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A framework for applying geophysics


 

What's down there?Seven steps to applying geophysics for any geological or geotechnical problem

Whether the problem involves characterizing a few cubic metres, or a volume of ground hundreds of metres thick underlying tens of square kilometers, the following seven steps invariably will be addressed at some point in the project. All seven tasks are inter-related so the distinction between the steps can become blurred. For example, the geoscience problem will determine an appropriate interpretation procedure, which in turn will place constraints upon the survey design and choice of processing steps. Also, data processing, interpretation and synthesis are often particularly tightly related. However it is useful to think in terms of these seven steps because they form a framework which can be employed for any application of geophysical work to applied geoscience problems.

1.
Setup:
Establish the geoscience objectives, consider conventional practice, and identify how geophysics might contribute.
2.
Properties:
Characterize materials that can be expected and establish the likely physical property contrasts.
3.
Surveys:
Determine a suitable geophysical survey, and design an effective and efficient field survey. Identify possible sources of error, noise and mis-interpretation.
4.
Data:
Carry out the field survey, taking all necessary actions to ensure compelete, high quality, and cost effective data sets.
5.
Processing:
Plot the data, and apply appropriate processing and analysis.
6.
Interpretation:
Interpret results in terms of physical property distributions, and then in terms of the original geoscience objectives.
7.
Synthesis:
Combine interpretations with prior knowledge about the problem, and with other relevant information. Decide if your results are adequate for the particular problem. Iteration is usually necessary.

This sequence of images summarizes the process:

1. Setup

2. Properties
3. Surveys
 gathering data in the field
4. Data
raw data

7. Synthesis:
- Is the result adequate?
- Do results correlate with prior
   and  alternative information?

Iteration back to previous steps is expected  before finalizing the work.

6. Interpretation
5. Processing
model produced by inverting data

Top of pageA few more details for each of the seven steps are outlined in the following table:

1.
Setup:
Establish the geoscience objectives, consider conventional practice, and identify how geophysics might contribute.
Details of setting up the problem depend upon which of the four general task types is involved.
- Buried object location
- Mapping "apparent" physical properties
- Identifying boundaries where physical property values change
- Mapping detailed locations and depths of actual physical property values
2.
Properties:
Characterize materials involved and establish the likely physical property contrasts.
Understanding how physical properties relate to geophysical work is crucial. The most important relevant physical properties are:
1. Density;   2. Compressional wave and shear wave velocities;   3. Magnetic susceptibility;   4. Electrical conductivity (or resistivity);   5. Electrical chargeability;   6. Dielectric permittivity
3.
Surveys:
Determine a suitable geophysical survey, and design an effective and efficient field survey. Identify possible sources of error, noise and mis-interpretation.
Successful application of geophysical techniques depends upon careful survey design and data acquisition. Matching surveys to suit the relevant physical properties is important, and forward modelling may contribute towards building appropriate expectations for data quality, noise levels, and suitablility to the task.
4.
Data:
Do the field survey taking all necessary actions to ensure compelete, high quality, and cost effective data sets.
Geophysical data can be gathered inside boreholes, using ground-based systems, or from aircraft. Field procedures must permit acquisition of high quality data, yet they must be economical, safe, and reliable.
5.
Processing:
Plot the data, and apply appropriate processing and analysis.
In nearly all cases interpretation can not proceed until some form of data processing has been applied. This may be as simple as a calibration, or it may involve multiple numerical processing steps or data inversion.
6.
Interpretation:
Interpret results in terms of geological or geotechnical objectives.
The goal of interpretation is to draw conclusions from the geophysical data. Two types of conclusions were introduced, the first being an understanding of physical property distributions, and the second being a geological understanding derived from models of physical property distributions.
Non-uniqueness was identified as a key characteristic of most geophysical interpretations.
7.
Synthesis:
Correlate with prior and alternative information, and decide if your results are adequate for the particular problem.
Synthesis means making sure geophysical results agree with everything else that is known about the problem. Also a judgement must be made about the effectiveness and completeness of the geophysical results.

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