Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2023 / Paper 1 / 6 / p

Let a positive incident stress wavelet be $w(t)$ and measure return times relative to a receiver just above the reservoir. The top interface gives a negative P return of amplitude $-3/19$ and no basal P return. For a separately generated and measured S wave, the top return is also negative, $-3/19$, while the basal return is positive, $1/9$ at the lower interface.

If $h$ is reservoir thickness, the basal return arrives $2h/V_{P2}$ or $2h/V_{S2}$ after the top return; no numerical $h$ is supplied. Including downward and upward transmission through the top boundary, but ignoring seismic attenuation, spreading and seismic multiples, its observed S-wave amplitude isThus the synthetic S trace has an earlier negative wavelet and a later, smaller positive wavelet; the synthetic P trace has only the earlier negative wavelet. The figure chooses $h=250\mathrm{m}$ solely to separate the pulses visually. Receiver electronics or a displacement convention can reverse the displayed trace polarity, so the convention must accompany the sketch.

A marine hydrophone streamer with a purely normal-incidence P source does not directly record the hypothetical S trace. A seabed multicomponent receiver and suitable S excitation or oblique-incidence seismic mode conversion are required. This distinguishes the calculated solid-layer response from what a particular acquisition system can measure.