Soil body conductivity measuring method based on active heating optical fiber method
Technical Field
The invention belongs to the technical field of conductivity test, and particularly relates to a soil body conductivity measuring method based on an active heating optical fiber method.
Background
Conductivity is an important electrical parameter of rock and soil, and is the basis of electrical (magnetic) exploration in geophysics. In an electric power system, the soil resistance value is a parameter commonly used in lightning protection grounding engineering, and the size of the soil resistance value directly affects the grounding resistance value of a grounding device in the grounding engineering, the ground potential distribution of a ground grid, the contact voltage, the step resistance and the like. The size of the grounding resistance value is one of important indexes for judging whether the grounding condition in engineering meets the requirement of lightning protection safety specifications or not, when the system operates in a single-pole ground loop mode for a long time, the large current continuously enters the ground through the grounding electrode to cause the soil around the grounding electrode to generate heat, so that the evaporation of soil moisture is caused, and the normal operation of the grounding system can be influenced in serious cases. Resistivity can also be used to assess the corrosivity of soil, also because the rate of metal corrosion is directly related to the resistivity of soil. Therefore, the conductivity can be accurately measured, and the method has practical significance in the aspects of electric power grounding engineering and lightning disaster risk assessment and also has important engineering application value in the aspect of corrosion prevention of underground metal facilities.
There are two main methods for engineering conductivity measurement. One method is to measure the conductivity of the soil from the extract. The method measures the conductivity of the extracting solution at a fixed temperature by extracting an air-dried soil sample, but the method does not make sense of the measured conductivity because the performance of undisturbed soil is damaged by extracting the soil sample. The other method is to calculate the conductivity of the soil by utilizing the principle of the conductivity of the side of an external voltage. The method utilizes the measurement of the external voltage, assumes the constant external voltage and determines the conductivity through the ratio of the current to the voltage, neglects the instability of the voltage, and fundamentally forms system errors. In summary, the current method for measuring the conductivity has the problem that the accuracy of measurement is not high due to damage to soil samples and instability of voltage.
Disclosure of Invention
The invention aims to provide a soil body conductivity measuring method based on an active heating optical fiber method, which solves the problem that the accuracy of measurement is not high due to damage to a soil sample and instability of voltage in the conventional method for measuring the conductivity.
The technical scheme adopted by the invention is as follows;
a soil body conductivity measuring method based on an active heating optical fiber method is carried out according to the following steps:
step 1: embedding the heating optical cable in the soil body for electrifying heating, measuring the thermal resistance coefficient rho of the soil body according to a linear heat source, and then providing a temperature characteristic value TtWill TtA linear function with the volume water content theta of the soil; finally, obtain TtAn empirical model of θ;
step 2: establishing a capillary model, and combining the capillary model with an empirical model to express TtThe relationship between the conductivity of the soil and the conductivity of the soil,resistivity rho according to saturated soilsatResistivity p with soilsTo establish a theoretical model for solving the soil conductivity based on the soil saturation, then converting the theoretical model into the relation between theta and the soil conductivity, and combining the theta with an empirical model to obtain TtAnd (3) calculating the conductivity of the soil according to the formula (1) of the conductivity of the soil:
wherein, TtAs a characteristic value of temperature, ECaFor volumetric conductivity in soil, EC0The k is a constant and is a limit value of the volume conductivity in the soil and the conductivity corresponding to the soil liquid limit index, and B can be determined by experiments to be an empirical parameter.
The present invention is also characterized in that,
in step 1, a temperature characteristic value T is provided according to the following formula (2)t:
Wherein, T
0Is the initial temperature, q' is the heating power of the heat source in unit length, p is the thermal resistivity,
is a time interval [ a, b]Is a logarithmic sum over time, d is a constant;
the linear function of one degree is formula (3):
Tt=k1θ+k2in the formula (3),
wherein k is1、k2A constant calibrated for the experiment;
the empirical model is the following equation (4):
wherein k is1,k2,k3,k4,k5To be at leastConstants, theta, calibrated by experiment0The water content is a limit;
in step 1, the empirical model is obtained by measuring the temperature characteristic value T of the soil under different soil moisture contents thetatUsing least squares regression to calculate TtAnd theta, and the limit water content theta of the soil0And determining, namely dividing the soil into a dry stage and a wet stage, and calibrating the function relation of the soil by respectively adopting linear fitting and logarithmic fitting.
In step 2, the capillary model combines the property coefficients of the soil to form the following formula (5) by using the differential principle:
ECa=ECw(aθ2-bθ)+ECsin the formula (5),
wherein, ECaFor volumetric conductivity in soil, ECsFor surface conductivity in soil, ECwThe conductivity of medium water in soil, a and b are constants to be measured;
Ttthe relation with the soil conductivity is shown as formula (6):
wherein, EC0Is the limit value of the volume conductivity in the soil, the conductivity corresponding to the soil liquid limit index, k1,k2,k3,k4,k5,k6Are constant and can be determined by experiment.
