In many cases, the seismic response of bottom-simulating reflectors is characterised by low frequencies called “low-frequency shadow”. Generally, this phenomenon is inter- preted as attenuation due to partial saturation with free gas. Actually, this frequency loss may have multiple causes, with a normal moveout stretch as a possible candi- date. To analyse this phenomenon, we compute synthetic seismograms by assuming a lossy bottom-simulating layer, with varying quality factor and thickness, bounded by the upper hydrate-brine/gas-brine and lower gas-brine/brine interfaces. First, we estimate the shift of the centroid frequency of the power spectrum as a function of the travelled distance of the seismic pulse. Then, we perform one-dimensional numerical experiments to quantify the loss of frequency of the seismic event below the bottom- simulating reflector as a function of the quality factor of the bottom-simulating layer and its thickness (due to wave interference). Then, we compute shot gathers to obtain the stacked section, with and without the normal moveout stretch correction and with and without the presence of wave attenuation in the bottom-simulating layer. The results indicate that the low-frequency shadow due to the normal moveout stretch is stronger than that due to attenuation and may constitute a false indicator of the presence of gas. In fact, often, the low-frequency shadow overlies events with higher frequencies, in contradiction with the physics of wave propagation. This is particu- larly evident when the low-frequency shadow is so extensive that the presence of high frequencies below cannot be justified by the acquisition geometry.

Seismic attenuation, NMO stretch and low-frequency shadows underlying BSR events

Gei D;Picotti S
2018

Abstract

In many cases, the seismic response of bottom-simulating reflectors is characterised by low frequencies called “low-frequency shadow”. Generally, this phenomenon is inter- preted as attenuation due to partial saturation with free gas. Actually, this frequency loss may have multiple causes, with a normal moveout stretch as a possible candi- date. To analyse this phenomenon, we compute synthetic seismograms by assuming a lossy bottom-simulating layer, with varying quality factor and thickness, bounded by the upper hydrate-brine/gas-brine and lower gas-brine/brine interfaces. First, we estimate the shift of the centroid frequency of the power spectrum as a function of the travelled distance of the seismic pulse. Then, we perform one-dimensional numerical experiments to quantify the loss of frequency of the seismic event below the bottom- simulating reflector as a function of the quality factor of the bottom-simulating layer and its thickness (due to wave interference). Then, we compute shot gathers to obtain the stacked section, with and without the normal moveout stretch correction and with and without the presence of wave attenuation in the bottom-simulating layer. The results indicate that the low-frequency shadow due to the normal moveout stretch is stronger than that due to attenuation and may constitute a false indicator of the presence of gas. In fact, often, the low-frequency shadow overlies events with higher frequencies, in contradiction with the physics of wave propagation. This is particu- larly evident when the low-frequency shadow is so extensive that the presence of high frequencies below cannot be justified by the acquisition geometry.
Bottom simulating reflector; NMO stretch; low-frequency shadow
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.14083/1768
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact