SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
January 2012 Vol.55 No.1: 143–148 doi: 10.1007/s11430-011-4335-6
Joint elastic-electrical properties of sediments in the Yellow Sea HAN TongCheng1,2*, LIU BaoHua1,2, KAN GuangMing1,2, MENG XiangMei1,2 & DING ZhongJun1 2
1 The First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China; Key Laboratory of Marine Sedimentology and Environmental Geology, State Oceanic Administration, Qingdao 266061, China
Received February 10, 2011; accepted September 5, 2011; published online November 26, 2011
We measured in the laboratory compressional wave velocity and electrical resistivity on 434 sediment samples collected from the Yellow Sea to study the joint elastic-electrical properties of marine sediments. Porosity was found to reduce both elastic velocity and electrical resistivity of the marine sediments in a non-linear fashion; velocity showed an approximate linear increase with increasing logarithm of resistivity. Various effective medium models either implemented or developed were compared with the new dataset. The model results showed that the combined self-consistent approximation and differential effective medium model using critical porosity of 0.6 and 0.5 for velocity and resistivity respectively gave a reasonable description of the joint elastic-electrical behaviors of the marine sediments. The joint elastic-electrical properties of the marine sediments established would be used to estimate resistivity from measured velocity and vice versa, and could also be suitable for detection of gas hydrate or other suitable targets from joint seismic-resistivity surveys. joint elastic-electrical properties, marine sediments, Yellow Sea, effective medium models Citation:
Han T C, Liu B H, Kan G M, et al. Joint elastic-electrical properties of sediments in the Yellow Sea. Sci China Earth Sci, 2012, 55: 143–148, doi: 10.1007/s11430-011-4335-6
Marine sediments on the shallow surface of seafloor are an important interface between sea water and the rocks beneath in terms of their transitional physical properties. The acoustic properties (mainly sound speed and attenuation) of marine sediments are key factors that affect sound spatial structure, underwater sound communication, underwater objection locating, and underwater sonar performance, and are therefore being significant research topics of military oceanography and military geophysics that play an important role in national coast defenses [1, 2]. On the other hand, the measurements of electrical properties (e.g. electrical resistivity or its inverse electrical conductivity) of marine sediments provide valuable information for marine engineering [3] and resource [4, 5] investigations and sea-
*Corresponding author (email:
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floor environmental monitoring [6]. Recent development in in-situ seafloor sediments acoustic equipment [1, 7] and the controlled source electromagnetic (CSEM) techniques [8] has made the accurate joint elastic-electrical measurements of marine sediments possible, which can give a better evaluation of the sedimentological properties (e.g. porosity and brine or gas content) provided that the inter-relationships among sediment engineering and the joint elastic-electrical properties of marine sediments are known. There has been joint elastic-electrical research on sedimentary rocks (e.g. reservoir sandstones) [9, 10], but unfortunately none relevant work on marine sediments can be found in the open literature. We aim in this paper to establish the relationship between compressional wave velocity (Vp) and electrical resistivity () of marine sediments (referred to as the joint elastic-electrical properties) based on comprehensive laboratory earth.scichina.com
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measurements on 434 marine sediment samples collected from the Yellow Sea. We first present the control of porosity on Vp and respectively, with various effective medium models either implemented or newly developed compared with the new dataset; we finally give an explicit relationship between Vp and with a satisfactory effective medium model for the joint elastic-electrical properties of marine sediments.
