. The cooperation and mutual inspection with laboratory research methods will improve the quality of site geotechnical characteristics evaluation.
Geophysical logging method studies and determines the physical state and physical and mechanical properties of rock and soil, based on the differences of physical properties of rock and soil, as shown in Table 5- 1-6 and Table 5- 1-7. It can be seen from the table that resistivity logging, acoustic logging and nuclear logging can be used to analyze the lithology of rock and soil in borehole profile, determine its porosity, density and water saturation, estimate permeability and study the physical and mechanical properties of rock and soil. The basic principle and determination method are introduced below.
Table 5- 1-6 Lithology Analysis Results of Geophysical Logging Borehole Profile (A)
Table 5 1-7 Lithology Analysis Results of Geophysical Logging Borehole Profile (B)
1. Porosity of rock and soil
The gap between the skeleton minerals of rock and soil is called porosity, and the volume ratio between rock and soil and its pores is called porosity.
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Among them, Wφ is the volume occupied by rock and soil pores; W is the geotechnical volume; Porosity φ is expressed as a percentage.
Geophysical logging method is to determine the porosity of rock and soil according to the difference between the physical properties of space medium filled with pores and the physical properties of rock and soil solid phase (skeleton mineral particles). The physical properties of rock and soil solid phase are determined by the composition and separation of mineral particles. The basic principles of resistivity logging, acoustic logging, gamma-gamma density logging and correlation curve method for calculating rock porosity are introduced below.
(1) resistivity logging
Taking pure sandstone as an example, the principle of measuring rock porosity by resistivity logging is explained. The skeleton minerals of pure sandstone are mainly chronological and feldspar, which are similar in electrical properties and almost non-conductive. However, formation water or drilling mud filtrate filled in pores is ionically conductive, and the difference between them is quite obvious, which can be derived according to the square volume model in Figure 5- 1- 10.
L=Lma+Lφ
W=Wma+Wφ=L2Lma+L2Lφ=L2(Lma+Lφ)
porousness
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According to the model shown in fig. 5- 1- 10, considering the different sorting and roundness of the particles constituting the rock skeleton minerals, the pore structure and the degree of channel bending are also different, therefore, the model can be equivalent to the form shown in fig. 5-11,and it is considered that the skeleton and pores are when the current flows.
Fig. 5- 1- 10 cubic volume pure sandstone model
Fig. 5-1-1equivalent model of cubic pure sandstone
Let Rt, Rma, Rw, ρt, ρma and ρw respectively represent the resistance and resistivity of rock and soil, skeleton minerals and formation water filled in pores, and Lw, Sw and Ww respectively represent the length, cross-section and volume of channels through which current flows, then there are
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Considering that the skeleton minerals are partially nonconductive, that is, Rma→ ∞, only the formation water filled in the pores is the current channel. At this time,
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therefore
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In the formula, it is difficult to determine the tortuosity of the channel in practice, so Arnay puts forward the following empirical formula.
or
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Where f is called formation factor or relative resistivity, which reflects the porosity and pore structure of rock and soil, m is called cementation factor, which is related to tortuosity C, and α is a parameter reflecting lithological characteristics. It can be seen that f is a function of lithology, porosity and tortuosity of conductive channel, which is more closely related to the latter. Therefore, only after the values of α and M are measured by experiments can the F value of each working area be obtained. According to the experience at home and abroad, F has the following relationship:
When the sand layer,
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When sandstone is formed,
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When porous carbonate rocks are relatively pure,
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According to formula (5- 1-24), porosity values can be obtained from the above categories by using formation water resistivity ρw and formation resistivity ρt obtained from resistivity logging data.
(2) Acoustic logging
For pure sandstone, the acoustic wave propagation speed of mineral particles such as quartz and feldspar is much faster than that of formation water, and this difference is the physical premise of calculating the porosity of rock and soil by acoustic logging. The acoustic velocity recorded by acoustic logging is the average value of sliding wave propagation along the formation near the borehole wall. In compacted and cemented pure sandstone, the propagation of sound wave at the interface between mineral particles and pore water can be ignored because of its small porosity (capillary diameter is 0.05~0.002mm). At this time, it can be considered that the sound wave propagates in a straight line in the rock, and its propagation time is equal to the sum of the time when the gliding wave passes through the fluid in the rock skeleton and pores.
