Identification, character and geological significance of lineaments Data acquisition, identification of lineaments and uncertainties

The use of lineaments in the modelling of steeply dipping deformation zones has developed succes-sively during the site investigation work at Forsmark. Up to and including SDM version 1.2 /SKB 2005a/, lineaments were identified on the basis of anomalies in predominantly airborne magnetic data, high-resolution topographic data and bathymetric data /Isaksson et al. 2004, Isaksson and Keisu 2005/. Electromagnetic (EM and VLF) data were of limited, additional help. Both fixed-wing airborne geophysical data, which were generated predominantly by the Geological Survey of

Sweden (SGU), and similar helicopter airborne data, with considerably higher resolution (Table 5-4), were used (Figure 5-17). Linear arrays of low velocity anomalies in the older refraction seismic data close to the nuclear power plant were also interpreted as lineaments along the surface that separates the crystalline bedrock from the Quaternary cover (see also section 5.2.9). Ground magnetic and EM (slingram) data in restricted areas inside the candidate area were also evaluated in /SKB 2005a/.

Short method-specific lineaments, identified on the basis of a single data set with specific attributes (e.g. low magnetic lineament with high confidence), short co-ordinated lineaments that represent a combination of different types of lineament, and longer linked lineaments that integrate the co-ordinated segments into a single inferred lineament were acquired /Isaksson and Keisu 2005/.

The methodology, results and uncertainties in the technique used were addressed in /SKB 2005a/.

Table 5‑4. Characteristics of geophysical surveys used for the detection of lineaments in the Forsmark area. The resolution of the data obtained from ground surveys in the north‑western part of the candidate area is five times greater than that obtained from helicopter airborne measurements, which, in turn, is four times greater than that obtained by the Geological Survey of Sweden from fixed‑wing airborne measurements in connection with their standard mapping activities.

Type of survey Contractor Line

spacing Station

spacing Survey

direction Survey

elevation Grid

resolution Airborne, fixed-wing

(magnetic, VLF) SGU 200 m 17 or 40 m EW 60 m

(30 m older data) 40×40 m Airborne, helicopter

(magnetic, EM, VLF) NGU 50 m 3 m NS and EW 45 m 10×10 m

The interpretation of lineaments is subjective in character and serious questions arise concerning the reproducibility of the results, not least the trace length of an individual structure. Furthermore, there are intrinsic problems that bear on the geological character of a lineament; a deformation zone or anisotropy related to rock type and/or pervasive ductile strain in the bedrock? The low topographic relief in the area also gives rise to serious difficulties in the interpretation of digital topographic data.

Bearing in mind these uncertainties, an alternative interpretation of lineaments was carried out inside and immediately around the candidate area by an independent working group /Korhonen et al. 2004/.

Figure 5‑17. Coverage of airborne and ground geophysical data used for the detection of lineaments in the Forsmark area. Only magnetic data have been used in the interpretation of lineaments during model stages 2.1, 2.2 and 2.3.

A comparative study of the two lineament interpretations showed, in principle, good reproducibility /Johansson 2005, SKB 2005a/. However, some discrepancies in the two interpretations, especially for shorter lineaments and lineaments identified with a lower degree of confidence, were recognised.

Naturally, these discrepancies are enhanced when different method-specific lineaments are combined into linked lineaments.

On the basis of reflection seismic data along c. 16 km of continuous seismic profiles (see section 5.2.8) in combination with refraction velocities calculated from refraction seismic data at three sites, /Bergman et al. 2004b/ estimated the thickness of the Quaternary cover sequence along the profiles. This study demonstrated the generally poor correlation between the topographies of the ground surface and the crystalline bedrock surface. The intrinsic difficulties with the interpretation of the topographic data motivated the use of solely low magnetic lineaments in subsequent (after ver-sion 1.2) geological modelling work. Use of a single type of lineament also reduced the uncertainty in the evaluation of the critical attribute trace length. In addition, ground magnetic data with a resolution five times greater than the helicopter airborne data (Table 5-4) were acquired during model stages 2.2 and 2.3 inside the target area and in some offshore areas (Figure 5-17). These data provided a radical improvement for the interpretation of lineaments in these areas. However, the complementary data acquired during stage 2.3 were not available for use in the stage 2.2 geological modelling work (see also section 5.5.3).

Integration of low magnetic lineaments derived from both the high-resolution ground magnetic data and the airborne helicopter data inside and immediately around the target area has dominated subse-quent work during model stages 2.2 and 2.3 /Isaksson et al. 2006a, 2006b, 2007/. General estimates of the uncertainty in the spatial position of a point along a lineament interpreted from these two data sets are ±10 m and ±20 m, respectively, e.g. /Isaksson and Keisu 2005, Isaksson et al. 2006a/.

Low magnetic lineaments and their geological significance

Two types of low magnetic lineament are present inside the Forsmark tectonic lens in the north-western part of the candidate area (Figure 5-18):

• Magnetic minima that are discordant to the tectonic foliation, rock units and the banded magnetic anomaly pattern.

• Magnetic minima connections that are concordant with the same geological and geophysical features and are locally folded.

Excavations of lineaments classified as magnetic minima inside the Forsmark tectonic lens (see also section 5.2.5) show that they represent either steeply dipping fracture zones (4 out of 5 cases) or are related to a swarm of dykes of Group D granite and pegmatite with low magnetic susceptibility (1 out of 5 cases). The steeply dipping fracture zones are associated with wall rock hydrothermal alteration that consists of intense hematite dissemination in the bedrock and is optically visible as red staining (Figure 5-19). This is identical to the alteration observed along possible deformation zones in the boreholes (cf. sections 5.2.2 and 5.2.3). These findings provide strong support to the change in strategy that favoured the sole use of low magnetic lineaments in the identification of steeply dipping fracture zones. Magnetic minima with a span in trend of 25−70°, and a dominant trend of 50−65° are present inside the north-western part of the candidate area, i.e. the target area /Isaksson et al. 2007/.

Inside the Forsmark tectonic lens, lineaments classified as magnetic minima connections are inferred to be related primarily to lithological contrasts that are aligned parallel to the ductile tectonic folia-tion in the bedrock /Stephens et al. 2007/. However, the occurrence of minor fracture zones along the tectonic foliation cannot be excluded as a contributory factor. By contrast, in the ductile high-strain belts outside this lens, where all structures are concordant, it is generally impossible to distinguish magnetic minima connections that are simply related to lithological contrasts from those that are related to deformation zones.

Figure 5‑18. Low magnetic lineaments inferred from the integration of high-resolution ground magnetic and airborne helicopter data. The SFR underground facilities and the cooling water tunnels from reactors 1-2 and 3 are shown with pink lines and white filling, the candidate area with a thick dot-dashed, magenta line, and roads and drill sites in red. © Lantmäteriverket Gävle 2007. Consent I 2007/1092. After /Isaksson et al. 2007/.

A faintly discordant anomaly of dyke-like character, which consists of combined magnetic minima and maxima, has been identified on the basis of the high-resolution, ground magnetic data in the south-westernmost part of the detailed survey area, close to Bolundsfjärden (Figure 5-18). Areas with low magnetic intensity and with low relief (Figure 5-18) are inferred to be related to intersec-tions of magnetic minima. Areas with a diffuse magnetic pattern may indicate a deeper bedrock source and/or the presence of fractured and altered surface rock.

5.2.8 Character and geological significance of seismic reflection data