Data processing Flashcards
Why is data processing done?
To emphasize and enhance wanted anomalies and signals
What does imaging mean?
Data transformation by data processing.
No forward model response calculated. No comparison of measured data and model response.
What is fourier analysis?
A wiggly line, a profile, can be reproduced by adding together a series of waves. Only certain wavelengths are used – harmonics. Based on what the target is shorter or longer wavelengths can be rejected to show the anomaly.
So, fourier analysis is sorting features by their widths, wavelengths, from which we then can select the ones we want. Removing wavelengths improves the signal-to-noise ratio.
Fourier analysis requires the whole signal or profile to be analysed
What is a wavelength?
The distance between two peaks in a sinusoid. Symbol used is lambda.
What is the amplitude?
The maximum deviation from the undisturbed position for a sinusoid function
What is a sinusoid?
A curve described by the mathematical sine function.
What is cosine?
A function that exists together with or follows the sine function.
What is a harmonic?
It’s half a wavelength. The amount of each harmonic can be calculated independently, which is of great importance.
What is a problem with fourier analysis?
Each component due to different geological bodies contain a range of wavelengths, therefore the separation of wanted and unwanted anomalies is only partial.
Shorter wavelengths often imply bodies close to the surface but not always. A broad anomaly can for example be due to a narrow body at depth or a broad body near surface.
What are signals in time measured in?
The frequency is measured in units of Hertz.
Hertz = the number of times a complete cycle repeats in 1 second.
What does digital filtering mean?
Separating wanted from unwanted by size. Can be applied to values taken at a regularly spaced sampling interval, either along a line or on a grid.
Digital filtering needs only a few successive readings at a time, those in the “window”. Gives a less complete separation of wavelengths than fourier analysis.
What does smoothing, low pass, high pass and band pass mean in digital filtering?
Smoothing - done to reduce jaggedness
Low pass filter - lets through wavelengths longer than some value.
High pass filter - lets through wavelengths shorter than some value.
Band pass - lets through only a range of intermediate wavelengths.
What does the phenomena called aliasing mean?
Sometimes the sampling interval is exactly equal to the wavelength which makes all the readings the same, so there seems to be no variation.
When the sampling interval is only a little different from the wavelength the signal appears to have a very long wavelength. Called the apparent wavelength. This production of spurious wavelengths is called aliasing.
Once the sampling interval exceeds half the wavelength the apparent wavelength is always longer than the true one.
What is the Nyquist wavelength?
It is the critical wavelength. It’s twice the sampling interval.
It is possible to design your digital filter. One aspect that has an effect is the sampling interval. How should it be chosen?
The sampling interval is chosen to be somewhat shorter than the shortest wavelength we wish to retain; a quarter of this length is a suitable sampling distance.
