Data Reduction and Interpretation
BACKGROUND
This will be your first exposure to the interpretation of data from a survey where
the source can be controlled to focus on a specific objective.
In such surveys, the design of the survey is especially important because many (if
not most) non-geologic sources of error can be eliminated by use of proper instrumentation
and careful field procedures.
The accuracy of controlled source data is affected mostly by geologic noise and depends on
questions such as whether the geology satisfies the theoretical assumptions upon which
the interpretative process depends.
In this exercise, the process will be similar to that used in the previous interpretive
exercises.
Once again, the emphasis is on codifying the process, gaining a conceptual understanding
of the physics, and identifying sources of error.
OBJECTIVES
There are four learning objectives:
- Continue to refine your understanding of the interpretative process and the role of
modeling, validation, and sensitivity analysis,
- Develop a conceptual understanding of the physical parameters in a resistivity survey
and how they are related to the data,
- Gain experience in manual inversion of data in the case where the solution must be
built from the surface down, and
- Codify the sources of ambiguity and create rules for the interaction of model
parameters when inverting resistivity data.
To continue with this exercise, you first need to
generate a resistivity data set using your survey parameters
and download them to your computer.
PROCEDURE
Unlike the gravity and magnetics observations we have collected, the observations derived from
this controlled source experiment require no additional processing before interpretation.
We can, therefore, proceed directly to the interpretation process.
- With your resistivity observations in hand, download the appropriate interpretation script pointed to below.
The data file you received should contain two columns.
The first column contains the potential electrode spacing for Schlumberger soundings or the electrode
spacing for Wenner soundings, in meters. The second column contains the apparent resistivity computed
from the voltage, current, and array geometry specified in the survey design or by instrumentation.
Schlumberger or Wenner Arrays
- Follow the instructions provided in the script to load and plot your resistivity observations.
In all cases, note that
this involves modifying the script, usually by simply replacing a file name with one that contains your observations.
Because we have saved information concerning each sounding in separate data files, you will have to load and
interpret each separately.
- You may now begin to model the observed apparent resistivity curves. The form of the models you will use
are identical to those you used in designing the resistivity survey; they consist of layered models, two layers
over a halfspace maximum. Thus, you need to adjust layer thicknesses and resistivities to attempt to match
the observed resistivity soundings.
- 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.
- 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 the following:
- A brief review of the basis for the survey design (statement of the problem),
- A summary of the data acquisition and interpretation procedures (you may want to refer to a flow
chart in the appendices), and
- A clear and concise statement of your 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 the acquisition parameters used, and
- A narrative discussion of how and why you chose the "possible" models for each anomaly.
As always, remember that the your report is also 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.
