AUTHOR=Amoroso Ortensia , Napolitano Ferdinando , Hersir Gylfi Pall , Agustsdottir Thorbjorg , Convertito Vincenzo , De Matteis Raffaella , Gunnarsdóttir Sveinborg Hlíf , Hjörleifsdóttir Vala , Capuano Paolo 

TITLE=3D seismic imaging of the Nesjavellir geothermal field, SW-Iceland

JOURNAL=Frontiers in Earth Science

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.994280

DOI=10.3389/feart.2022.994280

ISSN=2296-6463

ABSTRACT=<p>We present a detailed seismic imaging of the harnessed Nesjavellir geothermal area, SW-Iceland, which is one of several geothermal fields on the flanks of the Hengill volcano. We map the <inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>, <inline-formula id="inf2"><mml:math id="m2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>, and <inline-formula id="inf3"><mml:math id="m3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio using seismic data recorded in 2016–2020 and compare them with both a resistivity model of the same area and the rock temperature as measured in boreholes. The results show that the shallower crust (depth less than 1 km) is characterized by low <inline-formula id="inf4"><mml:math id="m4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> and <inline-formula id="inf5"><mml:math id="m5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>, and high <inline-formula id="inf6"><mml:math id="m6" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio (around 1.9). Shallow low resistivity values at similar depths in the same area have been interpreted as the smectite clay cap of the system. At depths between 1 and 3 km the observed low <inline-formula id="inf7"><mml:math id="m7" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio of 1.64–1.70 correlates with high resistivity values. In this area, characterized by temperatures larger than 240°C, the seismicity appears to be sparse and located close to the production wells. This seismicity has been interpreted as induced by the production in combination with naturally occurring earthquakes. At depths greater than 4 km, high <inline-formula id="inf8"><mml:math id="m8" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio of 1.9 correlates well with low resistivity values. In the valley of Nesjavellir, a deep-seated conductive body, domes up at about 4.500 m b.sl. and coincides spatially with a significant high <inline-formula id="inf9"><mml:math id="m9" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio anomaly (&gt;1.9). Above these anomalies an elevated temperature is registered according to borehole temperature data. This is proposed here to be caused by hot 600°C–900°C cooling intrusives, close to the brittle ductile transition—probably the heat source(s) of the geothermal field above. These anomalies are at the same location as the last fissure eruption in Hengill almost 2,000 years ago. The NNE-SSW trending, deeper seismic cluster at 3–6 km depth is located at the edge of this high <inline-formula id="inf10"><mml:math id="m10" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> anomaly. The heat source of the Nesjavellir geothermal field is most likely connected to this most recent volcanism as reflected by the deep-seated low resistivity body and high <inline-formula id="inf11"><mml:math id="m11" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>P</mml:mi></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:mfrac></mml:mrow></mml:math></inline-formula> ratio, located beneath the deep fault that connects the flow path of the high temperature geothermal fluid, resulting in an actively producing reservoir.</p>