Alireza Malehmir

Controlled Source Boat-towed Radio-magnetotellurics for Site Investigation at Äspö Hard Rock Laboratory, Southeastern Sweden

Summary The radio-magnetotelluric (RMT) method has traditionally been used for land investigations. However, with the development of the boat-towed RMT system, this method is used on shallow water. The lowest frequency of the RMT method is about 14 kHz and in addition water resistivity is quite low in some cases, therefore controlled source measurements is naturally considered for data acquisition. In order to resolve a fracture zone under a brackish water body, the controlled source boat-towed RMT (CSRMT) approach was tested. CSRMT and RMT one-dimensional inversions were carried out separately to analyze galvanic distortions and source effects in our dataset. Serious distortions observed in both inversions as well as the two-dimensional (2D) structure observed in our previous study made us consider 2D inversion for modeling the data. Due to the sufficiently large distance between transmitter and receivers, the CSRMT data were inverted using a 2D inversion code originally designed for plane-wave RMT data. Occam and damped Occam schemes were used in our 2D inversions for CSRMT and RMT data. The results show that CSRMT can better resolve the fracture zone than RMT. This study further illustrates the use of the boat-towed RMT system and particularly when combined with controlled source.

Using Supervirtual First Arrivals for Improving the Seismic Imaging of Deep Deposits - Well Worth the Effort

Different applications of seismic interferometry have arisen in the last decade, however the potential of this technique to improve reflection seismic processing in hardrock environments has not been regarded explicitly. Therefore, in this paper we investigate the potential of retrieving the first arrivals originally hindered by high noise level in the exploitation of controlled-source data acquired over the apatite-iron deposit at Grangesberg (Sweden) and its mining-induced structures. The supervirtual first arrivals generated using interferometry methodologies allowed first-breaks to be picked more extensively than in the original data. Revised static corrections significantly improved the linearity of the first arrivals and continuity of reflections in the source gathers. Especially, reflections considerably enhanced in the source gathers stacked constructively in the final seismic section. Comparison with geologic data, supported by reflection-traveltime forward modelling, indicates that these reflections represent the deep (> 700 m) and unmined part of the deposit. Other reflections at shallower depth are interpreted as anthropogenic faults possibly located at lithological contacts (pegmatites). Even though the potential of first-arrivals retrieval is likely case-dependent, this study illustrates that interferometry may substantially improve the accuracy of field static corrections and subsequent stack for hardrock imaging and deep mineral exploration.

CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel

More than 7% of the Scandinavian landmass is covered with fresh-water bodies in the form of lakes and rivers. This poses a unique challenge to carry out electromagnetic survey on shallow-water bodi ...

Bedrock and Fracture Zone Delineation UsingDifferent Near-surface Seismic Sources

To delineate the bedrock surface and a fracture zone intersected by a well at c. 50 m depth, a seismic survey wasconducted using four different near-surface seismic sources. These were a 5-kg sledg ...

Seismic and Electrical Resistivity Investigations for the Planning of a Tunnel in Oslo Outskirts

Oslo municipality is presently planning bus and car tunnels to facilitate its accessibility and increase traffic efficiency. Urban environment is usually a challenge for geophysical pre-investigations because of the various sources of noise, vibrations and restriction both in time and space. These technical challenges were overcome with the use of a newly developed seismic streamer specifically designed for noisy urban areas, from an industry-academia partnership. A total of 3.5 km long seismic data along 14 profiles were acquired for the tunnels pre-investigation with the main goals of (1) obtaining information about depth to bedrock, (2) detecting potential weakness zones, and (3) optimizing the number of drillings and their locations for a follow-up study. In addition, six electrical resistivity tomography profiles were acquired near the planned tunnel alignments. Inversion of first breaks and electrical resistivity data provides a seamless depth to bedrock interface that is in most places in good agreement with the nearby geotechnical soundings. In addition, the geophysical sections reveal the bedrock undulation character and provide some indication of weakness zones. This case study also illustrates that if the pre-investigation had been based only on boreholes, it would have overseen a potential difficulty during excavation.

High-resolution Reflection Imaging for the Planning of a Double Train-track Tunnel in the City of Varberg, Sweden

A newly developed broadband MEMs-based seismic landstreamer system was employed for the planning of a double train-track tunnel in the city of Varberg, southwest Sweden. In addition to the refraction analysis and velocity tomographic modeling, reflection processing of the data was considered given the good quality of the data and realization of reflections in some raw shot gathers. Bedrock is strongly reflective in most cases and only at occasions when reaches near the surface it disappears in the reflection section. Bedrock undulation is clearly noticeable in most reflection sections and at one occasion it appears to be strongly diffractive. The diffraction signature is now known to be associated with a buried water tank used in fire emergency situations. Reflection seismic data greatly complements tomographic models and may support a deep bedrock at where the tunnel is planned to be excavated in the downtown Varberg.

Reflection Seismic Characterization of the Grängesberg Iron Deposit and Its Mining-induced Structures, Central Sweden

Reflection seismic investigation has been conducted on the Grangesberg apatite iron deposit. At the time of closure in 1989, the mine was operated at about 650 m below the surface. Mining activities might be resumed in the next years, which require better understanding of (1) the ore geometry and (2) the fault network which has developed up to the surface from excavated zones at depth. Two E-W oriented reflection lines with a total length of 3.5 km were acquired. The seismic lines intersect the Grangesberg ore body and open pit, as well as several of the mining-induced faults. A weight drop mounted on an hydraulic bobcat truck was used as a seismic source; both cabled and wireless receivers were used for the data recording. Preprocessing of the data first required the cable- and wireless- recorded datasets to be merged before stacking all data available at each shot point. The dataset exhibits several shallow reflections which are likely to occur on steep lithologic or tectonic structures. Other deeper reflections are recorded; careful processing will be carried out in order to preserve such events in final stacked sections and help with refining the geological model of the area.

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.

COSC Geophysical and Geological Site Investigations

Drilling of the first borehole, about 2.5 km deep, for the continental scientific deep drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides) is scheduled to begin in the summer of 2013. Here we present the project, a 3D interpretation of seismic data, combined with surface geology and potential field data, used for locating the most suitable drill site and planning of the drilling. An evaluation of the seismic interpretations by constrained 3D inverse modeling of potential field data shows a good fit to observed data, further supporting the choice of the drill site.

New Insights on Large-scale Structures at the Forsmark Site, Central Sweden - Reprocessing of High-resolution Seismic Data

We present reprocessed data from seven profiles from the Forsmark site, the location where Sweden will store its spent nuclear fuel. The reprocessed data show improved images of the crust below 1 km since lower frequencies were preserved in the new processing sequences. These sections together with data from a longer more regional profile to the south of the area indicate that there may be a previously unidentified larger southwesterly dipping deformation or fracture zone in the southern part of the Forsmark area, well away from the planned repository.