Low‐frequency microtremor anomalies at an oil and gas field in Voitsdorf, Austria

Abstract

A B S T R A C TResults of a passive microtremor survey at an oil and gas field in Voitsdorf, Austria, are presented. The survey consists in six parallel profiles approximately 9 km long over two hydrocarbon reservoirs. For each profile the seismic wavefield was recorded synchronously at 11 in-line stations. The measurements were conducted with broadband seismometers and lasted, for each profile, at least 12 hours overnight. Data interpretation is based on a comprehensive data set and on the analysis of four different spectral attributes. These attributes quantify the characteristic features of the wavefield's Fourier spectra in the low-frequency range (<10Hz). One attribute quantifies the spectral energy in the vertical wavefield component, another attribute quantifies the maxima in vertical-to-horizontal spectral ratios and two attributes describe the frequency shifts of peaks within the spectra of vertical and horizontal wavefield components. Due to temporal variations of the signals we combine the long-term measurements (several hours of continuous records) of multiple profiles. This procedure considerably enhances the consistency of each spectral attribute and makes them suitable to quantify lateral variations of the wavefield. The results show that using a combination of several attributes significantly increases the reliability of detecting anomalies in the microtremor wavefield that are presumably caused by hydrocarbon reservoirs. A numerical study of two-dimensional seismic wave propagation is applied to investigate the peak frequency shift attributes. The results of the study indicate that the attributes may contain information on the depth of hydrocarbon reservoirs, assuming that the reservoir acts as a (secondary) source of low-frequency seismic waves.

Spectral analysis of ambient ground-motion—Noise reduction techniques and a methodology for mapping horizontal inhomogeneity

“…1) and is therefore expected to be strongly affected by anthropogenic signals. In fact, Lambert et al (2009) have already suspected (based on results of the 2007 data) that this anomaly is caused by a strong and unstable source, possibly related to cultural activities. This view is now supported with the more comprehensive data-set of 2008.…”

“…4b, suggest that some stations exhibit elevated V/H-amplitudes around 2 Hz. It was suggested that these or similar features could carry information on reservoir-related modifications in the wave-field (Dangel et al, 2003;Lambert et al, 2009;Saenger et al, 2009;van Mastrigt and Al-Dulaijan, 2008). Four spectral seismic attributes are used in this study to quantitatively describe these features and to map their distribution in space and time.…”

“…As a result, most of the measurement locations are not coincident with the stations of the survey in 2008 but are shifted up to 1 km in the cross-line direction. A detailed description of the survey in 2007 is given in Lambert et al (2009). Differences between the 2007 and 2008 data can be explained by (i) temporal changes in noise conditions, (ii) sensors not being at the exact same position for each survey and associated different local surface noise or near-surface effects and (iii) differences in data processing.…”

“…Green and yellow colors illustrate the topography which varies between 400 m and 550 m above mean sea level. A description of the Voitsdorf oil and gas field and the geological setting of the area can be found in Lambert et al (2009).…”

“…The attributes chosen for this study are Attributes 1 to 4 from Lambert et al (2009) because they quantify the features of interest identified in the previous section. Fig.…”