Reply to comment on ‘Low‐frequency microtremor anomalies at an oil and gas field in Voitsdorf, Austria’ by Marc‐André Lambert, Stefan M. Schmalholz, Erik H. Saenger and Brian Steiner, Geophysical Prospecting 57, 393–411

Abstract

We welcome the opportunity to respond to Professors Green and Greenhalgh's analysis of our work in this journal. In addressing their observations we hope to contribute towards a better understanding of our work in general.Green and Greenhalgh (2009) commented not only on Lambert et al. (2009) but also on peer-reviewed papers, conference proceedings and magazine articles authored by at least one of us, thereby combining formative early work with more advanced results in their critique. Additionally, they commented on publications that none of us has co-authored (e.g., Dangel et al. 2003;Walker 2008), addressing questions to a scientific community beyond our research group. This reply necessarily focuses only on the work that at least one of us has co-authored.Green and Greenhalgh (2009) asserted that our conclusion that "the results indicate that passive low-frequency spectral analysis can increase the probability of locating reservoirs significantly" is unfounded. We identify three main arguments in their commentary: i) that our attributes are not applicable to reservoir detection, ii) that the results of our numerical study do not fit the data and iii) that our attributes are in fact sensitive to shallow structures and we inadequately consider these potential shallow effects in our work. We first respond to these three main points and then to additional criticism, which is included as discussion within their commentary. Finally, we describe the objectives of our research to put it in the correct perspective and to address Green and Greenhalgh's (2009) as- * Green and Greenhalgh (2009) expressed the view that our Attributes 1 and 2 do not indicate the two reservoir locations. Based on our qualitative analysis of the correlation between these spectral attributes and reservoir locations, we agree with them. In Lambert et al. (2009) we explicitly stated: "Attributes 1 and 2 are not sensitive for the northern reservoir", "For Attribute 1, temporal variations are rather large and, therefore, the observed anomalies are less significant" and "Attribute 2 behaves very stable in time . . . the profiles do not show clear lateral anomalies". We clearly stated that the profiles of Attributes 1 and 2 are not useful for detecting the two reservoirs at this site. We do not see the supposed contradiction in Lambert et al. (2009) because we did not claim that all attributes must show anomalies above both reservoirs in order to increase the probability of detecting reservoirs. Attributes 3 and 4Attributes 3 and 4 are based on more stable frequency values rather than on amplitude values, which can vary substantially in experiments utilizing an uncontrolled source. Green and Greenhalgh (2009) wrote that "Lambert et al.'s (2009) simplistic physical exploration is both implausible and inconsistent with (our) observations". Our numerical study was in fact a feasibility (or plausibility) study intended to show that seismic signals emitted by a subsurface source in a homogeneous, C 2009 European

On the use of microtremors for hydrocarbon detection

“…(, ) and Steiner, Saenger and Schmalholz () opened a lively debate in the geophysical literature, involving Green and Greenhalgh (, ) and Lambert et al . (), among others. Spectral anomalies in the frequency band between 1 and 6 Hz were presented as empirical tools for hydrocarbon detection in a few case histories (van Mastrigt and Al‐Dulaijan ; Saenger et al .…”

Microtremor spectra: a proven means for estimating resonant frequencies and S-wave velocities of shallow soils/sediments, but a questionable tool for locating hydrocarbon reservoirs

