Hans Palm

Deformation of the Baltic continental crust during Caledonide intracontinental subduction: Views from seismic reflection data

Deep seismic profiling in Norway and Sweden provides documentation of the amount and style of deformation of the Baltic crust in response to Caledonian intracontinental subduction. The seismic data demonstrate that the preserved Caledonide allochthon in central Scandinavia is 15 to 20 km thick, considerably greater than the 2 to 5 km predicted from surface geologic data. Much of the thickness of allochthonous rocks results from imbrication of the Baltic basement within a thin-skinned-style thrust complex. Seismic data across the Skardora antiform, one of the basement windows in the central Caledonides, indicate that the window represents an antiformal stack of thrust sheets involving basement and cover rocks. These data suggest that many of the basement windows in the Scandinavian Caledonides may result from similar thrust culminations and that the basement windows do not expose autochthonous Baltic basement. Documentation of the response of the Baltic crust during intracontinental subduction provides new constraints on reconstructions of the Scandinavian Caledonides and insights into the style of deformation occurring in other collisional orogens.

3-D structure below Ävrö Island from high‐resolution reflection seismic studies, southeastern Sweden

Two 1-km-long perpendicular seismic reflection lines were acquired on Avro Island, southeast Sweden, in October 1996 in order to (1) test the seismic reflection method for future site investigations, (2) map known fracture zones, and (3) add to the Swedish database of reflection seismic studies of the shallow crystalline crust. An east‐west line was shot with 5-m geophone and shot point spacing, and a north‐south line was shot with 10-m geophone and shotpoint spacing. An explosive source with a charge size of 100 g was used along both lines. The data clearly image three major dipping reflectors and one subhorizontal one in the upper 200 ms (600 m). The dipping reflectors (to the south, east, and north‐west) intersect or project to the surface at or close to where surface‐mapped fracture zones exist. The south‐dipping reflector correlates with the top of a heavily fractured interval observed in a borehole (KAV01) at about 400 m. The subhorizontal zone at about 100–200 m correlates with a known fracture zone in the same borehole (KAV01). 3-D effects are apparent in the data, and only where the profiles cross can the true orientation of the reflecting events be determined. To properly orient and locate all events observed on the lines requires acquisition of 3-D data.

Deep crustal seismic reflection profiling across two major tectonic zones in southern Sweden

Abstract A seismic reflection profile has been shot in Varmland, southwestern Sweden, across two major tectonic zones, the Protogine Zone and the Mylonite Zone. The crustal bedrock units separated by the tectonic zones are clearly distinguished in the seismic profile by changes in the reflection character in the upper 5–6 km. The Protogine Zone is represented by a 15–20 km wide band of dipping structures. The Mylonite Zone dips too steeply to be seen directly and a lack of clear reflectors at depth makes a continuation down through the crust hypothetical. In the uppermost 5–6 km between the two tectonic zones, stronger and more continuous reflections can be seen, and are interpreted as being connected with “hyperite” intrusions, which have been mapped at the surface. The lower crust shows generally little reflectivity, with only short and weak reflectors. An exception occurs at mid-crustal depths where a number of clearly recognizable bands of reflected energy are suggested to be major shear zones formed during Sveconorwegian-Grenvillian thrusting towards the east. A slight increase in reflectivity in the depth range 42–48 km correlates well with the Moho depth determined from refraction seismic studies. The possible relationships between data acquisition parameters and the observed scarcity and low strength of reflections from the deeper crust have been considered in some detail. It is shown that the use of 28 Hz single geophones instead of 10 Hz geophone strings along a major part of the profile did not reduce the signal-to-noise ratio for deep reflections. However, the varying source characteristics, in combination with the different quality of the recordings, made computation of the residual static corrections difficult, which may have resulted in a partly destructive stacking of seismic traces. In this way, the low reflectivity of the deeper crust in this area is further brought out in the stacked seismic sections.