Let t, tma, tf, T, tma, tf, V, Vma and Vf represent the propagation time, time difference and velocity of sound waves in rocks, skeletons and pore fluids respectively. With figure 5- 1- 10 models are:
t=tma+tf
or
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Multiply the area a at both ends of the above formula and divide it by the same volume w, and you will get it.
t =( 1-φ)δTMA+φδTF
From this, the Willie equation is obtained.
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For example, the timely TMA = 51.5μ s/ft1ft = 0.3048m (international system of units).
The TF of fresh water is 200μ s/ft, and that of formation water with NaCl concentration of 100× 10-6 is 189μ s/ft. Substituting these parameters into equation (5- 1-25) can be obtained according to the t value recorded by acoustic logging.
(3) Gamma-gamma density logging
Because the grain density (mineral density) of timely feldspar is more than twice that of fluid in pores (σ Ma of mineral is 2.648g/cm3, σ F of fresh water is1g/cm3, and σ F of NaCl solution with concentration of 1000× 10-6 is/kloc-0.
The scattered gamma intensity recorded by density logging directly reflects the volume density σH related to the electron density of rock and soil. The pure sandstone volume model shown in Figure 5- 1- 10 is still used, and GH, Gma, Gf, σH, σma and σf are used to represent the weight and density of fluid in rock and soil, skeleton and pores respectively, so there are
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In the formula, σma and σf are the known densities of fluids in the skeleton and pores, respectively, and σH is given by the observation results of density logging. The total porosity φ of rocks is measured by density logging.
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Combined with the results of density logging and acoustic logging, the rock secondary porosity index SPI can be obtained. It is known that total porosity φ is the sum of primary porosity φp and secondary porosity and fracture degree (SPI), which can be used to evaluate the development degree of secondary porosity and fracture in rocks, that is,
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Where φp is given by acoustic logging results.
(4) Correlation curve method
The so-called correlation curve method is to use the correlation curve (gauge plate) between different physical parameters of a certain geotechnical type and porosity to solve porosity. Fig. 5- 1- 12 shows the relationship between resistivity ρt and P-wave propagation velocity VP and density σ and porosity of mudstone (curve 3), siltstone (curve 2) and sandstone (curve 1). Knowing the resistivity, P wave propagation velocity and density values of these rocks and soil types, we can calculate the porosity from the measuring plate.
Fig. 5- 1- 12ρS, VP, σ and porosity φ.
1-sandstone; 2— Siltstone; 3- mudstone
In practice, in order to calculate the porosity value reliably, comprehensive geophysical logging method should be adopted to determine it.
2. Lithological analysis of rock and soil
In modern logging technology, the logging data can be digitized by computer, and the lithology of the borehole geological profile composed of sand and argillaceous rock and soil can be analyzed, and the argillaceous content, sand content and porosity of the formation can be obtained respectively, and output in the form of graphs, as shown in Figure 5- 1- 13. The map is divided into two parts, the left part is the observation result (GR) of natural gamma intensity varying with depth h, and the right part is the result of lithologic analysis. The short line pattern indicates the mud content of the stratum, the pitted surface pattern indicates the sand content of the stratum, and the blank indicates the porosity of the stratum.
Fig. 5 Lithological analysis results of-1-13 gr-CNL combination
(According to Cai Berlin 1987)
The principle of rock and soil lithology analysis is to analyze the lithology of sand and mud layer by using volume model method and rock intersection triangle. Input logging curves can be divided into the following three types: ① combination of natural gamma logging (GR)- gamma-gamma density logging (DEN); ② Combination of natural gamma logging (GR) and neutron logging (CNL); ③ Gamma-gamma density logging (DEN)- neutron logging (CNL) combination, and acoustic logging (AC) can replace neutron logging.
(1) adopts the combination of GR and DE ⅳ.
1) Calculate the shale content Wsh with GR curve: If the natural radioactivity intensity in the stratum is mainly related to the shale content in the stratum, and the natural radioactivity intensity of the rock skeleton (such as timely and feldspar) is quite weak, calculate the relative value of natural gamma intensity △G first, that is,
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Where GR is the natural gamma logging reading of the interpretation layer; GRmax and GRmin are the natural gamma logging readings of mudstone and pure sandstone in Duan Chun, respectively (Figure 5- 1- 14). Then calculate the shale content Wsh with the following formula
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Figure 5- 1- 14 Natural gamma logging readings of pure mudstone and pure sandstone
Where, c is the empirical coefficient, for the old formation, c=2, and for the new formation, c = 3.7 ~ 4.
At present, natural gamma logging is the main method to obtain shale content (volume percentage).