In step 2, the theoretical model is formula (7):
wherein S isrIs the soil saturation, B is an empirical parameter, rhosIs the resistivity of the soil, psatIs the soil saturation resistivity.
The invention has the beneficial effects that: the soil body conductivity measuring method based on the active heating optical fiber method utilizes the optical fiber temperature to deduce the accuracy of the water content, combines with a theoretical model of the water content and the conductivity with higher accuracy, and further deduces the relation between the temperature characteristic value and the conductivity of the optical fiber heating method, so that the measured resistivity of the optical fiber embedding position can be more accurate.
Detailed Description
The soil body conductivity measuring method based on the active heating optical fiber method of the invention is further described in detail by the following specific implementation modes:
a soil body conductivity measuring method based on an active heating optical fiber method is carried out according to the following steps:
step 1: embedding the heating optical cable in the soil body for electrifying heating, measuring the thermal resistance coefficient rho of the soil body according to a linear heat source, and then providing a temperature characteristic value TtWill TtA linear function with the volume water content theta of the soil; finally, obtain TtAn empirical model of θ;
step 2: establishing a capillary model, and combining the capillary model with an empirical model to express TtDependence on the conductivity of the soil, according to the resistivity rho of the saturated soilsatResistivity p with soilsTo establish a theoretical model for solving the soil conductivity based on the soil saturation, then converting the theoretical model into the relation between theta and the soil conductivity, and combining the theta with an empirical model to obtain TtAnd (3) calculating the conductivity of the soil according to the formula (1) of the conductivity of the soil:
wherein, TtAs a characteristic value of temperature, ECaFor volumetric conductivity in soil, EC0The k is a constant and is a limit value of the volume conductivity in the soil and the conductivity corresponding to the soil liquid limit index, and B can be determined by experiments to be an empirical parameter.
Further, in step 1, a temperature characteristic value T is provided according to the following formula (2)t:
Wherein, T
0Is the initial temperature, q' is the heating power of the heat source in unit length, p is the thermal resistivity,
is a time interval [ a, b]Is a logarithmic sum over time, d is a constant;
the linear function of one degree is formula (3):
Tt=k1θ+k2in the formula (3),
wherein k is1、k2A constant calibrated for the experiment;
the empirical model is the following equation (4):
wherein k is1,k2,k3,k4,k5Is a constant which can be calibrated by experiment, theta0The limit water content is shown.
Further, in step 1, the empirical model is obtained by measuring the temperature characteristic value T of the soil at different soil moisture contents thetatUsing least squares regression to calculate TtAnd theta, and the limit water content theta of the soil0And determining, namely dividing the soil into a dry stage and a wet stage, and calibrating the function relation of the soil by respectively adopting linear fitting and logarithmic fitting.
Further, in step 2, the capillary model combines the property coefficients of the soil to form the following formula (5) by using the differential principle:
ECa=ECw(aθ2-bθ)+ECsin the formula (5),
wherein, ECaFor volumetric conductivity in soil, ECsFor surface conductivity in soil, ECwThe conductivity of medium water in soil, a and b are constants to be measured;
Ttthe relation with the soil conductivity is shown as formula (6):
wherein, EC0Is the limit value of the volume conductivity in the soil and the conductivity k corresponding to the soil liquid limit index1,k2,k3,k4,k5,k6Are constant and can be determined by experiment.
Further, in step 2, the theoretical model is formula (7):
wherein S isrIs the soil saturation, B is an empirical parameter, rhosIs the resistivity of the soil, psatIs the soil saturation resistivity.
Examples
A soil body conductivity measuring method based on an active heating optical fiber method is carried out according to the following steps
Firstly, the method comprises the following steps: active heating method for measuring water content
The active heating method is to electrify and heat a heating optical cable embedded in the soil body, and calculate the thermal resistance coefficient rho of the soil body according to the diffusion speed of a linear heat source formed after the optical cable is heated to the surrounding soil body, wherein the thermal resistance coefficient rho of the linear heat source and the surrounding soil body satisfies the following formula (8):
T-T0=(q′ρ/4π)ln(t+t0) + d formula (8)
Wherein, T0Is the initial temperature, T is the measured temperature value, T is the elapsed time, q' is the heating power of the heat source per unit length, p is the thermal resistivity, T is the measured temperature value0D is a constant and is a time parameter related to the size of the heat source and the contact surface of the heat source and the surrounding medium;
providing a temperature characteristic value TtAs in formula (2):
wherein, T
0Is the initial temperature, q' is the heating power of the heat source in unit length, p is the thermal resistivity,
is a time interval [ a, b]Is a logarithmic sum over time, d is a constant;
wherein the initial temperature T0D, and the sum of logarithms over time can all be considered constant, with the thermal resistivity ρ being a linear function of water cut. Therefore, the temperature characteristic value TtIt can also be expressed as a linear function equation (3) with respect to the water content, as follows:
Tt=k1θ+k2formula (3)
Wherein k is1、k2Is a constant calibrated by experiments.
