Controlled Source Electromagnetic (CSEM) surveying is an emerging geophysical technology that offers great potential to aid in pore‐fill discrimination when seismic direct hydrocarbon indicators are lacking. CSEM though, is generally considered a deep‐water technique because of the so‐called "airwave" problem in a shallow marine setting. Moreover, shallow water proves to be a very noisy electromagnetic environment. In an effort to extend the envelope of CSEM into shallower water, we carried out a calibration survey over a known hydrocarbon field in 40m of water. In this paper, we will show how we dealt with the issues and risks associated with shallow water CSEM operations and how we succeeded in recovering a reliable resistivity image of the subsurface.
Abstract The Tjårrojåkka area is located about 50 km WSW of Kiruna, northern Sweden, and hosts one of the best examples of spatially and possibly genetically related Fe-oxide and Cu-Au occurrences in the area. The bedrock is dominated by intermediate and basic extrusive and intrusive rocks. An andesite constrains the ages of these rocks with a U-Pb LA-ICPMS age of 1878±7 Ma. They are cut by dolerites, which acted as feeder dykes for the overlying basalts. Based on geochemistry and the obtained age the andesites and basaltic andesites can be correlated with the 1.9 Ga intermediate volcanic rocks of the Svecofennian Porphyrite Group in northern Sweden. They formed during subduction-related magmatism in a volcanic arc environment on the Archaean continental margin above the Kiruna Greenstone Group. Chemically the basalts and associated dolerites have the same signature, but cannot directly be related to any known basaltic unit in northern Sweden. The basalts show only minor contamination of continental crust and may represent a local extensional event in a subaquatic back arc setting with extrusion of mantle derived magma. The intrusive rocks range from gabbro to quartz-monzodiorite in composition. The area is metamorphosed at epidote-amphibolite facies and has been affected by scapolite, K-feldspar, epidote, and albite alteration that is more intense in the vicinity of deformation zones and mineral deposits. Three events of deformation have been distinguished in the area. D1 brittle-ductile deformation created NE-SW-striking steep foliation corresponding with the strike of the Tjårrojåkka-Fe and Cu deposits and was followed by the development of an E-W deformation zone (D2). A compressional event (D3), possible involving thrusting from the SW, produced folds in the central part of the area and a NNW-SSE striking deformation zone in NE.
The crust and uppermost mantle in the Danish Basin are investigated by modelling the P-wave velocity distribution along the north–south trending seismic profile ESTRID-2. Seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust, from ∼10 km in the northern part of the profile, to ∼2 km depth in the southernmost part. The crystalline crust shows very high P-wave velocity in the central part of the profile, with ∼6.7 km s−1 at depths as shallow as 12 km, and ∼7.3–7.5 km s−1 in the lowermost crust. These values confirm previous results obtained along the orthogonal ESTRID-1 profile and the Eugeno-S profile 2. This high velocity zone in the middle to lower crust is interpreted as a mafic intrusion, which explains a positive gravity anomaly of ∼50 mGal (Silkeborg Gravity High). The total length of the intrusion is at least 80 km in the east–west direction and ∼25– 35 km in the north–south direction. The estimated thickness of the intrusion, from its top to the Moho level is ∼18–20 km, which gives a total minimum volume of ∼40–50 000 km3. The reflectivity properties of the Moho discontinuity are variable along the profile. Below the intrusion, the PmP signal is very weak, due to the small velocity contrast between the lowermost crust (∼7.4 km s−1) and uppermost mantle (∼7.6–7.7 km s−1). The main Moho reflection has a ‘reverberative’ character to the south of the intrusion. This feature is interpreted by layering at the Moho level, possibly due to magmatic underplating. The occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional/transtensional tectonism in the Tornquist Fan area in the Late Palaeozoic. The extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust. The ascending magma may have been injected at upper-middle crustal levels and, during the late phases of the development, ‘squeezed’ laterally along the Moho.
