We provide direct experimental comparison of the optoacoustic imaging performance of two different 64-element linear detector array (LDA) units based on polyvinylidene difluoride (PVDF) films. The first LDA unit was based on traditional flexible circuit (FC) technology and consisted of an FC glued to the nonmetalized signal surface of a 28-μm-thick PVDF film providing 300  /  80-μm axial resolution/lateral resolution (AR/LR) and 0.4-kPa noise equivalent pressure of its single element. The other LDA unit was manufactured using a technology of low-temperature photolithographic etching (PE) of a signal electrode onto a 25-μm-thick PVDF film providing 300  /  40-μm AR/LR and 1 kPa noise equivalent pressure. As compared with a previously reported LDA unit based on a 100-μm PVDF thick film, the main advantage of using the thinner PVDF films was 10-fold improvement in axial resolution, whereas the main drawback was 10-fold increased noise equivalent pressure. In terms of in vivo imaging performance, higher bandwidth of PE LDA probe was more important than the higher sensitivity of FC LDA unit.

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Biot's coefficient is one of the key parameters in estimating effective stresses, leading to understanding of the three stresses spatial distribution, namely vertical, minimum and maximum horizontal. Ultimately, these stresses shape up the behavior of a geomechanics model (either in 3D or in 1D). Thus, the robustness of any geomechanics model significantly depends on the precision of Biot's coefficient estimation. The proposed technique allows evaluating isotropic and anisotropic Biot's coefficients based on the log responses independent of the geological environment. The methodology is based on elastic moduli-minimization. In isotropic case, Bulk rock frame and Bulk rock grain moduli minimization produce the best fit to the measured Density, DTP and DTS. Then, isotropic Biot's coefficient can be computed directly. In the case of anisotropy, additional control on lamination is required. This can be achieved by comparing estimated laminated and dispersed clay volumes based on the anisotropic rock-physics model and derived from the Thomas-Stieber plot or any alternative lamination analysis technique. Anisotropy modeling allows to produce five independent VTI elastic moduli and as a result to compute anisotropic Biot's coefficient. The methodology has been tested in several fields: clastic (Western Siberia, Norwegian offshore, Argentina unconventional) and carbonates (Brazil, Middle East, North Sea chalks). It produces reliable results in all cases. This study shows good agreement of the Biot's coefficient computed from the proposed methodology with measurements of core-based Biot's coefficients. In practice, core-based Biot's coefficient measurements are rarely available and quite often done on a few samples, taken in the reservoir section only. The proposed methodology allows reliable estimates of Biot's coefficient for the entire wellbore section, where density and sonic logs are available. It utilizes a minimization technique instead of using geomechanics correlations. Thus, it is applicable for any rocks and geological settings and is not bounded to the area or formation compared to correlations specific to the particular formation. The novelty of the method is in the process of elastic-moduli minimization based on logs and allows direct extraction of the Biot's coefficient. Previous works were either concentrating on principles of the laboratory Biot's coefficient measurements or focusing on the correlations derived from core tests. Correlation derivation requires a significant number of core tests conducted for the same geological settings. However, the proposed methodology requires a few core samples for Q.C. purposes only.

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