A passive low‐frequency seismic experiment in the Albertine Graben, Uganda

Abstract

A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low‐frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water‐bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface‐wave (fundamental mode) dispersion curve as well as the observed horizontal‐to‐vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low‐frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.

Sedimentary basins investigation using teleseismic P‐wave time delays

“…The absolute time delays defined here can be considered a point measurement of P‐wave velocity and can be used in parallel with other complementary approaches, that is modelling of surface waves dispersion in ambient noise measurement (Martini et al . , ) and receiver function analysis (Licciardi and Piana Agostinetti ; Piana Agostinetti et al . ).…”

“…Ambient noise measurements can be used to constrain the dispersion of the surface waves passing through the sedimentary strata and therefore, by inversion methods, to map the structure of a basin (Martini et al . , , and references therein). Moreover, seismic waves generated by large magnitude earthquakes occurring at very large distance from the target area (so‐called teleseisms , with sources located at more than 3000 km from the recording seismic stations) also carry valuable information for constraining the elastic properties of the basin of interest (Srinivas et al .…”

Integrated geophysical exploration in onshore frontier basins

“…This study is discussed in detail in Martini et al . (, ), and we refer to these publications for the data acquisition and analysis.…”

“…Where data are available (velocity, density or depth of basement), the inversion can be constrained and the results are therefore more reliable. For more details on the technique we refer to Martini et al (2013Martini et al ( , 2015, and references therein.…”

Effective Fluid Bulk Modulus in the Partially Saturated Rock and the Amplitude Dispersion Effects

“…Kazantsev () and previous authors concluded that the physical mechanisms behind the low‐frequency microtremor anomalies are unclear in the literature. Several authors argued that these anomalies could be related to the anthropogenic noise, geological structure, surface waves, human errors during interpretation, and so forth ( e.g.,Ali et al, ; Hanssen & Bussat, ; Martini et al, ; Woith et al, ). In this study we demonstrate that these anomalies could be explained by the transfer of the low‐frequency energy of ocean waves (0.1–1 Hz) to higher frequencies (2–6 Hz) by a partially saturated reservoir due to a nonlinear amplitude dispersion effect caused by a static contact angle hysteresis.…”