Geophysics 437: Application of Methods in Environmental and Exploration Geophysics
Fall 2008
Instructor: V. Kravchinsky
TA: Dennis Rippe
Assignment 4, Weight 20%, due Friday, November 21 (5:00 pm)

Modeling of the gravity and magnetic sources

a) Use your gravity filtered data to model an anomaly using the Talwani’s 2-D algorithm. The gravity model should be simple and defensible using geophysical and geological (to download the Warner county geological report in the PDF format) reasoning. Plot the result. Explain which filtered gravity profile you decided to choose as most representative and why you preferred particular type of filter. How did you choose the density and shape of your model objects? How much can you vary the density and depth of your model, and still obtain a reasonable fit? Give short geological interpretation of your geophysical data and suggest where to drill a borehole to target a potential aquifer in the area of the survey.

b) Give a short geological interpretation of your magnetic data if you know that diabase and lamprophyre dykes of Late Cretaceous age are common in the region. Sometimes diamonds can be found in lamprophyre dykes (for example, see http://en.wikipedia.org/wiki/Lamprophyre, http://www.turnstone.ca/lampro.htm and http://gsc.nrcan.gc.ca/diamonds/canada/index_e.php). Suggest one-two drilling sites on a basis of your interpretation if geological assumptions are that some of these dikes could be lamprophyre. Apply approximate methods of depth and size determining (modified Parasnis’ method and symmetrical anomaly method) as well as forward modeling using dipole approximation (see Excel file Dipole_Solution.xls). Compare results.

c) Describe in details one of two topics: 1) Talwani’s 2-D forward gravity modeling algorithm starting with the Hubbert’s integral; 2) Magnetic dipole forward modeling (to see most of the class’ notes you may go to the link on Total-Field anomalies).

Materials to hand in:
1) Submit your report by email and bring in the hard copy. Recommended volume of the report including figures is 12±2 pages. Please NO hand writing formulas in the report.
2) Submit your code and input files by email to me or TA.

Downloads:
talwani_10.m This is a function to read the file and output the file as well as menu that helps to select vertexes for the model. You need to copy this function and paste it into your Matlab code.
GravityDataforTalwani_mod.m This includes density and number of vertices and the first column is the distance along profile and the second column is the depth for all vertices (from 1 to 5).
Test_GravityDataforTalwani.m First column: Distance along profile; Second column: Bouguer anomaly in mgal

Talwani-59 2-D gravity.pdf
Hubbert-1948.pdf
To read about Canadian kimberlites and their ages see Heaman-03-kimberliteCanada.pdf.