Keywords

1 Introduction

For many countries “there is currently limited detailed understanding and data on the national offshore seabed characteristics” [1], thus greater efforts are needed to improve understanding of the subsoil profile underlying the planned offshore wind energy areas to support development decision-making. While PetroVietnam as the leading national company for oil and gas exploration and production still wonders about its suitable position in the supply chain of renewable energy industry, the private sector has been proactively involved in developing wind farms from small to large scale in offshore area, which used to be the traditional operation space of the former. The question is whether or not a win-win cooperation can be made between a state-based organization like Vietnam Petroleum Institute (VPI) and a private company like Technical World (TW) with their own advantages and disadvantages in the period of energy transition. The objective of this joint research is to perform a near-shore site investigation and its data analysis with an emphasis on seismic logging data acquired at a shallow water depth at the Ca Mau offshore wind farm site with the help of the rock physics diagnostic (RPD) approach, which is more common for oil well log data analysis.

2 Rock Physics Diagnostic (RPD) Method

“Rock physics is defined as science exploring quantitative relations between various properties (attributes) of the natural rock” [2]. It has started as a “velocity-porosity” science with applications mainly in oil and gas exploration, in particular for well log data analysis and static reservoir modeling. It has developed to “velocity-porosity-mineralogy” and then “velocity-porosity-mineralogy-fluid” science. Rock physics studies rocks “as a holistic object whose various properties (attributes) are extracted from experiments simulating processes such as elastic wav propagation, fluid and electrical transport etc.” [2]. Below are given a number of fundamental equations used in rock physics modeling (RPM):

$$V_{p} = \sqrt {\frac{{K + \frac{4}{3}G}}{{\rho_{b} }}}$$
(1)
$${V}_{s}=\sqrt{\frac{G}{{\rho }_{b}}}$$
(2)
$$M={\rho }_{b}{{V}_{p}}^{2}$$
(3)
$$G={\rho }_{s}{{V}_{s}}^{2}$$
(4)
$$K = M - \frac{4}{3}G$$
(5)
$$AIp={\rho }_{b}{V}_{p}$$
(6)
$$\vartheta =\frac{1}{2}\frac{{\left({V}_{p}/{V}_{s}\right)}^{2}-2}{{\left({V}_{p}/{V}_{s}\right)}^{2}-1}$$
(7)
$$\lambda =K-\frac{2}{3}G$$
(8)

Where: Vp, Vs are the compressional and shear velocity (m/s); K, M, G are the bulk modulus, compressional modulus and shear modulus (GPa), respectively; AIp is the acoustic impedance (m/s*g/cc); \(\vartheta\) is Poisson ratio; \(\lambda\) is Lame’s constant (GPa).

When building out a rock physics model (RPM) one has to detect various velocity-porosity trends and assign each of them to appropriate depth intervals and depositional sequences. This process is called rock physics diagnostic (RPD). For newly deposited sediments a chart of VP versus porosity (F) with the Reuss elastic bound is very useful in deciphering rock texture [3]. For this study the methodology follows the chart as shown in Fig. 1.

Fig. 1.
figure 1

Flow chart of rock physics diagnostic (RPD).

Full size image
Fig. 2.
figure 2

Study site and seismic borehole locations [4].

Full size image

The study wind farm site belongs to Dam Doi district, Ca Mau province, with a shallow water depth from 2.5 to 3.5 m. It is about 35 km far away from Ca Mau city to the East, about 3 km from Ganh Hao town - Bac Lieu province to the South as seen in Fig. 2 with the locations of six PS seismic boreholes [4] that were conducted as a part of an integrated geotechnical-geophysical near-shore investigation, which included the drilling and sampling works, laboratory geotechnical testing (grain size analysis, Atterberg limits, water content, bulk density, direct shear test, triaxial test, and consolidation test), in-situ geotechnical testing including CPTU, field vane shear.

3 Results and Discussion

Fig. 3.
figure 3

(a) The near-shore subsoil profile at the Ca Mau wind farm site and (b) Elastic characteristics of the seabed at the Ca Mau wind farm site.

Full size image
Table 1. (a) Geotechnical characteristics of the study site seabed and (b) Elastic characteristics of the study site seabed.
Full size table

As the results the geotechnical characteristics of the study site seabed are presented in Fig. 3a and Table 1a, while those of elastic characteristics are presented in Fig. 3b and Table 1b, respectively. In this study the whole range of elastic properties such as the ratio between P and S-wave velocities (Vp/Vs), Acoustic impedance (AIp), Poisson’s ratio (\(\vartheta\)), Lame’s constant (\(\lambda\)), bulk modulus (K), compressional modulus (M) and shear modulus (G) could be determined using Eqs. 1 to 8.

Fig. 4.
figure 4

RPD charts constructed for the study site: (a) Vp vs F in comparison with the North Sea soil data, updated from [4]; (b) M vs. F, (c) AIp vs F; (d) Vp/Vs vs AIp.

Full size image

Based on the results of an integrated geotechnical and geophysical analysis the subsoil profile up to 80 m deep was completely characterized, consisting of five soil layers, i.e., very soft to soft clay (0–17 m), stiff to very stiff clay (17–41 m), medium dense to very dense clayey sand (41–55 m) having a medium sand of about 2 to 3 m thick in the upper part, stiff to very stiff with two sublayers from 55 to 65 m (stiff fat to lean clay) and 65 to 55–80 m (hard lean clay). Some rock physics diagnostic (RPD) charts were constructed for the study site as shown in Fig. 4a–d. In Fig. 4a the data points of the Ca Mau soils are located close to the curve of Ruess’s bound, which indicates of very young sediments, in particular the very soft clay on top. It is interesting to see that the five soil layers identified by geotechnical characteristics could be well defined by elastic characteristics as seen in Fig. 4b–d and Table 1a and b.

4 Concluding Remarks

The seabed characteristics of an offshore wind farm site in Ca Mau, Vietnam, were completely characterized by an integrated geotechnical-geophysical investigation and data analysis. The subsoil profile up to 80 m deep under the sea bottom consists of five well defined layers. Layer 1 of very soft to soft clay on top has highest porosity (0.55–0.7) and Vp/Vs (8.3–14), lowest M (3.5–4 GPa) and AIp (2.3–2.5 km/s*g/cc). In contrast, Layer 5 of very stiff to hard clay is characterized by the highest M (5–6 GPa), AIp (3–3.3 km/s*g/cc), and lowest Vp/Vs (4–5); Layer 4 has a M (5.2–5.7 GPa) higher than that of the layer 2 (4.7–5.2 GPa), whereas Vp/Vs of layer 4 (4.7–5) is smaller than that of the layer 2 (5.8–7.8); The layer 3 has smallest porosity and highest velocity compared to those of the other layers as seen in Table 1b, indicating a mixture of clay-sand. It is first time the RPD method was applied for a shallow seismic logging survey, whose results agree well with the results from geotechnical testing, thus it can be a value-added tool for further nearshore site investigations for offshore wind farms’ construction along the coast of Vietnam.