The numerical dispersion error has two main contributions, one from the spatial
and one from the temporal discretization. The temporal dispersion error can be
controlled by the chosen time step (up to the stability limit often dictated by
geometrical constraints) and time integration scheme. On the other hand, the
spatial error of the spectral element scheme can be controlled by element size
and polynomial order.
In finite-element slang, h usually refers to the grid spacing or size of the
elements - even if the element sizes may vary within the domain.
In contrast, p refers to the polynomial degree of the finite-element test
Consequently, h-refinement means to decrease the element size, i.e., use more
elements to cover the same domain, and, respectively, p-refinement means to
increase the polynomial degree within the elements.
While the total number of elements remains the same in the latter case, both
strategies increase the total number of degrees of freedom (dofs).
On the first glance, it may look as if both strategies give a very similar
result. However, the differences are important.
One key motivation behind the spectral-element method is that a higher
polynomial degree quickly reduces the approximation error. Hence, for larger
values of p fewer grid points per wavelength are typically sufficient to
obtain the same accuracy.
So why should we not always do p-refinement and choose p as high as
There are two main limiting factors. The algorithmic intensity increases with
higher p as more dofs contribute to the computation of the test functions or
their gradients within the elements.
Furthermore, the model parameters are smooth within the elements by
construction, and discontinuities in the medium properties can only occur
across element boundaries. Thus, in media with sharp interfaces the size of the
layers may limit the maximum element size.
Therefore, one needs to find a good balance between element size and
polynomial degree. In practice, p=4 is the most widely used choice for p.
Finally, as both methods increase the number of dofs, the spacing between grid
points decreases, which in turn necessitates a smaller time step due to the CFL
condition. Note that even if the total number of dofs is the same for both
cases in the example above, the location of the grid points is slightly
different, which might have an effect on the stability limit of the time step.
The total spatial dispersion error increases linearly with the propagation
distance from the source to the receiver, when the distance is expressed in
number of wavelengths. It is thus useful to consider the error that is
accumulated for each cycle that the waves go through and this can be derived
approximately as a function of the number wavelengths per element nλ
and polynomial order p:
In practice, the following figure showing this relation can be used to
determine nλ, which is needed to compute the local minimum element
size from the maximum frequency and the velocity model and a mandatory input to
all automatized meshing routines in salvus.
For example, if we consider an error of 1% on the seismogram acceptable and
want to propagate waves for 100 wavelength in distance, the dispersion error
per cycle should be 10−4, indicated by the blue dashed line. Using a 4th
order scheme then suggests to use about 2 elements per wavelength in the mesh.
To be more explicit for the most popular order p=4, the following figure
shows the total dispersion error for various propagation distances in terms of
The meaning of 'maximum frequency' is not well defined as the source time functions fall of smoothly towards high frequencies and the maxium frequency then corresponds to a chosen amplitude ratio. For the seismology community, this lead to the rule of thumb that one element per wavelength at the maximum frequency is sufficient for many applications. This is because the seismograms contain very little energy at this frequency.
The choice for the time step has two main critieria: firstly, explicit time
stepping schemes as typically used in the spectral element method are
conditionally stable. This means that there exists a maximum time step at which
the simulation becomes unstable and this limit is provided by the CFL
Here C is the Courant number that is specific to the time scheme, D is the
dimension of the problem, Ω is the spectral radius that depends on the
spatial discritization scheme, h is the element size and c is the velocity.
The minimum of this expression on the whole domain is a necessary condition for
stability and is used by default if the user does not provide a time step.
However, this may not be a sufficient criterion for the required accuracy.
As a second criterion the temporal dispersion error needs to be limited similar
to the spatial error discribed above. The temporal dispersion error can be
For the 2nd order Newmark scheme that Salvus uses the constants take the values
B=1/24 and q=2. The following figure shows dispersion error per cycle as a
function of the time step Δt times the frequency f:
To be more explicit, we again show the total dispersion error for several
In practice it is advisable to pick a time step that leads to a similar order of magnitude in temporal and spatial dispersion error to obtain the most efficient schemes. For models with small geometrical features, the CFL condition may be limiting this choice. This is for example the case for global simulations where the crustal thickness dictates small elements.