoint (2.5m from Channel 1 or 12).

(iii) Mark in the direct and refracted segments on the forward and reverse records.

(iv) Use the slope of the direct-wave segment to estimate the v1 velocity. How do theestimates at each end of the spread compare.

(v) Use the slope of the refracted-wave segment to estimate the apparent v2 velocitiesin the forward and reverse directions, and hence estimate the true v2 velocity.Comment on possible dip in the refractor.

(vi) Calculate the critical angle of refraction appropriate to this recording.

(vii) Measure the intercept at each shot point, and hence estimate the depth to refractorat each shotpoint.

(viii) Use these depths to estimate refractor dip, and compare this with your answer toQ1(v) .

Seismic Refraction Practical ERTH3021 Page 1


2/3

2. Reciprocal Analysis of 12-channel refraction record.

(i) Working on the UQ lakes records from Q1, estimate the reciprocal time (totalshot-to-shot time).

(ii) For those geophones receiving both forward and reverse refractions, computethe time-depth and velocity functions. Tabulate these as a function of geophonecoordinate.

(iii) Sketch the velocity function versus geophone position, and use this plot to ob-tain an estimate of the true v2 velocity. How does this compare with the slope-intercept estimate ofv2.

(iv) From the time-depth values, estimate the refractor depth in the central part of the

spread. How does this compare with the depths obtained in the slope-interceptanalysis.

(v) Concisely comment on possible geological interpretation of the layers detectedby the seismic refraction.

3. Testing of linear-regression function

(i) Download the function linearfit.py from the erth3021 download site.

(ii) Test this function using the (x, tv) data from Q2(iii). Check the derived v2 estimateagainst that obtained manually.

4. Slope-intercept examination of 120-Channel refraction data.

You will be provided with travel-time picks from a 120-channel reversed refractionrecording, acquired as part of a civil-engineering roadbuilding project. The line wasrecorded to assist in the placement of a road cutting. We will interpret these data usingthe reciprocal method. Because of the volume of data, we will automate the processwith Python programs.

(i) Download the arrival-time data file (NE14B-12.picks) and familiarise yourselfwith the header information, and file structure.

(ii) Write a python program to construct a time vs distance plot for these data.

(iii) Note the range of geophones which record reversed refraction arrivals.

(iv) Use the linearfit function to estimate the average v1 and v2 velocities.

(v) Use intercepts to estimate refractor depths at the shotpoints.

(vi) Estimate the reciprocal time for use in the reciprocal analysis.



3/3

5. Reciprocal-Method interpretation of 120-channel refraction data.

(i) Write a python program to plot the time-depth (tG) and velocity-function (tV)for these data. Note that you should only plot these functions over the range ofgeophones recording both forward and reverse refractions.

(ii) Estimate the slope of the tv plot, using the linearfit function, and a running win-dow approach. The length of the running window will be a compromise betweenresolution (short window) and stability (long window). Hence produce a thirdsubplot, showing the variation of the v2 velocity along the line.

(iii) Combine your tG, v1,and v2 results to add a fourth subplot which shows the re-fractor elevation along the line. This calculation should allow for any major vari-

ations ofv1

andv2

along the line.(iv) Provide a concise geotechnical interpretation of the geophysical results. This

should summarise bedrock profile and rippability. At what location along theline would you position a transverse road cutting, to achieve the most economi-cal construction.


erth3021 seismic refraction prac

Documents