Multiple (seismology)
In seismology and seismics, multiple is a type of seismic wave that is reflected through one or more seismic layers several times before they reach a measuring point.
background
A rock layer within which the propagation speed of the seismic wave does not change or changes continuously to a small extent is regarded as a seismic layer . In the latter case, the term gradient layer is used. The seismic layer is defined upwards and downwards by layer boundaries at which the seismic speed changes abruptly or there is a sufficiently large impedance contrast.
If a seismic wave hits such a layer boundary, part of the energy it transports - analogous to optics - is refracted into the adjacent layer , while another part is thrown back into the layer (reflected). This division of the wave energy creates wave trains that run through the ground in different ways. The measurement of the associated transit times (with seismometers or geophones ) allows conclusions to be drawn about the speed structure, which in turn reflects geological processes and phenomena.
Multiple
Seismics is primarily interested in the first reflected inserts of a layer boundary in the seismogram in order to derive the sequence of seismic layers. However, wave trains can also be reflected multiple times within a layer (see figures), which are then referred to as multiple reflections (or multiple for short ). In the seismogram, these appear as a series of time-shifted inserts, some of which have strong amplitudes and thus can disrupt or even completely cover the more interesting reflections from deeper layer boundaries.
In the illustration on the right, some multiple routes are outlined in color. The thin black lines in the figure sketch direct routes through the individual layers, which generate the actual useful signal in the seismogram. Even this small selection illustrates the complexity of possible disruptive multiple reflections in the seismogram. However, since the wave energy decreases with the path ( damping ) and also partially flows into the refracted wave every time it hits a layer boundary, the amplitude decreases with every reflection, so that higher multiples are no longer registered.
Use
In seismology, multiples can also be useful in some areas. Due to the distance to the energy source (usually earthquake ), the multiple phases generated near the source can be neglected. To investigate speed structures in the area below the measuring device, u. a. the method of the receiver functions is used. This uses wave trains that have been converted into another wave type at interfaces. Again, multiple reflections (with or without simultaneous conversion) can occur here as well. Examples are shown in the figure on the left.
The propagation delay of the multiples relative to the direct waves depends directly on the seismic velocities and the depth of the bed boundaries involved. However, if a reliable speed model is not available, the transit time differences of the multiples can be used to check modeled speed structures.
