Magnetic Data Reduction and Interpretation

OBJECTIVES

There are four learning objectives:

• Continue to gain experience with geophysical data presentation and modeling.
• Develop an understanding for the kinds of corrections applied to magnetic data.
• To begin to develop an understanding of the power of interpreting observations derived from different types of geophysical surveys.
• Get exposure to the problem of matching magnetic anomalies with possible geological sources

To continue with this exercise, you first need to generate a magnetic data set using your survey parameters and download them to your computer. Like the gravity observations collected previously, we must do some data enhancement of the magnetic observations prior to interpreting them. Unlike the gravity observations, these enhancements tend to be less involved.

There are two milestones in the process of accomplishing the objectives defined above:

• Process the data to remove or minimize all sources of gravity variations that can be explained by sources that are not related to the target of interest.
• Identify all possible geological models that explain the gravity anomaly that result from the above.

PROCEDURE

For this exercise you will need the magnetics observations generated from your survey, a spreadsheet, and the Java package pointed to below. Once you have these, you can begin on the following procedure.

• Process the data to remove or minimize all variations not deriving from the target.
  • Load the generated magnetics observations into your spread sheet and generate plots of the observed magnetization in nT versus position along each line for every line of magnetic data collected in your survey.

    The data file you received should contain five columns. The first and second columns list the x and y coordinates of the observation point using the standard survey coordinates defined by the client. The magnetic stations listed in the file start in the southwest corner of the survey area. Locations increase in x first and then values of y increase next. That is, the first set of locations listed are those corresponding to the southern most line in the survey (where lines are defined to run from west to east) and then proceed northward for the additional lines. The second column gives the time at which the magnetic observation was made in minutes from the start of the survey. The fourth column specifies the magnetic field intensity observed at that location. The fifth column gives the magnetic field intensity observed at the base station at the time corresponding to the station observation.

  • Using your spreadsheet, correct the observed magnetic observations for the diurnal variation of the magnetic field by simply subtracting the base station observations from the field observations.
  • Correct the data for long-wavelength variations caused by deep geological structure. As was the case in estimating the regional gravity anomaly, there are a number of different ways you can estimate the regional magnetic anomaly.

    One of the simplest is to fit a some low-order function to the data (say a plane or some smoothly varying surface). Then subtract the magnetic anomaly predicted by this function from the observed magnetic values. Another way to estimate the regional magnetic anomaly is to use magnetic observations from other, more spatially dispersed, magnetic surveys. Data of this nature is collected on a routine basis and models of the long-wavelength variations of the earth's magnetic field are widely available. We will use this technique to estimate and correct for the regional magnetic anomaly. One source for deriving these long-wavelength magnetic field variations is the US Geological Survey.

    • Before proceeding, you will need to know the latitudes, longitudes, and elevations of the corner points defining the survey area.
    • Connect to the USGS Geomagnetics Branch. Note that this is a telnet connection. You must log into this connection using the login name QED.
    • Once you are logged in, proceed to the option GEOMAG (type m) and confirm that you do want to run GEOMAG.
    • Read the documentation presented. You are interested in computing the values of the total field intensity at the corners of your survey. These values are labeled F in GEOMAG's output. Select Option 1 to display field intensities (the same as the "magnitudes", "amplitudes", or "strength" which we have been using).
    • Proceed through the other program inputs. Use field model USCON95 and today's date. When entering latitudes and longitudes, all latitudes and longitudes are entered as positive values. You are then prompted as to whether these latitudes and longitudes represent north or south latitudes or east or west longitudes.
    • Compute the magnetic field intensity at each corner of the survey area.
    • If the USGS site is unavailable, try the fundamentally similar site at the British Geological Survey. This is an ordinary Web page, so there is no need to log in, but you will need to know the corner coordinates, of course.
    • If neither site is running, an undated set of corrections is contained in this file. If you use these, justify your actions.
  • You are only interested in the difference in these values from survey corner to corner. Choose one corner of your survey and subtract the computed value of the magnetic field intensity determined at this corner from that determined at all four corners. You should have a remainder of zero at the corner you've chosen.
  • Using these reduced values, linearly interpolate the computed field intensity over your survey area and subtract the interpolated values from the observed values. These values now represent the magnetic anomaly generated from local geological structure only.
• Preliminary Data Presentation and Interpretation
  • Plot the resulting residual magnetic field versus position along each line in your survey. Are there interpretable magnetic anomalies present?
  • Identify geological models that should be considered in interpreting the data.
    • Evaluate whether an anomaly associated with the target of interest can be seen in the data.
    • Identify all anomalies that appear to derive from local geological features.
    • Determine which geological models could be the source of those local anomalies.
• Formal Data Interpretation
  • From your spread sheet, output a data file that contains three columns. The first two columns should be the x and y coordinates of each magnetic station. The third column should contain your residual magnetic anomalies. Do not output column names as the first line in the file.
  • To interpret the magnetic observations download the script pointed to below to your machine.

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  Magnetic Model

   

  Note: The modelling applet may not work with some platform/browser/Java combinations. If the sliders are not visible, or do not work, or if clicking-and-dragging does not work in the control panel, the best solution is to change platforms (computers). In the iMac lab - use Internet Explorer for this modelling. If you have access to a Windows machine, use that for the modelling (it will only take a few minutes). There are Windows machines in Room 315. You need only save your reduced data set as a text file (use Save-As and select the tab-delimited text option in Excel, for instance), and use a floppy disk to transfer it to a new machine. The Java modelling system is believed to work fully on Windows 95/98 platforms, running Internet Explorer or Netscape of version 4 or later. Macintosh machines should have the upgrade to Macintosh Runtime Java 2.1.2 or later installed, and Internet Explorer 4.* should then function.

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  • Follow the instructions provided in the script to load and plot your residual magnetic field into the script.
  • Beginning the parameters of the structure derived from your reinterpretation of the gravity profile, model the magnetic profile and estimate the trend and the susceptibility of the buried dyke.
  • Once you have found a preferred model(s), estimate the uncertainties in the model parameters. Do this by systematically varying the model parameters about your preferred values and find all values that fit the observed data to within the data uncertainties.
  • If you did not have the gravity data available to constrain some of the dyke's parameters, what could you infer from the magnetic data alone?
  • Finally, are there other models that could fit the data equally as well as your preferred model that have very different parameters? If these models are geologically plausible, describe what they are and give your rationale for choosing a preferred model(s).

OUTCOMES

The final report, should be in the form of a summary report to your client. The heading can be in standard memo format. The summary report should include:

• A brief review of the basis for the survey design (statement of the problem),
• A summary of the data-processing and interpretation procedures (you may want to refer to a flow chart in the appendices), and
• A clear and concise statement of your preliminary interpretation, and an indication of the action that will be required to refine and validate that interpretation.

As usual, the body of the report must be no longer than two pages. However, it is important to provide enough information (in the appendices) for the client's geophysical staff or consultant to be able to check any of your work. This would include:

• A tabulation of the field data.
• A description of each processing step, including formulas and outcomes.
• For any "standard" corrections that were not done, a description of how they normally would have been done and an explanation of why they were not necessary in this case.
• A narrative discussion of how and why you chose the "possible" models for each anomaly.

As always, remember that the your report also is a sales document; in this case, instead of selling your services, it is selling your competence and the quality of your work. Also remember that your clients are busy executives that probably are out of touch with the technical state of the art. It must communicate quickly and effectively, but it also should convey a sense of competence and professionalism.