Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 6 / e

An explosive seismic source gives an impulsive, potentially high-energy broad-band signal and can couple effectively to competent material. It can be useful where strong penetration is needed and heavy vibrating vehicles cannot reach the site. Disadvantages include destructive ground disturbance, permitting and specialist handling, variable source coupling and signature, and the cost of repeated source preparation. The comparison does not require a prescription for charge size or emplacement.

Vibroseis spreads controlled energy over a longer frequency sweep. It is repeatable, permits tailored bandwidth and repeated sweeps, and its known signal supports correlation processing and rejection of unrelated noise. It avoids the impulsive ground disruption of an explosive source. Disadvantages include vehicle access and terrain restrictions, dependence on baseplate coupling, limited force and low-frequency performance, possible harmonic distortion, and strong near-surface waves. For a fractured shallow geothermal site, test which source produces usable coherent reflections or arrivals in the desired band; nominal source energy alone does not establish data quality.

At each source position, couple the vibrator to the ground, transmit a known frequency sweep $s(t)$ and record the ground response on synchronized geophones. Record a pilot or measured source-force reference and sufficient listening time for delayed arrivals. Repeat sweeps where beneficial and retain geometry and timing information.

If the Earth’s impulse response is $h(t)$, a linear model is $d=s\ast h+n$. Vibroseis correlation cross-correlates the record with the known sweep:The sweep’s autocorrelation $R_{ss}$ is a compact, approximately zero-phase wavelet, so the long chirp is compressed into identifiable delayed events. In the frequency domain this multiplies $D(\omega )$ by $\overline{S(\omega )}$, yielding $H(\omega )|S(\omega ){|}^2$ plus noise. It is pulse compression, not an exact recovery of $h$ at frequencies never transmitted. Spectral shaping or stabilized deconvolution can further compensate the finite source bandwidth and sidelobes.^[ref]