Data Interpretation - Seismology

BACKGROUND

Like resistivity, refraction seismic observations incorporate the use of a controlled source. Also, like resistivity, there is very little, if any, data reduction that needs to be done to the observations before interpretation. Assuming random noise can be effectively reduced by stacking individual sources, the accuracy of the data is affected most by geologic noise and depends on questions such as whether the geology satisfies the theoretical assumptions upon which the interpretative process depends.

This exercise is 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 five 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 refraction seismic survey and how they are related to the data,
  • Continue to gain experience in manual inversion of data where the solution must be built from the surface down,
  • Begin to recognize the necessity of acquiring refraction seismic observations in two directions along the same line, 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 refraction seismic data set using your survey parameters and download them to your computer.

PROCEDURE

Like resistivity data, refraction seismic observations require little, if any, preprocessing. We can, therefore, proceed directly to the interpretation process. The initial stage of the interpretative process, which is not replicated in your suite of observations, is the picking of first arrival times from the raw seismic traces. As we saw in the Observations section , this task can be quite challenging. Because the times of first arrival can often be difficult to unambiguously pick, this process should be considered part of the interpretative process. Thus, even at this level, a geophysicist brings to the table his own prejudices and expectations about what the data should show.

With this warning, let's assume that the arrivals derived from your observations have been chosen with relatively high reliability.

  • First, you need to identify whether or not you are dealing with dipping horizons and to get a preliminary estimate of the structure. You will only be able to identify dipping beds if you acquired seismic observations along the same line in two directions. If you didn't do this, you will have to assume that the beds are horizontal. Import all of the observations derived from the same line into your favorite spreadsheet and interpret using the slope-intercept method. If dipping beds are indicated, note the direction of dip and estimate the size of the dip for each.
  • Now, download the modeling program zip below.
  • As was the case for the resistivity observations, these scripts will allow you to interpret the observations one source location at a time. Using your preliminary estimates of the structure derived above, now attempt to refine these estimates by modeling all of the source/receiver combinations acquired. Remember, you must come up with consistent models. That is, if the subsurface can be approximated by dipping layers, then the dips and depths implied by different sources acquired over the same line should be the same.
  • 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 type-written pages. It is important, however, 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.