CN-115980829-B - Method for widening air gun focus frequency band by delay excitation construction wide pulse wavelet and application

CN115980829BCN 115980829 BCN115980829 BCN 115980829BCN-115980829-B

Abstract

The invention belongs to the technical field of air gun focus data identification in the marine seismic exploration process, and discloses a method for widening an air gun focus frequency band by using a delay excitation structure wide pulse wavelet and application thereof. The method comprises the steps of firstly calculating the time for reaching the peak value of the main pulse after the excitation of the capacity air guns, and then adjusting the excitation time of the air guns with different capacities according to the obtained time, so that the time for reaching the peak value of the main pulse of each capacity air gun in the array generates corresponding time delay, thereby constructing wide pulse wavelets with wider main pulse waveforms and approximate trapezoids, and realizing the purpose of guaranteeing the high frequency of a seismic source and expanding the low frequency. According to the invention, by utilizing the relation between the capacity of the air gun and the time of the main pulse peak value, the delayed excitation time of the air guns with different capacities is regularly adjusted, the wide pulse wavelet is constructed, the aim of expanding to low frequency on the basis of guaranteeing high frequency is realized through comparison, and finally, the air gun seismic source with rich high and low frequency and strong energy downloading capability aiming at the middle-deep geological target is obtained.

Inventors

LV DINGYOU
MING JUN
HUANG JIANGBO
ZHANG ZHIJUN
ZHANG ZHONGQIAO
YAO JIAN
XIONG YU

Assignees

中海石油(中国)有限公司
中海石油（中国）有限公司天津分公司

Dates

Publication Date: 20260505
Application Date: 20221223

Claims (10)

