S U M M A R YPure 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.

We have developed a 2.5D finite-element modeling (FEM) method for marine controlled-source electromagnetic (CSEM) applications in stratified anisotropic media. The main feature of the method is that delta sources are used to solve the governing partial differential equations for cases with and without a resistive target and to obtain the difference of these two solutions as the scattered field from the target. The total field is then the sum of the analytical background field calculated with a 1D modeling method and the difference or scattered field mentioned above. Compared with a conventional direct solution (using delta sources directly in a 2.5D formulation), the new method has smaller near-field error as a result of the source singularity and smaller boundary reflections. The new method does not require a dense mesh in the source region, which thereby reduces the total number of variables to be solved. In this way, the modeling time can be kept within a few minutes for some cases. We show that the maximum relative error of the calculation can be kept within 2% for targets at depths of approximately [Formula: see text]. The method is valid for stratified anisotropic media. The anisotropic modeling examples show that (1) marine CSEM is predominantly sensitive to target vertical resistivity and not to target horizontal resistivity, provided that the targets are thin, horizontal, high-resistivity layers and (2) marine CSEM is sensitive to the horizontal resistivity of the conductive sediments surrounding the target (e.g., the overburden).

SUMMARY 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 layer.