1 Laboratory measurements 1.1 Sample collection and treatment The target area of this research is the central South Yellow Sea (latitude from 33°46′23″N to 35°58′54″N and longitude from 120°35′50″E to 123°29′19″E), where the water depth is in the range of 15 to 80 m and shows a gradual increase from the coast to the open sea. The 434 sediment samples were collected from the Yellow Sea in June 2009 using gravity corer with the internal PVC liner diameter of 10.4 cm. All the samples were carefully sealed and transported to the laboratory for measurements. Since the original samples were relatively too long for the acoustic measurements (the length of the samples can reach 3.4 m in maximum), the samples were cut into pieces with a length of approximate 30 cm (this number adjusts according to the practical length of the original sample; each of the short sample was regarded as a separate sample), on which the joint elastic-electrical measurements were performed. 1.2
Elastic velocity measurements
Both compressional and shear wave velocity of the sediment samples was measured in the laboratory; however, in this paper, we prefer to focus on the compressional wave velocity (Vp), which is believed to be more accurate and reliable. The compressional wave velocity was measured using the bench-top acoustic system (Figure 1) comprising the bench, the digital sonic meter (WSD-3 digital sonic meter developed by Chongqing Benteng Digital Control Technology Institute), and the transducers. The length of the sediment sample and the travel time of the compressional wave were measured with accuracy of ±0.1 mm and ±0.1 s respectively, so that the accuracy of the compressional wave velocity was estimated to be better than ±0.1% for a typical sample with a length of 30 cm and velocity of 1500 m/s. The compressional wave velocity was measured at the frequency of 25, 50, 100, 150, 200, and 250 kHz respectively; however, we chose to present the 100 kHz data that are representative and most reliable.
Figure 1 A picture showing the bench-top acoustic system in measuring the compressional wave velocity of a sediment sample.
1.3
Electrical resistivity measurements
Of all the 434 sediment samples, the electrical conductivity (the reciprocal of electrical resistivity) was randomly measured on 109 samples using the Field Scout EC 110 Meter that has a measurement range of 0.00–199.9 mS/cm with accuracy of ±2%. The measurement was done by inserting the probe into the center of the sample in the interval of 2 cm, and the final electrical conductivity was calculated by averaging the values measured at each intervals. The experiments were conducted in a temperature-controlled laboratory at around 25°C (the electrical resistivity of sea water with salinity of 35 g/L is 0.189 m at 25°C) to minimize the influence of temperature on varying electrical resistivity.
2 Experimental results 2.1
Elastic velocity and porosity
The measurement result is given in Figure 2 showing the control of porosity (porosity = 1/(1+1/e), where e is the void ratio that is defined by the ratio between the void and solid volume of the sediments, which are measured by weighting the wet and dry sample respectively provided the density of each phase is known) on the compressional wave velocity (Vp in m/s) of the marine sediments. With increasing porosity, Vp decreases gradually in a nonlinear fashion, confirming the observations of other researchers on loose sediments [11, 12], but contradicting that on cemented sandstones, where an approximate linear correlation between porosity and Vp is observed [13, 14]. In low porosity cemented sandstones, the propagation of compressional waves is dominantly via the inter-connected solid sand frame, in which case, a small increase in porosity will dramatically decrease the bulk and shear moduli of the rock resulting in a rapid reduction of the wave velocities. This may also be the case for sediments with relatively low porosities. However, with further increase in porosity, the sand
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grains become floating in the brine (in case of full brine saturation) which is now load-bearing; in this case the increase in porosity will have a less effect in reducing the moduli of the sediments and thus cause a gentler decrease in the velocity. The observation in Figure 2 confirms this analysis. To simulate the relationship between Vp and porosity observed from our measurements, we implement a series of commonly employed effective medium models that relate elastic wave velocity to porosity on assumption of homogeneous medium using the parameters given in Table 1 for a two-phase medium (i.e. quartz and brine). The models include Wyllie time-average equation [15, 16], Gassmann’s equation [17], the self-consistent approximation (SCA, for spherical grains) [18], the differential effective medium model (DEM, for spherical inclusions) [19], the combined self-consistent approximation and differential effective medium model (SCA/DEM, for spherical phases) [20], and the Hashin-Shtrikman (HS) bounds [21]. The comparison of the model results with the laboratory data is shown in Figure 3. All the models fall within the HS upper and lower bounds (HS+ and HS respectively), indicating their validity with an exception that the SCA model gets lower than the HS lower bound when porosity is higher than 0.6 since the SCA model is only valid in the porosity range 0–0.6 [18]. The combined SCA/DEM model using critical porosity of 0.6 is indistinguishably coincident with the HS lower bound, both of which give a good fit to the experimental data. This confirms the observation by Ellis [22] that the critical porosity of 0.6 for the combined SCA/DEM model is valid for
Figure 2 Elastic measurement result showing the relationship between porosity and the compressional wave velocity of all marine sediment samples.
