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Introduction to Geophysics Assignment No. 3 Answer BOTH questions Please read each question CAREFULLY It is MOST IMPORTANT that you explain what you are doing 1. A Wenner electrode configuration has been used in an electrical sounding experiment to determine the depth and resistivity of various subsurface layers. The separation of the electrodes was expanded in steps from 1 m to 50 m (see attached Table 1) and the corresponding potential difference measured to the nearest + 0.001 volts. The current was maintained at a constant value of 0.25 amps throughout the experiment. (a) Plot the apparent resistivity (a) vs electrode spacing on the attached log-log graph paper. How many layers are present? (b) Determine the value of 1, the resistivity of the surface layer (c) On Figure A attached, plot the ratio a / 1 vs the ratio of the electrode spacing to a “guessed’ thickness, h, for the surface layer – repeat this process until your values lie on or are parallel to the lines in Figure A (d) From (c) determine a thickness for the upper surface layer (e) Determine a value for the resistivity of the layer beneath the surface layer. (f) Estimate the electrode spacing that corresponds to the inflection point in your plot from (a). Compare this value of electrode spacing to your value of thickness in (d). Comment on this comparison. 2. Select a research article from a scientific journal (NOT from the textbook and NOT from a website!) that describes a study to determine the location of a paleomagnetic pole for a particular continent for a particular time (e.g. North America in the Early Cretaceous). Write a one-page summary of the article, which includes the major findings and discusses (albeit briefly) any special assumptions made that limit the validity of the resulting interpretation. Attach the abstract from the original article to your answer. Do not include any mathematics in your 1-page summary. You may illustrate your answers with diagrams from the article or elsewhere (diagrams will not be considered part of the 1-page length limit) Note Do NOT simply paraphrase the original article's abstract in your 1-page summary. TABLE 1 Electrode Spacing (m) Potential Difference (V) Apparent Resistivity (-m) 1 0.800 2 0.420 4 0.280 6 0.155 8 0.125 10 0.120 15 0.105 20 0.100 25 0.098 30 0.086 40 0.076 50 0.064

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