the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2026
Rayleigh-wave focal spot imaging of the polymetallic Kylylahti mine area in the Outokumpu mineral district, Finland
Abstract. The green energy transition requires the development of new seismic imaging approaches to locate and characterize deposits of critical raw materials efficiently and cost-effectively. Here, we apply the Rayleigh-wave focal spot imaging approach to dense array data collected in the Kylylahti polymetallic mine area in the Outokumpu mineral district, Finland, and demonstrate the efficiency of the method in a hardrock environment. We cross-correlate one month of ambient-noise records from the 994 vertical-component stations deployed over an area of ∼10.5 km2 following standard signal processing operations. The anatomy of the correlation wavefield includes spurious arrivals associated with the local mining operations and with a mine located to the SW of the array. From the narrow-band zero lag amplitude distribution of the cross-correlations we get the Rayleigh-wave focal spots which we parameterize with a Bessel-function model to estimate the local phase velocity at each station. The employed model accounts for the anisotropic background illumination and is constrained using focal spot data in the short 1λ distance range. In the 3 − 6 Hz frequency range we obtain phase velocity maps that correlate well with the Kylylahti formation, which is resolved with lower phase velocity values compared to the host rock. The lower phase velocities are associated with abundant soapstone, while higher phase velocities are indicative of the surrounding mica schist. The similarity of the phase-velocity features to the structure resolved by an independently obtained P-wave velocity model, together with the agreement between the inferred Kylylahti formation boundaries and the contours of the geological model, suggest that the focal spot method is a valuable, low-cost passive imaging tool for modern mineral exploration that can complement established approaches.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Solid Earth.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4298', Anonymous Referee #1, 28 Sep 2026
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RC2: 'Comment on egusphere-2026-4298', Anonymous Referee #2, 01 Oct 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-4298/egusphere-2026-4298-RC2-supplement.pdf
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- 1
Tsarsitalidou and colleagues report on a case-study of what they call “Rayleigh-wave focal spot imaging”. It may be seen as a creative implementation of the SPAC method, pioneered by Keiiti Aki in 1957. With SPAC, an estimate is made of the local surface-wave dispersion curve by using ambient noise. Where SPAC is applied to a local array of stations to allow azimuthal averaging, Tsarsitalidou et alii apply it to a network of stations. Each station together with its surrounding stations forms an array in this network. So, to each array in this network the SPAC method is applied. The authors describe this massive implementation of SPAC to an existing passive dataset acquired in a mining area in Finland. The authors show that the method is effective in finding areas with relatively low shear-wave velocities, which are linked to areas with different mineral composition. Overall, the manuscript is well written. Nevertheless, a few clarifications are needed.
In Section 2.2 it is mentioned that ‘Each station consisted of tightly colocated six 10 Hz vertical geophones’. First, put ‘six’ before ‘tightly’. Second, please explain what this really means. What is the spatial layout of these arrays and for what purpose are they used? In the reminder of the paper, these arrays are not mentioned anymore. So, third, please explain how this array-data is dealt with during the processing.
Implementing the method yields phase-velocity maps. For different minerals in the area, P-wave velocities are known from petrophysical measurements on core samples. These mean P-wave velocities are within 2% from each other. The stated values are 5540, 5550 and 5630 m/s (lines 289-299). S-wave velocities are not known. For one of the minerals, however, it is listed that it has a high vp/vs ratio and hence a relatively low vs. For the other minerals, no vp/vs ratio or vs velocity is given. Nevertheless it is concluded that ‘the significant difference in vs between talc and mica schist explains the low Rayleigh-wave velocities observed on the formation with respect to its surroundings’. Such statement needs to be rewritten into a hypothesis instead of a conclusion. Similarly, in the abstract a statement like ‘the lower phase velocities are associated with abundant soapstone’ need to be softened with at least the word ‘likely’. In the discussion in fact an alternative possibility is mentioned (faulting).
L10 ‘short 1 labda distance range’. In this context it is not clear how the word ‘short’ is meant. Also better to write out labda in an abstract. Hence, I would replace ‘short 1 labda’ by ‘one wavelength’.
Line 23: I would use the more conventional term ‘ambient noise tomograpy’ here instead of ‘passive surface-wave tomography’.
Line 50: the acronym ‘ERA-MIN COGITO-MIN’ is only partly reflected in the previous text. ‘ERA-MIN’ is missing. And, to make the second part clearer, a few extra capitalizations are needed: Cost→COst, Mineral → MINeral.
Lines 69-71, “Our results demonstrate that focal spot imaging … surveys”. These conclusive statements do not belong in an introduction. Please remove them.
Line 85, “The massif (hereafter referred to as Kylylahti formation) ..”. It is not clear to me what is the purpose of renaming ‘Kylylahti massif’ to ‘Kylylahti formation’.
Line 99: vibroseismic sources → vibrator sources
Line 102: COGITO-MIN survey → COGITO-MIN active survey
Line 105: passive noise correlation surface-wave tomography → ambient noise tomography
Line 108, ‘receiver gathers’. A receiver gather is a collection of records from many different sources observed at one receiver. It is unclear to me why the data would be ordered as receiver gathers.
Line 111: standard deviation of the signal distribution → standard deviation from the mean noise level
Lines 121-122, “Ambient-noise tomography as well as the spatial autocorrelation imaging used here are based on the cross-correlation of seismic noise recorded at two stations”. This is true, but nevertheless it might be confusing to some readers that a ‘spatial autocorrelation’ is achieved by performing a ‘cross-correlation’. It could be helpful to add a few clarifying sentences.
Line 122: empirical estimate → estimate
Line 130: signals → cross terms (or spurious terms)
Line 170, ‘J_0 and J_1 functions’. These functions are stated without explaining the meaning of J_0 and J_1.
Line 173, ‘Here, too, we perform this focal spot parameterization (Tsarsitalidou et al., 2026)’. The reference can be removed here since the relevant references (including this one) were already added to the previous sentence.
Line 180, ‘Only the real part of Eq. (1) is used for focal spot parameterization’. Please explain why.
Line 181, ‘wherek_ang’. Here a space is missing
Line 189, ‘using M=k_1 ...6’. This text may be removed as M does not exist anymore in Eq (2).
Line 204: three station → three stations
Line 209: South → south
Lines 213-214, ‘Overall … source’. This statement does not fit with the preceding. In the preceding all kinds of noise sources are discussed that are likely relevant for the different focal spots. Only for the focal spot in the NW it was identified that the mine is likely the dominant noise source.
Line 224, ‘residual distribution’. In this section the residual is defined as model minus measurement. Note that commonly the definition is the other way around, center staging the measurement.
Line 245: lower seismic velocities → lower P-wave seismic velocities
Line 330: Chamarczuk et al. (2022) → (Chamarczuk et al., 2022)
Line 365, ‘Theoretical depth resolution’. Not clear what is meant with this statement. Do you mean ‘better depth resolution’? If so, please add a REF because it is not something that is shown in this manuscript.
Line 375, ‘first application of the focal spot imaging method in a hardrock environment, ...’. This statement is unlikely true since it was mentioned that the method was applied to USArray and AlpArray data. There is definitely some hardrock in the Alps.
Caption of Figure 2: here SW means both ‘southwest’ as ‘surface waves excited by mine activity’. This can easily be fixed by writing out southwest.
Caption of Figure 4: write out the meaning of RSS.