Table 1 Physical properties of the components used in the effective medium modelsa) Bulk modulus Shear modulus K (GPa) (MPa) Quartz 36.6 45 Brine 2.32 0 a) * Value for 35 g/L brine at 25°C.
Medium
Density d (g/cm3) 2.65 1.01
Resistivity (m) 105 * 0.189
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Figure 3 Comparison of the measured compressional wave velocity with various effective medium models.
unconsolidated sediments whereas the value of 0.5 should be utilized for consolidated sandstones [20]. Gassmann’s equation [17] fits the data when porosity is higher than about 0.5, and the rest of the models predict far higher an elastic velocity than our measurements. 2.2
Electrical resistivity and porosity
Figure 4 shows the effect of porosity on controlling the electrical resistivity of marine sediments. As expected, resistivity decreases with increasing porosity because electrical conduction via the connected conducting pore fluid (i.e. brine) dominates the overall conductivity and hence the resistivity. Compared with elastic velocity, electrical resistivity is more sensitive to parameters as saturation, porefilling materials (e.g. bubbles and gas hydrate) and temperature and can therefore provide complementary information to elastic measurements. It is also due to the above reasons that the electrical resistivity data are more scattered. The scattering of the resistivity data in Figure 4 might be a result of loss of brine when the samples are cut or a fluctuation in temperature of the laboratory (see discussion). In a similar way to model the relationship between electrical resistivity and porosity, we implement a range of effective medium models including Archie’s equation [23], the complex refraction-index method (CRIM) model [24], the electrical SCA [18] and DEM model [19] (both for spherical grains), and the HS bounds [25], and develop a two-phase electrical combined SCA/DEM model using a critical porosity of 0.6 for spherical shaped inclusions. Figure 5 shows the comparison of the model results with the laboratory data. Using cementation coefficient m = 2 (tortuosity factor a = 1 according to Glover [26]), Archie’s equation coincides with the CRIM model, which proves to give a reasonable fit to the laboratory data. This also confirms the feasibility of Archie’s equation in modeling electrical resistivity of un-
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Figure 4 Electrical measurement result showing the relationship between porosity and electrical resistivity of all marine sediment samples.
Figure 5 Comparison of the measured electrical resistivity with various effective medium models.
consolidated marine sediments. The electrical SCA model only fits the measurement results in the porosity range of ~0.4 to 0.5, and the model prediction gets too low once porosity becomes higher than about 0.5. The electrical DEM model, assuming brine as the matrix and quartz as inclusion (note quartz is assumed as background matrix and brine is inclusion in the elastic DEM model), always predicts lower resistivity values than the experimental data. The electrical combined SCA/DEM model using critical porosity of 0.5 turns out to give the best fit to the data than all the other models while using critical porosity of 0.6 predicts lower resistivity values than practical. All the models mentioned above fall between the electrical HS bounds.