1. A method for broadening the air gun source frequency band by constructing a broad pulse wavelet by delayed excitation, the method comprising the steps of: s1, simulating air gun wavelets with different capacities according to a Van der Waals nonideal air gun wavelet model, and setting model initial conditions; s2, executing a simulation process according to initial conditions of the set model; S3, analyzing the simulated air gun wavelets, and counting the time t f from the excitation to the main pulse peak value of the air gun wavelets with different capacities; s4, calculating the difference value between the time from the excitation to the main pulse peak value and the minimum time t 0 of the air guns with different capacities according to the simulation result ; S5, obtaining the calculated As the delayed excitation time of air guns with different capacities in the air gun array, then carrying out air gun wavelet simulation to generate air gun array wavelets with wide pulse shapes; s6, performing spectrum analysis on the obtained air gun array wavelet.
2. The method of claim 1, wherein in step S1, the equation for the van der waals nonideal gas gun wavelet model is expressed as: (1) Wherein a and b are Van der Waals constants , , In order to be an effective thermodynamic temperature, In order to be a universal gas constant, In order to be a mass of gas, Is in the form of a volume and, Is air gun pressure; effective temperature Depending on the high pressure gas within the chamber: (2) In the formula, , Is the water temperature; according to the law of conservation of energy, the balance between the energy obtained by the heat propagation loss of the bubble and the mass transfer of the bubble substance and the change in the internal energy of the bubble is: (3) In the formula, Is the temperature of the air bubble and the temperature of the air bubble, Is the pressure of the air bubbles and, Is the mass of the gas in the bubble, Represents the internal energy of the air bubble, And Respectively a constant pressure specific heat capacity and a constant bulk specific heat capacity, Is the rate of heat transfer through the bubble wall, For a unit interval of time of movement of the bubble wall, For the transfer of heat per time interval of the bubble to the surroundings, Is the amount of decrease in energy per unit time in the bubble, As the amount of change in the volume of the bubbles per unit time, Heat transfer coefficient as the amount of change per unit time of mass in a bubble The bubble heat loss rate was determined by fitting the model to the experimental data and expressed as: (4) In the middle of Is the bubble temperature And ambient water temperature The temperature difference between the two is calculated, Is the bubble radius; Using the van der waals non-ideal gas equation, the internal energy of a non-ideal gas is a function of gas temperature and volume: (5) the full differential equation is expressed as: (6) Furthermore, the first law of thermodynamics can be converted into: (7) In the formula, , For a molar heat capacity at a constant pressure, Is the rate of change of the amount of gaseous material; the throttle constants of air guns of different capacities are related to the size of the air chamber and are expressed as: (8) In the formula, Is a port throttle constant independent of capacity, The flow rate of the gas through the port of the air gun depends on the pressure difference inside and outside the air gun in any given time, and the release rate of the gas is expressed as: (9) In the formula, Is the amount of gaseous material released into the bubbles, Is the total amount of gas in the gas chamber, Is the ratio of the amount of gas in the bubble to the total amount, the throttle constant , The capacity of the air chamber is set, The mass of the gas in the gas chamber, Is the pressure of the air gun and the air pressure of the air gun, Is the bubble pressure; The equation of motion of the bubble wall is expressed as: (10) In the middle of Is the radius of the air bubble, And The velocity and acceleration of the bubble wall respectively, Is the velocity of the sound wave in the fluid medium, Is the enthalpy difference of the bubble wall, Is the static water density at infinity, Is the pressure of the air bubbles and, Is hydrostatic pressure at infinity, and the vertical rising speed of the air bubble during the rising process is expressed as: (11) In the formula, Is the bubble depth, g is the gravitational acceleration constant, Is bubble radius, hydrostatic pressure The expression of (2) is: (12) In the formula, Is a standard atmospheric pressure of air and, Is the air gun depth, and at 1m from the air gun, the air gun wavelet signal is expressed as: (13) the pressure field around any arbitrary bubble is superposition of hydrostatic pressure plus time-varying pressure field generated by the bubble, and the effective hydrostatic pressure at each bubble of the ith is; (14) In the formula, Is the hydrostatic pressure of the water, Is the sum of the pressure contributions of all other air guns in the air gun array, Is the first Air bubble induced pair Hydrostatic pressure disturbance of individual bubbles, item Air bubble induced pair Time delay of individual bubbles and pressure characteristics on a distance scale: (15) In the middle of Represent the first Bubble and the first Bubble spacing between individual bubbles.
3. The method of claim 1, wherein in step S1, the model initial conditions are set to be: step 1.1, condition 1, initial value of air gun pressure Setting the working pressure; step 1.2, condition 2, initial temperature in the bubble is set to ; Step 1.3, condition 3 initial volume of bubbles An initial radius of ; Step 1.4, condition 4, initial velocity of bubble wall is ; Step 1.5, condition 5 bubble initial pressure The initial temperature is water temperature The initial mass in the bubble is ; And 1.6, setting the placement positions (x, y, z) of the air guns.
4. The method of claim 1, wherein in step S2, the simulation is performed according to the initial conditions of the set model by: Step 2.1, inputting initial conditions of a Van der Waals nonideal gas gun wavelet model; step 2.2, starting the time cycle and calculating the bubble volume at time t=k ; Step 2.3, calculating the bubble pressure at the time t=k by using the equation (1), ; Step 2.4, calculating the heat loss rate of the bubbles through an equation (4), ; Step 2.5, calculating the release rate of the gas by an equation (9), ; Step 2.6, calculating The rate of change of the volume of the bubbles at the moment in time, ; Step 2.7, calculating by equation (7) The rate of change of temperature within the bubble at the moment, ; Step 2.8, calculating enthalpy difference of the bubble wall, ; Step 2.9, solving the equation (1) for time The differentiation of (c) yields the rate of change of bubble pressure, ; Step 2.10, enthalpy difference is calculated with respect to time Is obtained by a differentiation of (a) and (b), ; Step 2.11, calculation by equation (10) The rate of change of velocity of the bubble wall at the moment in time, Acceleration of the bubble wall; Step 2.12, pair , , , , , Solving for time Is obtained by a differentiation of (a) and (b), , , , , , ; Step 2.13, since the air gun wavelet simulation is an iterative process, the bubble wall radius, the bubble wall speed, the gas temperature and the mass of the gas in the bubble are obtained through the second-order Taylor series expansion: , , ,and ; step 2.14, expressing bubble pressure as a function of enthalpy, bubble wall velocity and bubble radius: , distance from the center of the bubble to the far field point; step 2.15, repeating steps (2.1) to (2.14) until ; Step 2.16, calculating far-field wavelet sound pressure of the air gun, including sea surface ghost reflection: , representing the reflection coefficient of the sea surface, It is the distance between the air gun and the hydrophone that, Is the distance between the sea surface mirror image of the air gun and the hydrophone, Is the air gun signal passing through And Is a time delay of (a) to (b).
5. The method of claim 1, wherein counting time t f from excitation to main pulse peak of different volume air gun wavelets in step S3 includes counting time t f from main pulse peak after excitation of 45 cu.in/70 cu.in/100 cu.in/150 cu.in/250 cu.in volume air gun.
6. The method for broadening an airgun source bandwidth as defined in claim 1 wherein in step S4, a difference between a time from an excitation to a main pulse peak of the airguns of different capacities and a minimum time t 0 is calculated The method comprises the following steps: calculating 70cu.in\100cu.in\150cu.in\250cu.in the main pulse is reached after the air gun with capacity is excited peak time to 45cu.in capacity air gun to main pulse peak time difference 。
7. The method of claim 1, wherein in step S5, the air gun array wavelet generating the wide pulse pattern is specifically 39 guns in the air gun array, wherein the number of the air guns is 33, the total volume of the air guns is 40cu.in, and the volume and the number of the single guns are 45cu.in, 70cu.in, 100cu.in, 150cu.in and 250cu.in respectively.
8. The method for broadening the air gun source bandwidth as defined in claim 1 wherein in step S6, the spectrum analysis of the obtained air gun array wavelet comprises calculating the main pulse peak value, the ghost value and the bubble pulse peak value of the air gun array wavelet, performing the spectrum analysis of the wavelet by Fourier transform, obtaining the effective bandwidth of the wavelet main pulse by taking the maximum amplitude-6 dB as the standard for judging the effective bandwidth, and obtaining the wavelet main pulse main frequency.
9. A high volume air gun embodying the method of any one of claims 1 to 8 for broadening the air gun source frequency band by constructing a broad pulse wavelet by delayed excitation.
10. Use of a method of widening the frequency band of an air gun source by constructing a broad pulse wavelet by delayed excitation according to any one of claims 1-8 in a marine field broadband stereoscopic viewing system, and marine geological investigation and oil and gas exploration equipment.

