geopsy.org

Posted: **Mon Jan 04, 2021 12:51 am**

Dear Marc,

I have a couple of questions about the H/V toolbox in Geopsy. Under the Processing tab, there is the option to apply a taper- I am choosing to apply a Cosine taper. I have seen in previous versions of the software an option for modifying the width of the cosine taper (i.e. giving a percentage) that is not available on my version of the software. Is this option no longer available? (see below). Also, is there no longer the ability to explicitly set the smoothing constant value (b) for the Konno-Ohmachi smoothing?

My current version of software (no option to set tapering width or smoothing constant value):

: attach_1.png (25.05 KiB) Viewed 11222 times

Previous version- image from Wikipedia (with option for setting Cosine taper width and smoothing constant):

: attach_2.png (39.44 KiB) Viewed 11222 times

Finally, under the Output tab in the H/V toolbox, the option for setting the 'number of samples' was previously available- but is no longer visible. Can sample number still be edited in the current version of Geopsy? I see it has been replaced by a 'step value/step count' option - but I am not sure if this refers to the number of samples (see below).

My version of Geopsy (cannot set number of samples):

: attach_3.png (22.8 KiB) Viewed 11222 times

Previous version of Geopsy (can set number of samples) - image from Wikipedia:

: attach_4.png (18.28 KiB) Viewed 11222 times

Any help would be greatly appreciated.

Many thanks,

Luke

Posted: **Mon Jan 04, 2021 8:59 am**

Hi Luke,

Taper

Cosine taper is a cosine variation over the whole time window. Only the sample at the middle of the time window is unaffected. There is no parameter for such window function. To get the same results as in the previous releases, you have to choose Tukey taper: a cosine function over a parametrisable range at the beginning and at the end of the time window. There you can adjust the percentage.

Smoothing

The classical Konno-Ohmachi is obtained by, obviously, selecting the Konno-Ohmachi window but also by choosing a log width. The original b constant value is inversely proportional to the width of the smoothing window, which was somehow counter-intuitive. A value of b=40 corresponds to a 20% log width. A tooltip is displayed if you stop with the mouse anywhere on the smoothing box. It provides the following equivalences:

Code: Select all

b=20 ==> width=40%
b=40 ==> width=20%
b=80 ==> width=10%

Note that it is not an exact equality but very close. To be precise, the exact formula is

Code: Select all

width=10.0^(pi/b)-1.0

Hence more precise values are:

Code: Select all

b=20 ==> width=43.575%
b=40 ==> width=19.823%
b=80 ==> width=9.4636%

The scale type is a new option that was not available previously. H/V amplitudes have always been averaged on a log scale (geometrical average) since SESAME. However the original smoothing algorithm was summing amplitudes on a linear scale. This sounded like inconsistent. The Log scale type is probably better. This is now the default value.

Frequency samples

Effectively, sample number is replaced by two options:

Step value: defines a fixed step between samples. For a log scale, the default value is 1.025. This is the common ratio of a geometric progression. A sample value multiplied by the step value provides the next value.
Step count: you can specify a fixed number of samples as in the previous releases

The first option has the advantage that it fully controls the density of samples. The minimum and maximum values can be changed without affecting the density of samples which is not the case with the second option. Additionally samples are always aligned to 0 and 1 for linear and log scales, respectively. Hence you can change the minimum and the maximum frequency, you will always get the same values, which is easier for comparing curves obtained with various processing.

I hope I answered all your questions.

Best regards,

Marc

Posted: **Mon Jan 04, 2021 4:08 pm**

Hi Marc,

Thank you very much- that makes much more sense now.

Luke

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