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rine sediments is always more difficult and thus less done; however, the resistivity can be predicted from the in-situ measurement of elastic properties, which are relatively easier, provided a robust relationship (under various conditions, e.g. partial saturation, varying gas hydrate and clay content) between them is already known. Furthermore, the joint use of elastic and electrical properties is expected to give a better discrimination of the petrophysical parameters than the properties used separately [10]. Figure 6 shows the measurement results of the relationship between Vp and by color-coding porosity on a semi-logarithmic scale to keep consistency with published joint elastic-electrical properties of reservoir sandstones by Han et al. [27]. The logarithm of shows an approximate linear increase with increasing Vp, satisfying log()= 0.0019Vp3.0201 with squared correlation coefficient R2 = 0.7841, where Vp is in m/s and in Ωm. A good correlation is found between porosity and the joint elastic-electrical properties of the sediments, that is, with increasing porosity, both velocity and resistivity of the sediments decrease. Therefore, porosity is regarded as the link to interpret the positive relationship between Vp and : with increasing porosity, the bulk and shear moduli of the sediments reduce insulting in a decrease in the elastic velocity; on the other hand, increasing porosity of fully saturated sediments provides more conducting brine for the electrical conduction to take place, leading to a reduction in the electrical resistivity. Using porosity as the link in the method proposed by Carcione et al. [28], we combine the elastic and electrical models implemented or developed above to model the joint elastic-electrical behaviors of marine sediments. The comparison of the model results with the experimental data is shown in Figure 7. Carcione et al. [28] concluded that using the Gassmann’s equation for elastic velocity and the CRIM for electrical resistivity (Gassmann-CRIM in Figure 7) gives a reasonable fit to the well-log data obtained from the North Sea. However, our results show that the GassmannCRIM fails to describe the joint elastic-electrical properties
2.3 Joint elastic-electrical properties Joint elastic-electrical properties in this paper refer particularly to the relationship between compressional wave velocity and electrical resistivity of the marine sediments. The relationship once established can be used to estimate one of these two parameters from another if the latter is easier to obtain. For example, in-situ resistivity measurement of ma-
Figure 6 The cross-property relationship between electrical resistivity and compressional wave velocity of the marine sediment samples by colorcoding porosity.
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Figure 7 Comparison of the measured joint elastic-electrical relationship of the marine sediment samples with various effective medium models.
of marine sediments in the Yellow Sea. The elastic combined SCA/DEM using critical porosity of 0.6 and the electrical combined SCA/DEM utilizing critical porosity of 0.6 (SCA/DEM(0.6)-SCA/DEM (0.6) in Figure 7) does not predict the joint elastic-electrical properties of marine sediments either. However, the elastic combined SCA/DEM using critical porosity of 0.6 and the electrical combined SCA/DEM employing critical porosity of 0.5 (SCA/ DEM(0.6)-SCA/DEM(0.5) in Figure 7) successfully gives a reasonable fit to the laboratory data of the joint elastic-electrical behaviors of marine sediments in the Yellow Sea, indicating that it is possible to predict electrical resistivity of the sediments from the compressional wave velocity, and vice versa.
3 Discussion It looks odd that the compressional wave velocity of our marine sediment samples shown in Figure 2 is less than 1500 m/s when porosity is higher than about 0.6, because in our normal knowledge that the velocity of marine sediments should be higher than that of the brine, which is around 1500 m/s at room conditions. However, since the accuracy of our compressional wave velocity measurement is very high, the relationship between Vp and porosity given in Figure 2 is convincing, and the less than 1500 m/s velocity of the sediments is due to the fact that in the porosity range when the sand grains are becoming non-load bearing, the bulk and shear moduli of the sediments are more or less equivalent to that of the brine while the density of the sediments is higher than that of the brine, leading to a lower velocity of the sediment than the brine, which can be confirmed by the effective medium models in Figures 3 and 7 and by the measurement results of other researchers in the Yellow Sea area [29–31]. According to Lin [32], there is an ineligible amount of