High-resolution reflection seismic imaging of the upper crust at Laxemar, southeastern Sweden

Abstract A major cost in exploring the upper 1–2 km of crystalline crust with reflection seismics is the drilling required for explosive sources. By reducing the charge size to a minimum, shallow inexpensive shotholes can be drilled with handheld equipment. Here, we present results from a full-scale test using small charges for high-resolution seismic surveying over a nuclear waste disposal study site (not an actual site). Two 2–2.5-km-long crossing profiles were acquired in December 1999 with 10-m shot and geophone spacing in the Laxemar area, near Oskarshamn in southeastern Sweden. After standard processing, including dip moveout (DMO), several subhorizontal to moderately dipping reflections are imaged. Many of the dipping ones can be correlated to fracture zones observed in a ca. 1700-m-deep borehole where the profiles cross and/or to fracture zones mapped on the surface. The imaged fracture zones form a complex 3D pattern illustrating the necessity of having 3D control before interpreting seismic reflection data. Analyses of sonic and density logs from the borehole show that greenstones have significantly higher impedances than the more dominant granite found in the borehole (granite/greenstone reflection coefficient is +0.065). These greenstones may contribute to the reflectivity when associated with fracture zones. In some cases, where they are present as larger subhorizontal lenses, they may be the dominant source of reflectivity. A set of north-dipping (10°) reflectors at 3–3.5-km depth can be correlated to a similar set observed below the island of Avro about 3 km to the east.

Imaging of groundwater resources in glacial deposits using high-resolution reflection seismics, Sweden

Two high-resolution reflection seismic profiles were acquired in the Heby area of eastern Sweden over glacial deposits for the purpose of mapping groundwater resources. The majority of shot points were located in clay resulting in good quality data along most of the profiles. On stacked and migrated sections, the uppermost clay is about 20 m thick and is characterized by its subhorizontal reflectivity. Sand/gravel deposits below it contain more dipping interfaces and have a chaotic reflectivity pattern. Depth to bedrock is interpreted to be 90 and 65 m on the respective profiles and occurs in about a 100-m-wide trough on both profiles. Reflections from the tops of sandy gravel zones generally have higher amplitudes. Clear reflections from a thin silt layer (20 cm thick) at about 10-m depth are observed on one of the profiles. Elastic finite difference modeling and the observation of this reflection in shot gathers show that the reflection is not an artifact of the acquisition nor the processing. The modeling also shows that there is no marked low-velocity waveguide in the near surface, but that an effective low Q zone may be present. Comparison with refraction profiling on the other profile shows that there is better agreement between the reflection seismic results and penetration tests than the refraction results with these tests. Both profiles allow the thickness of the overlying clay layers to be determined, as well as the thickness of the underlying sand/gravel deposits. This is important for estimating the amount of groundwater resources in an area.

High‐resolution reflection seismics applied to detection of groundwater resources in glacial deposits, Sweden

A large proportion of Swedens ground water resources is contained in sand/gravel glaciofluvial deposits that overlie Precambrian crystalline basement and are covered by clay. The depth to the sand/gravel deposits varies from near-surface to about 100 m and their thickness is generally 10–20 m. Traditionally, due to logistics, short refraction seismic profiles and penetration tests have been used to localize the sand/gravel deposits below the clay and to determine their thickness. However, when the overlying clay deposits are thick or the glacial deposits show significant 2-D structure, refracting waves may not penetrate into the sand/gravel deposits or into the crystalline basement. Reflection seismic methods provide an alternative mapping method and a test profile was acquired over glacial deposits where the structure is well known. Standard processing shows, in general, excellent agreement between the seismic image and the structure determined from penetration tests.

The CABLES project: Imaging deep crustal structures in the Scandinavian Caledonides

The CABLES Project: Imaging deep crustal structures in the Scandinavian Caledonides (abstract)

High-resolution multicomponent hardrock seismic imaging of mineral deposits and their host rock structures

Although applied in the past, there are only a few cases demonstrating the advantages of multicomponent seismic data for mineral exploration. To illustrate this, a test survey using sixty 3C-digital sensors, spaced between 2 to 4 m and assembled in a 160 m long landstreamer, was carried out to provide information on shallow structures hosting mineralization and also a magnetic lineament with an unknown origin. The survey, totally about 1.3 km long, was complemented by Radio MagnetoTelluric (RMT) measurements. Although an explosive source was used to generate the seismic signal, the seismic data show good quality for all the three components. Supported by the RMT results, clear reflections are observed in the horizontal component data at about 25 m depth, one of them steeply dipping, likely associated with the magnetic lineament. Field static corrections were well estimated thanks to the close shot and receiver spacing and the broadband frequency content of the data. This study demonstrates that multicomponent seismic data can be useful for providing information on shallow structures and linking them to the surface geology. The vertical component data, however, show deeper penetration and better image the crystalline basement and its undulated/faulted surface at about 50 m depth.