Considering that the transmission channel of heat is generated continuously as the moisture content of the dried soil pores increases, the moisture content in the soil is very sensitive to the heat conduction in the soil; and the pores in the moist soil are gradually filled, so that a new heat transmission channel cannot be formed, and the change of the moisture content at the moment has little influence on the transmission of heat in the soil. Then, aiming at the problem that the conduction mechanisms of the soil are different under different volume water contents theta, the Caoding team carries out sectional treatment on the corresponding relation between the temperature characteristic value and the soil water content, and establishes the temperature characteristic value T for the cohesive soiltAnd (3) an empirical model of the volume water content theta, wherein the expression is shown in the following formula (4):
wherein k is1,k2,k3,k4,k5Is a constant which can be calibrated by experiment, theta0The limit water content is shown.
The piecewise function is suitable for measuring the water content of specific soil at different depths. The coefficient of the function is mainly determined by indoor tests, the temperature characteristic value of the soil is measured under different water contents, the relation between the temperature characteristic value and the water content is calculated by utilizing least square regression, the limit water content of the soil is roughly determined, the soil is divided into a drying stage and a wetting stage, and the function relation of the soil is calibrated by respectively adopting linear fitting and logarithmic fitting, so that the water content measuring and calculating function suitable for the soil is obtained.
Second, the relation between the temperature characteristic value and the conductivity
Firstly: establishing a soil conductivity model of liquid phase conductivity and water content
Assuming that the total resistance of each unit in the soil is the same, the volume water content theta and the volume conductivity EC of the soil are determinedaSurface conductivity ECsConductivity EC with aqueous medium in soilwThe following conductivity expression is formed in combination as shown in the following equation (5):
ECa=ECw(aθ2-bθ)+ECsin the formula (5),
wherein, ECaFor volumetric conductivity in soil, ECsFor surface conductivity in soil, ECwThe conductivity of medium water in soil, a and b are constants to be measured;
in order to express the relationship between the temperature characteristic value and the soil conductivity, the formula (5) is substituted into the formula (4), and basic data required to be extracted from the soil in the formula is subjected to normalization, so that the following formula (6) is obtained:
wherein, EC0Is the limit value of the volume conductivity in the soil and the conductivity k corresponding to the soil liquid limit index1,k2,k3,k4,k5,k6The coefficient k in the formula (6) can be determined by experiment as a constant1,k2,k3,k4,k5,k6Mainly utilizes the least square regression method to calculate and analyze the temperature characteristics in the measurement result according to the temperature characteristic values and the electrical conductivities measured under different water contents in the indoor testThe relation between the value and the water content and the relation between the measured water content and the conductivity are determined, the boundary conductivity of the soil is roughly determined through a relation curve, the soil is divided into a drying stage and a wetting stage, a function relation formula of the soil is calibrated through linear fitting and logarithmic fitting respectively, a temperature characteristic value-soil conductivity piecewise function is fitted, and the conductivity measuring and calculating method suitable for the soil is obtained.
Then: saturation presumption conductivity
Solving a theoretical model of the conductivity by the soil saturation:
wherein S isrIs the soil saturation, B is an empirical parameter, rhosIs the resistivity of the soil, psatIs the soil saturation resistivity.
The saturation resistivity of the soil can be measured directly from laboratory tests, and the empirical parameter B is determined from the solid content of the soil. The relation between the saturation and the soil resistivity can be converted into the relation between the volume water content and the soil conductivity, and the relation is substituted into the formula (4), so that the temperature characteristic value T can be obtainedtThe following relationship exists between the conductivity of the soil:
wherein k is1,k2,k3,k4,k5,k6Are constants, all can be determined by experiment, and B is an empirical parameter.
Coefficient k of formula (1)1,k2,k3,k4,k5,k6The method is mainly determined by indoor tests, corresponding empirical parameters B are selected according to temperature characteristic values, conductivity and components in soil which are measured under different water contents, the relation between the temperature characteristic values and the conductivity is calculated by using least square regression on measurement results, and a saturation-resistivity relation curve is listed. From temperature characteristic-water contentThe relation of the rates roughly determines the boundary conductivity of the soil, the soil is divided into a drying stage and a wetting stage, and the temperature characteristic value-soil conductivity two stages are calibrated by respectively adopting linear fitting and logarithmic fitting to obtain a conductivity measurement and calculation function suitable for the soil.
According to the soil body conductivity measuring method based on the active heating optical fiber method, the accuracy of water content is deduced by using the optical fiber temperature, the relation between the temperature characteristic value and the conductivity of the optical fiber heating method is further deduced by combining with a theoretical model of water content and conductivity with high accuracy, so that the resistivity of an optical fiber embedding position can be determined, the measuring accuracy is improved, the measurement is convenient, and certain practical significance is achieved.