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clay in the sediments of the Yellow Sea, which may affect both the elastic and electrical properties of sediments [13, 33] and thus influencing the joint elastic-electrical properties. Han et al. [10] experimentally found that there are two approximate linear groups in the joint velocity-resistivity plot of reservoir sandstones on a semi-logarithmic scale, and separated the two groups to be clay-rich and clean samples respectively. However, the joint velocity-resistivity of the marine sediments established in this paper appears in a single group; this is possibly due to the fact that we lack clean sediments (i.e. sediments without clay or containing small amounts of clay) in our experiments. Nevertheless, since our measurements are made on representative sediment samples collected from a particular area (i.e. the Yellow Sea), the relationships found are applicable to sediments in this area and caution should be exercised when applying these relations to sediments in other areas. As mentioned above, we measure the samples in the laboratory at bench-top conditions, which are apparently different from that in the sea bottom environment, where there are lower temperatures and higher pressures. There may also be a small variation in the saturation stage of the sediments during the transportation although the samples are well sealed. It is therefore that the relationships obtained in the laboratory may vary from that got in-situ. Indeed we can make a range of calibrations on the variables (e.g. temperature and pressure) to arrive at in-situ conditions, but this would add other uncertainties to the results. The best way to get the joint elastic-electrical relationships of marine sediments is to simultaneously measure elastic velocity and electrical resistivity of the sediments in-situ, to achieve which, the next step of our research is to develop an in-situ joint elastic-electrical measurement instrument of marine sediments. In addition to calculating resistivity from measured velocity or predicting velocity from resistivity of sediments in the Yellow Sea area, the joint elastic-electrical relationship and effective medium models developed in this paper can also be used for detection of gas hydrate or other suitable targets (e.g. gas bubbles which are a potential threaten to marine engineering security) residing in the sediments. If the plot of field-measured compressional wave velocity (from seismic survey or in-situ acoustic measurements) versus electrical resistivity (from marine resistivity measurements or potentially CSEM) follows the relation found in this paper, this is a possible indication that the clay-rich sediments are only filled with sea water; if the explored relation does not match that discovered in this paper, this could possibly suggest existence of gas hydrate in the sediments especially when the resistivity is higher. However, a precise appreciation of the content of gas hydrate needs further fundamental study, e.g. detailed laboratory joint elastic-electrical study of marine sediments with varying gas hydrate or gas bubbles at simulated in-situ conditions.
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4 Conclusions
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A large joint elastic-electrical dataset was successfully collected on representative marine sediments in the Yellow Sea. The effects of porosity on the elastic velocity, electrical resistivity and joint elastic-electrical properties of the sediments were investigated and a range of effective medium models either implemented or developed were compared with the new dataset. The analysis of the results leads to the following conclusions: (1) Compressional wave velocity decreases with porosity in a non-linear fashion, i.e. Vp decreases rapidly with porosity in the low porosity range and reduces gently in the high porosity range. The elastic HS lower bound and the elastic combined SCA/DEM model using critical porosity of 0.6 best describe the variation of Vp with porosity in our sediment samples. (2) In saturated marine sediments, electrical resistivity decreases with increasing porosity, indicating that conduction via the sea water dominates the conductivity of the sediments. The electrical combined SCA/DEM model using critical porosity of 0.5 gives a better estimation of the electrical response of the sediments than other model implemented in this paper and the newly developed SCA/DEM model where 0.6 is used as the critical porosity. (3) Compressional wave velocity of the sediments shows an approximate linear increase with logarithm of electrical resistivity in the form of log()=0.0019Vp3.0201 with squared correlation coefficient R2 = 0.7841. The combined SCA/DEM model using critical porosity of 0.6 and 0.5 for elastic velocity and electrical resistivity respectively gives a reasonable fit to the joint elastic-electrical properties of the sediments. The relation not only can be used to estimate resistivity from resistivity and vice versa, but also can be applied to the detection of gas hydrate from the joint velocity-resistivity measurements.
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25 We thank the crew for the help in collecting the sediment samples during the cruise in June 2009, and Li Shanshan for conducting part of the measurements in the laboratory. This work was supported by the Oceanic Special Public Sector Research Project (Grant No. 200805008). 1
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