Seismic imaging in the Krafla high-temperature geothermal field, NE Iceland, using zero- and far-offset vertical seismic profiling (VSP) data

Abstract

Among geothermal exploration methods, active surface seismic methods have played only a minor role to date. Especially in high-temperature volcanic systems, reflection seismic data often reveal poor delineation of volcanic features, due to the internal heterogeneity of volcanic sequences. To enhance the vertical resolution, one possibility is the application of downhole seismic methods like vertical seismic profiling (VSP). A test experiment was carried out in the Krafla hightemperature geothermal field, NE-Iceland, to assess the ability of VSP to image subsurface structures, such as fractures, zones of high permeability, magmatic bodies, and zones of supercritical fluids and steam. Logging in such hostile environments is technical challenging in many aspects, but mainly due to the high temperature impact on the downhole electronic components of the measuring equipment. This requires a thorough pre-examination and implementation of the measurement, especially to avoid delays and tool failures. This paper presents results of zero-and far-offset VSP data from the K-18 borehole from within the Krafla caldera, which reveal good correlation with the surrounding lithology. The raw three component seismic data display a good signal-to-noise ratio and dominant signal frequencies between 20 and 40 Hz, down to c. 2200mdepth, for air gun and explosive sources, respectively. A zero-offset source comparison was also conducted to assess the use of different impulsive sources for future VSP surveys in similar settings. By applying a standard VSP processing, we identified stratigraphic boundaries between lavas, hyaloclastites, and intrusions, which are in good agreement with existing well data. For the zero-offset VSP, both P-and S-wave velocity models were calculated and a depth-converted corridor stack was determined. In addition, multicomponent Kirchhoff depth migration and Fresnel volume migration were tested around the borehole. The 3D results are promising, but the specific shape and lateral extent of the reflectors could not be determined due to the restriction to only two sources and the insufficient spatial coverage (aperture). Our study demonstrates that vertical seismic profiles can clearly detect variations in the subsurface volcanic stratigraphy in hightemperature geothermal fields. A more detailed reservoir characterization can be achieved by further data integration, enhanced survey design including more source positions, and improved processing and imaging techniques, such as fullwaveform inversion.

Identification of Inhomogeneities: The Selected Coordinate Descent Method Applied in the Drilling Area

“…Thus, the profiling results [53,54], as well as our results, detected the state of subsurface velocity structures after a basic volcanic event, i.e., the eruption in 1984. In [53,54], the P-velocity model was calculated using (1) sonic log data collected within K-18 and (2) P-wave data generated by the explosive source (2 km outside of K-18) and air gun (over K-18). The processing of data (1) showed that the average velocity of the acoustic P-wave is 4.43 km/s.…”

Study of Transfer Characteristics of a Molecular Electronic Sensor for Borehole Surveys at High Temperatures and Pressures

“…on the surface. Further, the received signal is processed and a model near the well or interwell space is constructed [9,10].…”

Monitoring and Analysis of Geotemperature during the Tunnel Construction

“…To date, attempts to analyze the geotemperature field of tunnels based on field investigations have made some progress [6][7][8][9][10]. As a pioneering definition, when the buried depth of the tunnel is too large, or the temperature of the surrounding rock affected by magmatic activity is more than 30 • C, this kind of tunnel is called a high geotemperature tunnel [11].…”