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Geological Survey of Finland, Special Paper 59 Seismic ore exploration in the Outokumpu area, Eastern Finland: Constraints from seismic forward modelling and geometrical considerations SEISMIC REFLECTION SURVEYS IN THE OUTOKUMPU AREA Acquisition parameters of the Outokumpu seismic reflection data Altogether, 82 km of high-resolution seismic reflection profiles have been collected in the Outokumpu area (Fig. 1). From these, the vibroseis profiles OKU1, OKU2 and OKU3 were collected in 2002 as a part of the project FIRE (FInnish Reflection Experiment 2003–2006, Kukkonen et al. 2006) and vibroseis profiles V1, V2, V3, V7 and V8 and explosion profile E1 in 2008 as a part of the project HIRE (HIgh REsolution reflection seismics for ore exploration 2007–2010, Kukkonen et al. 2011, 2012). The project FIRE also includes a crustal-scale FIRE3 profile that crosses the Outokumpu area. SFUE Vniigeofizika, Moscow, Russia, worked as the seismic contractor for all these surveys. The seismic reflection survey profiles are displayed in Figure 1 on a geological map of the Outokumpu area, and the acquisition parameters for the high-resolution FIRE and HIRE surveys have been presented in Table 1. The acquisition of these Outokumpu seismic reflection data has been explained in more detail by Kukkonen et al. (2012). For the interpretation of seismic reflection data, it is essential to know the size of the structures that are resolvable given the acquisition parameters used in the surveys. The dominant frequency of the high-resolution FIRE profiles (OKU1, OKU2 and OKU3) is about 80 Hz, and that of the HIRE profiles (V1–V8 and E1) about 100 Hz. Using the Rayleigh quarter-wavelength criterion (Widess 1973) for an assumed average P-wave velocity of 6000 m/s (typical velocity for the Outokumpu area; see below for more), the vertical resolution, i.e., the ability to resolve the top and bottom of a layer, is about 20 m for the FIRE data and 15 m for the HIRE data. However, thin layers (down to 2–3 m) could be detectable if the signal-to-noise ratio and impedance contrasts are sufficiently high, and the boundary has sufficient lateral continuity (e.g., Yilmaz 2001). Using the Fresnel zone criterion (e.g., Yilmaz 2001) for given depths of 500 m and 1000 m, the horizontal resolution of unmigrated stacked data is respectively about 270 m and 390 m Table 1. Acquisition parameters for the Outokumpu high-resolution seismic reflection profiles shown in Figure 1 (modified from Kukkonen et al. 2012). Vibroseis profiles OKU1, OKU2 and OKU3 were collected in 2002 as a part of the project FIRE (FInnish Reflection Experiment 2003–2006, Kukkonen et al. 2006) and vibroseis profiles V1, V2, V3, V7 and V8 and explosion profile E1 in 2008 as a part of the project HIRE (HIgh REsolution reflection seismics for ore exploration 2007–2010, Kukkonen et al. 2011, 2012). FIRE HIRE Vibroseis OKU1, OKU2, OKU3 Vibroseis V1, V2, V3, V7, V8 Explosion E1 Recording system I/O-2 I/O-4 Acquisition geometry Symmetrical, Symmetrical, split spread split spread Spread length 7775–11825 m 5012.5 m 3500 m Number of active channels 312–474 402 280 Geophone type GS-20DX (Fres = 10 Hz) GS-20DX (Fres = 10 Hz) Number of geophones in a group 12 6 3 or 6 Geophone group length 25 m 12.5 m Geophone group spacing 25 m 12.5 m Seismic source 3 x Geosvip 15.4 t 3 x Geosvip 14.5 t 125–250 g dynamite Source spacing 50 m 25 or 50 m 50 m Number of sweeps/ shots/point 8 6 1 Shot hole depth 2.5 m Sweep frequencies 20–130 Hz 30–165 Hz Sweep length 12 s 16 s Listening time 18 s 24 s 6 s Recording time after correlation 6 s 6 s Sampling interval 1 ms 1 ms 35
Geological Survey of Finland, Special Paper 59 Kari Komminaho, Emilia Koivisto, Pekka J. Heikkinen, Hilkka Tuomi, Niina Junno and Ilmo Kukkonen for the FIRE data and 240 m and 350 m for the HIRE data. Nevertheless, objects smaller than the Fresnel zone can produce diffractions, and successful migration can collapse the Fresnel zone to approximately the dominant wavelength, thus increasing the horizontal resolution to at least the dominant wavelength (or half of that). However, it should be noted that the presence of migrationvelocity errors and noise adversely affects migration, and could significantly degrade the horizontal resolution. Data processing All the Outokumpu seismic reflection profiles have been commercially processed by the Russian company SFUE Vniigeofizika. Generally, these commercially processed sections are of good quality and have been used for the interpretations presented by Kukkonen et al. (2012) and Saalmann & Laine (2014). However, the quality can be further enhanced via reprocessing, as also previously documented by Koivisto et al. (2012) and Heinonen et al. (2013) with HIRE seismic reflection data sets from Kevitsa and Vihanti mining and exploration camps. Specifically, good control of the processing parameters is required for the comparisons between the real and simulated data. In this work, the seismic reflection profile OKU1 was chosen as the main focus of the reprocessing efforts, because the seismic forward modelling also focused along profile OKU1. Profiles OKU2 and OKU3 were also reprocessed with similar processing flows to that of OKU1. Reprocessing of seismic reflection profile V7, also conducted as a part of the overall project, has been reported by Aihemaiti (2014). In the reprocessing sequence of OKU1, specific emphasis was given on finding the optimal geometry of the CMP line (Common Mid-Point line, i.e., the processing line) for the crooked-line survey geometry, on static corrections in order to compensate for near-surface time delays caused by variations in the low-velocity weathering layer, and due to the highly variable bedrock velocities, on detailed velocity analysis. Amplitudes were first corrected for geometrical spreading, and each trace was balanced with a slowly time-varying scalar before and after stacking. This simple balancing procedure was preferred over the commonly used automatic gain control or other similar nonlinear processes, which destroy relative amplitude information and can alter the frequency content of the traces. The final sections were then migrated using the Stolt f-k migration algorithm (Stolt & Benson 1986) employing a 1D velocity model that was derived from the velocity analysis of the data by obtaining the optimal stacking velocity function for approximately horizontal reflectors. The same velocity Table 2. Processing sequence of profile OKU1. 36
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