<p>I have generated a synthetic seismogram using FDTD elastic velocity-stress formulation on a staggered grid. I can't understand origin of oscillations modulating on seismogram. 
<a href="https://i.sstatic.net/HggTo.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HggTo.jpg" alt="enter image description here"></a></p>

<p>Is it a result of dispersion? 
I have satisfied the grid dispersion criteria (10 grid per minimum wavelength) which keeps it below 5% but still it is very prominent.</p>

<p>Model description:
Velocities: Vp = 2000 m/s;  Vs = 1500 m/s;
Source:  Ricker (20Hz).
Grid spacing:  dx = dv = 1m;
time sampling: dt = 0.1msec</p>

<p><strong>Edit 1</strong></p>

<p>I have corrected one problem related to source (now I am using soft source) and it made initial pulse smooth. However its tail still have oscillations.
<a href="https://i.sstatic.net/j2BN0.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/j2BN0.jpg" alt="Pressure corrected"></a></p>

CC BY-SA 3.0
2016-04-14T23:06:21.343Z
0.1 ms seems like a very short interval, given the other parameters. Does it help at all if you use 1 ms? Make sure you are handling units properly (I find it best to do everything in seconds).
Matt Hall
CC BY-SA 3.0
2016-04-15T17:25:51.773Z
@kwinkunks I took care of units in first run itself. I carried out experiment with various values of 'dt' but all these oscillations increase with increase in time. I am using a Ricker wavelet of 30Hz frequency. Any suggestions about origin or cause of these oscillations.
Amartya
CC BY-SA 3.0
2016-04-21T15:20:22.247Z
Do you have any sharp edges that introduce oscillations after transformation?
Jesper
CC BY-SA 3.0
2016-04-22T16:39:55.487Z
@WayoftheGeophysicist I have taken a homogeneous model so it doesn't have any discontinuity. Additionally, all the edges have absorbing boundary layers so they won't be producing any reflections.
Amartya
CC BY-SA 3.0
2016-04-26T01:04:30.637Z
Have you considered taking a FFT of your data and identifying the wavelengths corresponding to the oscillation?
BarocliniCplusplus
CC BY-SA 3.0
2016-04-27T20:34:19.940Z
I am really hoping that someone with knowledge of numerical modeling will help with this question. I am very curious to see the answer. My guess would be numerical dispersion but there may be other things to consider. The fact that you used a staggered grid should help reduce this. Is there a relationship between the geometry of the grid and the expected frequencies of the dispersion artifacts? This is a very interesting question and I hope you get a good answer soon.
Antonio
<numerical-modelling><seismic>
Unwanted oscillations in Synthetic seismogram
Amartya
# Unwanted oscillations in Synthetic seismogram

(Asked by: username_0 on 2016-04-14)

I have generated a synthetic seismogram using FDTD elastic velocity-stress formulation on a staggered grid. I can't understand origin of oscillations modulating on seismogram.
[![enter image description here](https://i.sstatic.net/HggTo.jpg)](https://i.sstatic.net/HggTo.jpg)

Is it a result of dispersion?
I have satisfied the grid dispersion criteria (10 grid per minimum wavelength) which keeps it below 5% but still it is very prominent.

Model description:
Velocities: Vp = 2000 m/s; Vs = 1500 m/s;
Source: Ricker (20Hz).
Grid spacing: dx = dv = 1m;
time sampling: dt = 0.1msec

**Edit 1**

I have corrected one problem related to source (now I am using soft source) and it made initial pulse smooth. However its tail still have oscillations.
[![Pressure corrected](https://i.sstatic.net/j2BN0.jpg)](https://i.sstatic.net/j2BN0.jpg)

**Tags:** numerical-modelling, seismic

> **Comments:**
> **username_1**: 0.1 ms seems like a very short interval, given the other parameters. Does it help at all if you use 1 ms? Make sure you are handling units properly (I find it best to do everything in seconds).  
> **username_0**: @kwinkunks I took care of units in first run itself. I carried out experiment with various values of 'dt' but all these oscillations increase with increase in time. I am using a Ricker wavelet of 30Hz frequency. Any suggestions about origin or cause of these oscillations.  
> **username_2**: Do you have any sharp edges that introduce oscillations after transformation?  
> **username_0**: @WayoftheGeophysicist I have taken a homogeneous model so it doesn't have any discontinuity. Additionally, all the edges have absorbing boundary layers so they won't be producing any reflections.  
> **username_3**: Have you considered taking a FFT of your data and identifying the wavelengths corresponding to the oscillation?  
> **username_4**: I am really hoping that someone with knowledge of numerical modeling will help with this question. I am very curious to see the answer. My guess would be numerical dispersion but there may be other things to consider. The fact that you used a staggered grid should help reduce this. Is there a relationship between the geometry of the grid and the expected frequencies of the dispersion artifacts? This is a very interesting question and I hope you get a good answer soon.  

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## 0 answers