2) Calculate porosity φ with DEN curve:
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Where σsh is the density value of mudstone.
3) Calculate the sediment concentration WSD:
WSD= 1-Wsh-φ (5- 1-2 8)
(2) GR and CNL are combined.
1) Calculate the argillaceous content of rocks with GR curve, and the formula is the same as before.
2) Calculate porosity φ by CN L curve.
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Where φH is the neutron logging value, φma, φsh and φf are the neutron response values of rock skeleton, mudstone and pore water respectively, all of which are known values.
(3) Calculate the sediment concentration WSD with formula (5- 1-28).
According to the above principles, the logging teaching and research section of China Geo University has compiled a program, which can be used for lithologic analysis of sand and mud layers (Figure 5- 1- 13).
3. Water saturation and permeability of rock and soil
(1) Determination of water saturation of rock and soil
The water saturation Sw of rock and soil is the ratio of water volume Ww in rock and soil pores to total pore volume Wφ.
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There are two methods to determine water saturation from logging data, one is based on Arnay formula; The other is fast direct display method, such as crossplot method and overlapping method. Now introduce the calculation method. Therefore, the ratio I is introduced.
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Where Pt is the resistivity of partially water-bearing or waterless formation in pores; ρ0 is the resistivity of 100% saturated water formation. I is called resistivity rising rate, which removes the influence of formation pore structure and formation water salinity, and the ratio I is only related to water saturation, that is
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Where n is called saturation index, usually n=2.
Substituting the relationship between formation coefficient f and porosity into the above formula, the calculation formula of water saturation can be obtained, namely
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Where is the formation factor, and A and B are the empirical coefficients. It should be pointed out that the above formula can only obtain good calculation results for pure formations with uniform distribution of pores (intergranular or intergranular pores), but it is still applicable to formations with developed fractures and pores, but its accuracy is poor. For the argillaceous layer, it is necessary to modify the argillaceous influence of the formula, and its calculation formula is as follows
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Where ρsh is the resistivity of argillaceous material; a=0.62 .
(2) Permeability
The permeability of rock and soil refers to the ability of fluid (or gas) to pass through rock and soil under the action of pressure difference. The permeability k of rock and soil can be obtained by the following formula.
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Where L and S are the length and cross-sectional area of the sample, μ is the viscosity of the fluid, Q is the volume of the fluid passing through the sample per unit time, and δ P is the pressure difference. Millidarcy (MD)1MD (MD) = 9.87×10-10m2 (international system of units) is commonly used in practice.
As a unit of permeability.
Experiments show that when only one fluid passes through, the measured permeability (called absolute permeability) is only related to the pore structure of rock and soil, but not to the fluid properties. The permeability often mentioned in logging interpretation is absolute permeability. At present, it is inaccurate to calculate permeability with logging data. Figure 5- 1- 15 is an example of estimating permeability from resistivity logging results in a certain area. As long as the true resistivity of rock and soil is measured, the unknown permeability of rock and soil in the same area can be estimated by the correlation curve between resistivity and permeability.
Fig. 5- 1- 15 example of estimating permeability from resistivity logging results in a certain area
4. Rock and soil density
Rock and soil density is the ratio of rock and soil mass to its volume.
(1) mineral density σma
The mineral particle density of each geotechnical type depends on its mineral composition, while the mineral composition is little influenced by secondary factors and remains unchanged during the whole epigenetic evolution.
Mineral density is usually measured on laboratory specimens, and the measurement error is 0.0 1g/cm3. In addition, it can be calculated by the following formula.
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Where σH, σf and φ can be determined from logging data.
(2) saturated water rock mass density σH
In drilling, the density measured by density logging is generally equivalent to the geotechnical density σH of saturated water. Under favorable conditions, that is, the borehole wall is flat without reaming, the relative error of σH measured by density logging is about 1g/cm3. σH value can also be calculated by the following formula.
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5. Propagation speed of elastic wave in rock and soil
The methods of measuring elastic wave propagation velocity in rock and soil in borehole include acoustic logging, PS logging, formation detection method, cross-well method and so on. Using the measured P-wave velocity VP and S-wave velocity VS, combined with the density logging results, the elastic coefficient of rock and soil can be calculated, as shown in Table 5- 1- 1.
Chongqing Geological Instrument Factory has developed JBS-I portable digital logging system, including the downhole instruments and data processing software of the above logging method, which is suitable for hydrological engineering geological logging.