“…The HMS approach to analyzing 1-10 Hz microtremors is extremely simple: anomalously high vertical-component microtremor ground velocities are said to be recorded above hydrocarbon-rich sediments but not above water-saturated ones (Dangel et al, 2003;Schmalholz et al, 2006aSchmalholz et al, , 2006bGraf et al, 2007;Holzner et al, 2005Holzner et al, , 2007Lambert et al, 2007Lambert et al, , 2009Steiner et al, 2007Steiner et al, , 2008aSteiner et al, , 2008bSaenger et al, 2007aSaenger et al, , 2007bSaenger et al, , 2009Walker, 2008;Frehner et al, 2009;Holzner et al, 2009). High vertical-component velocities are variously defined by elevated values of the following spectral attributes: Elevated attribute values have been described as potential or direct hydrocarbon indicators (Holzner et al, 2005;Schmalholz et al, 2006a;Graf et al, 2007;Lambert et al, 2007Lambert et al, , 2009Saenger et al, 2007aSaenger et al, , 2009. Because ratios are less influenced by source effects than V or H alone (Nakamura, 1989;Fäh et al, 2001;Bonnefoy-Claudet et al, 2006), V/H has been emphasized in recent HMS papers (Graf et al, 2007;Lambert et al, 2006Lambert et al, , 2007Lambert et al, , 2009Saenger et al, 2007aSaenger et al, , 2007bSaenger et al, , 2009Walker, 2008).…”

“…High vertical-component velocities are variously defined by elevated values of the following spectral attributes: Elevated attribute values have been described as potential or direct hydrocarbon indicators (Holzner et al, 2005;Schmalholz et al, 2006a;Graf et al, 2007;Lambert et al, 2007Lambert et al, , 2009Saenger et al, 2007aSaenger et al, , 2009. Because ratios are less influenced by source effects than V or H alone (Nakamura, 1989;Fäh et al, 2001;Bonnefoy-Claudet et al, 2006), V/H has been emphasized in recent HMS papers (Graf et al, 2007;Lambert et al, 2006Lambert et al, , 2007Lambert et al, , 2009Saenger et al, 2007aSaenger et al, , 2007bSaenger et al, , 2009Walker, 2008). The generation of anomalously high vertical-component ground velocities within hydrocarbon reservoirs is a prerequisite of the HMS approach (Holzner et al, 2005;Walker, 2008;Saenger et al, 2009).…”

“…Because ratios are less influenced by source effects than V or H alone (Nakamura, 1989;Fäh et al, 2001;Bonnefoy-Claudet et al, 2006), V/H has been emphasized in recent HMS papers (Graf et al, 2007;Lambert et al, 2006Lambert et al, , 2007Lambert et al, , 2009Saenger et al, 2007aSaenger et al, , 2007bSaenger et al, , 2009Walker, 2008). The generation of anomalously high vertical-component ground velocities within hydrocarbon reservoirs is a prerequisite of the HMS approach (Holzner et al, 2005;Walker, 2008;Saenger et al, 2009). Several physical mechanisms for creating the necessary anomalous conditions within reservoirs have been proposed (Schmalholz et al, 2006a(Schmalholz et al, , 2006bGraf et al, 2007;Holzner, 2007Holzner, , 2009Saenger et al, 2007aSaenger et al, , 2009Walker, 2008), with the common driving force assumed to be the ever-present 0.1-0.2 Hz microseismic waves generated by meteorological phenomena and interactions of ocean waves with continental margins (Bonnefoy-Claudet et al, 2006;Berger et al, 2004).…”

“…Nearly parallel to, but quite distinct from developments and applications of the interferometric approach, an industry-university consortium has been promoting simple spectral attributes of microtremor recordings in the 1-10 Hz frequency range as tools for locating and estimating the thicknesses of relatively deep hydrocarbon reserves (Schmalholz et al, 2006a(Schmalholz et al, , 2006bGraf et al, 2007;Holzner et al, 2005Holzner et al, , 2007Lambert et al, 2007Lambert et al, , 2009Steiner et al, 2007Steiner et al, , 2008aSteiner et al, , 2008bSaenger et al, 2007aSaenger et al, , 2007bSaenger et al, 2009Walker, 2008;Frehner et al, 2009;Holzner et al, 2009). The proposed hydrocarbon microtremor spectra (HMS) approach requires oscillating fluids in deep sedimentary units to be the overriding influence on microtremor characteristics (Holzner et al, 2005;Walker, 2008;Saenger et al, 2009).…”