<< Click to Display Table of Contents >> Navigation:  Easy HVSR > The steps of the analysis > Spectrum H/V

 

Spectrum H/V

 

The "Spectrum H/V" window is divided into five sections: "Reliability curve H/V", "Reliability peak", "Model", "Automatic search" and "Results". The "Automatic search" is present only when is selected the option of automatic search as will be explained later.

In this window are collected all the features useful to the identification of possible stratigraphy associated with the spectral ratio H/V until now processed, and therefore useful to the identification of a reliable value of the parameter Vs30.

 

Reliability curve H/V

This section, along with the next one, collect the checks suggested by SESAME project as regards the reliability of the curve of the spectral ratio H/V and its peak until now processed. These checks are not binding on the identification of the model in the following sections, but if the user wants them to be verified is necessary to return to the previous steps of the analysis. For further clarification about the checks see the section on the description of the technique in this manual.

 

Reliability peak

As detailed in the description of the previous section, this is part of the checks required by the SESAME project.

 

Model

This section contains all the tools necessary to identify, store and evaluate the stratigraphic model that best approximates the behavior observed with the spectral ratio H/V obtained up to this stage of the analysis. This section provides two ways to search the model: manual and automatic.

 

Fields common to both modes:

 

Number of modes

This field allows you to set the number of the ellipticity curve concerning the modes to display the graphic of the spectral ratio H/V of the present model (or loaded) in the table of the model.

 

Show more models

Selecting this option is displayed on the graphic of the spectral ratio H/V the ellipticity curve relative to the stored models, listed and selected in the models table below.

 

Table of models

This table collects the models that are stored showing the name and the value of mismatch if calculated. The first column contains a check mark for each stored model that allows to exclude it from being displayed: if the check mark is present the ellipticity curve of the model is represented on the graphic of the spectral ratio H/V, otherwise it is excluded from display .

 

Store

This button saves the model in the main table of models, giving it a name that has as root "Model" and suffixed with a sequential number function of the number of models currently stored. If the storage is made in automatic search mode is also stored the value of mismatch.

 

Remove (table of models)

Removes the selected model from the list of store models.

 

  Add layer

This button adds a layer at the end of the model's stratigraphy.

 

Remove layer

This button removes the selected layer  form the model's stratigraphy.

 

Insert layer

This button inserts a layer to the model's stratigraphy immediately prior to the one selected in the table below.

 

Import stratigraphy XML

This buttons offers the possibility to import a stratigraphy from an XML file (previuosly created with other software provided by GeoStru).

 

Summarize

Launches the calculation procedure required to assess the ellipticity curve (or curves if it is set a number of modes greater than 1) of the model currently present in the table below. If the automatic search mode is  activated, the mismatch value is also calculated.

 

Manual search

This mode allows to search manually a model that approximates the behavior of the spectral ratio H/V calculated in the previous steps. This mode consists in inserting in the table below a number of layers that the user considers it appropriate, and inserting for each of them the geotechnical characteristics: the thickness of the layer expressed in meters, the unit weight (if submerged directly enter the dry unit weight) expressed in kN /m3, the Poisson's ratio and the shear wave velocity in m/s. By pressing the button "Summarize" the ellipticity curve is shown.

 

Automatic search

This mode allows to automatically search the geotechnical characteristics of the layers of the model in the table. By activating this mode it becomes visible the additional section called "Automatic search" in which some useful parameters for this search mode can be set.

N.B. The automatic search works exclusively on the research of the thickness and the speed of the shear waves.

 

Within the table of the model additional columns become visible immediately to the left and to the right of those of the thickness and the velocity of shear waves (ie, the parameters on which the search is performed). These new columns are used to indicate the variability limits of the parameter for the single layer, or identify a search range of the parameter and if that parameter must be actually considered as part of the search. The automatic search works only these two parameters as the SESAME project has been able to confirm, the unit weight and the Poisson's ratio have little influence on the variability of the model's response.

So, in this mode, for the thickness and the velocity of shear waves of each layer, it is possible to indicate whether the single parameter should be included in the search, and in that case what are its limits of variability. It is also to be considered that in the HVSR technique, in order that the procedure has its own validity, it is necessary to know the characteristics of the first layer. That's why Easy HVSR asks for confirmation to constrain the parameters of the first layer in the event that these, at the start of the automatic search procedure, have not yet been selected. This does not preclude proceeding equally with a search that includes the parameters of the first layer.

The thickness of the last layer is however constrained as it does not participate in the calculation as it represents the half-space lying below the stratigraphy, and that is searched for, possibly, only the propagation velocity of the shear waves.

Further consideration to take note of is the fact that the automatic search does not optimize the number of layers so that it remains at the sole discretion of the user.

In this mode the user can still make a manual search (through the same procedure as described above), with the only difference that in this way is also assessed the value of the mismatch.

Moreover, even if the automatic search provides a first solution, this can be relaunched from the solution just found, this is because the automatic search finds only one possible solution.

 

N.B.: Given the nature of the problem itself, namely non-definition of the source that generates the noise of the recording both in terms of amplitude and position, provides no reliability on the amplitude of the  H/V spectral ratio, for which reason the automatic search does NOT provide the exact solution in finding a stratigraphic model, but only a starting point from which to start a MANUAL "tuning" and "refining" of the model from the user, in which no method or contrivance can raise the ability to interpret which derives from the technical and practical experience of this kind of problems. So the automatic search in Easy HVSR is simply a tool of (notable) support for the user in interpreting the results.

 

Automatic search

In this section are presented some parameters and useful information to the automatic search of the stratigraphic model that best approximates the behavior of the  H/V spectral ratio.

 

Profile increasing velocities

This option, if selected, sets a constraint on the search of the shear wave velocity, which requires that the difference between the velocity of shear waves in two consecutive layers is equal to or greater than zero.

 

Mismatch value

This value indicates how well the identified stratigraphic model approximates the behavior deduced from the H/V spectral ratio according to a criterion for which a value of mismatch less than a larger one achieves a better match. In practice, if ideally this value was zero, it would be unequivocally identified the correct stratigraphy. This condition is clearly impossible even only taking into account the fact that all measurements made ​​are affected by error. So basically this value is useful for comparing multiple solutions of models with each other to choose the one with the lowest value.

 

Maximum number of iterations

Since the automatic search of the stratigraphic model is an iterative process, this value is used to set a maximum number of iterations. It has a purely applicative value in the sense that has no direct bearing on the calculation which could still end before reaching this limit value, and therefore a high value does not necessarily coincide with a better approximation of the solution.

In fact, the achievement of the maximum number of iterations (saturation of the calculation progress bar at the bottom right of the application window) means that the search did not produce a reliable result, and that the solution returned is only the best found during the search process.

 

Results

This section simply shows the results of the entire analysis procedure so that the user can make own considerations:

 

Graphical configuration

In the event that there is no layer in the model table, the graphical configuration of the right side of the application window assumes the same graphical configuration of the previous stage of analysis (Spectral analysis). Inserting even a single layer are displayed the following graphics:  at the top, the graphic of the H/V spectral ratio and eventual ellipticity curves of the model in the main table and those that may be stored and selected in the table of models; immediately below two graphics of which the one on the left is a graphical representation of the stratigraphic model present in the main table, while the one on the right is the trend of the profile of shear waves velocities with depth.

 

 

© Geostru