Application of surface-wave tomography to mineral exploration : a case study from Siilinjarvi, Finland

In order to assess the feasibility and validity of surface-wave tomography as a tool for mineral exploration, we present an active seismic three-dimensional case study from the Siilinjarvi mine in Eastern Finland. The aim of the survey is to identify the formation carrying the mineralization in an area south of the main pit, which will be mined in the future. Before acquiring the data, we performed an accurate survey design to maximize data coverage and minimize the time for deployment and recollection of the equipment. We extract path-averaged Rayleigh-wave phase-velocity dispersion curves by means of a two-station method. We invert them using a computationally efficient tomographic code which does not require the computation of phase-velocity maps and inverts directly for one-dimensional S-wave velocity models. The retrieved velocities are in good agreement with the data from a borehole in the vicinity, and the pseudo three-dimensional S-wave velocity volume allows us to identify the geological contact between the formation hosting most of the mineralization and the surrounding rock. We conclude that the proposed method is a valid tool, given the small amount of equipment used and the acceptable amount of time required to process the data.Peer reviewe

On the use of NGI's prototype seabed‐coupled shear wave vibrator for 1 shallow soil characterization - Part II: Joint Inversion of multi‐modal Love and Scholte surface waves

Seismic data generated by a novel, seabed-coupled, shear-wave vibrator (prototype) and recorded by a densely-populated, multicomponent ocean-bottom cable allowed several modes of propagation of Love and Scholte waves to be retrieved in a relatively wide frequency band. Both global dispersion curves and local dispersion curves are extracted in the frequency-wavenumber (f -k) domain and inverted with a multimodal joint Scholte and Love wave inversion algorithm. Monte Carlo inversion is used for a estimating the global S-wave velocity profile of the seabed sediments whereas laterally constrained inversion is used to detect lateral variations of the layer interface depths. The results are in agreement and allowed consistent full-waveform simulation to be performed. The investigation depth is limited to around 40 m due to the low shear wave velocities within the shallow clay laye

On the use of the Norwegian Geotechnical Institute's prototypeseabed-coupled shear wave vibrator for shallow soilcharacterization - I. Acquisition and processing of multimodalsurface waves

Pure shear wave data are only very rarely acquired for offshore site investigations and exploration. Here, we present details of a novel, seabed-coupled, shear wave vibrator and field data recorded by a densely populated, multicomponent ocean-bottom cable, to improve shallow soil characterization. The prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor, assuring seabed coupling in combination with self-weight penetration. The prototype is depth rated to 1500 m water depth, and can be rotated while installed in the seabed. The philosophy is to acquire fully complementary seismic data to conventional P- and P-to-S-converted waves, in particular for 2-D profiling, VSP (vertical seismic profiling) or monitoring purposes, thereby exploiting advantages of shear waves over compressional waves for determining, for example, anisotropy, small-strain shear modulus and excess pore pressures/effective stress. The source was primarily designed for reservoir depths. However, significant energy is emitted as surface waves, which provide detailed geotechnical information through mapping of shear wave velocities in potentially high resolution of the upper soil units. To fully utilize pure shear wave content, a proper analysis of surface waves is paramount, due to the proximity of surface wave propagation speed with shear wave velocities. The experiment was carried out in the northern North Sea in 364 m water depth. Cable dragging was necessary to obtain close receiver spacing (2.5 m effective spacing), with total line length of 600 m. Frequency-waveform transforms reveal both Scholte and Love waves. Up to six surface wave modes are identified, that is, fundamental mode and several higher surface wave modes. The occurrence of these two different dispersive surface wave types with well-resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy, which is discussed in an accompanying paper. The data presented in this paper are thus a unique (long and densely populated receiver array allows for multimodal Love and Scholte surface waves from the marine environment) but challenging (marine operations) marine data set