Description

Method for widening air gun focus frequency band by delay excitation construction wide pulse wavelet and application Technical Field The invention belongs to the technical field of air gun focus data identification in the marine seismic exploration process, and particularly relates to a method for widening an air gun focus frequency band by using a delay excitation structure wide pulse wavelet and application thereof. Background In marine geological investigation and oil and gas resource exploration, an air gun source is key equipment in a marine seismic information acquisition system, and particularly for deep target exploration in the ocean, low-frequency energy in the source is the most important. With the mass exploitation of deep oil and gas resources in the ocean, air gun seismic sources with abundant low-frequency energy are receiving more and more attention. A large volume air gun may contribute more low frequency signals, but the corresponding bubble oscillations are strong. At present, the aim of widening the wavelet frequency band of a seismic source and improving the downloading capacity of seismic waves of seismic exploration is fulfilled by mainly utilizing a multi-subarray plane and three-dimensional combination method at home and abroad by utilizing tuned subarray energy. The combination of multiple subarrays has obvious effect of improving the high frequency of the seismic source, but the effect of extending the frequency band of the seismic source to the low frequency is not obvious, and how to ensure the high frequency and simultaneously extend the low frequency is a problem to be solved in the field of geophysics. How to suppress sea surface ghost and improve the low-frequency energy of the seismic source and the seismic wave downloading capacity is a problem solved by the prior art. Through the above analysis, the problems and defects existing in the prior art are as follows: (1) The air gun seismic source downloading capability of the obtained land aiming at the middle-deep geological target is weak in the prior art; (2) The air gun focus formed by the prior art has low resolution ratio for exploration of the middle and deep layers and unclear structure depiction. (3) In the prior art, because the air gun excitation time with different capacities cannot be controlled to realize the extension of the low frequency of the seismic source and the widening of the sub-wave frequency of the seismic source, the generated subsurface transmission capacity seismic wavelet cannot be suitable for the related quality requirements of deep geological target exploration in shallow sea. Disclosure of Invention In order to overcome the problems in the related art, the disclosed embodiment of the invention provides a method for widening the frequency band of an air gun seismic source by adopting a delay excitation structure wide pulse wavelet and application thereof, which are used for a marine field wide-band stereoscopic observation system, exploration and acquisition of deep seismic reflection signals in offshore shallow water and marine geological investigation and oil and gas exploration. The technical scheme is that the method for widening the frequency band of the air gun focus by constructing a wide pulse wavelet through delayed excitation comprises the following steps: s1, simulating air gun wavelets with different capacities according to a Van der Waals nonideal air gun wavelet model, and setting model initial conditions; s2, executing a simulation process according to initial conditions of the set model; S3, analyzing the simulated air gun wavelets, and counting the time t i from the excitation to the main pulse peak value of the air gun wavelets with different capacities; S4, calculating a difference delta t i between the time from excitation to main pulse peak value of the air guns with different capacities and the minimum time t 0 according to the simulation result; s5, taking the calculated Deltat i as the delayed excitation time of air guns with different capacities in the air gun array, and then carrying out air gun wavelet simulation to generate air gun array wavelets with wide pulse shapes; s6, performing spectrum analysis on the obtained air gun array wavelet. In step S1, the equation for the Van der Waals nonideal gas gun wavelet model is expressed as: Where a and b are van der waals constants a= 0.1404m 6·Pa·mol-2,b＝3.764×10-5m3·mol-1,Tg, R g is a universal gas constant, m g is gas mass, V g is volume, and P g is air gun pressure; the effective temperature T g depends on the high pressure gas within the chamber: Tg＝Tw(1+Pg/Pc)(2) wherein P c＝139MPa,Tw is water temperature; according to the law of conservation of energy, the balance between the energy obtained by the heat propagation loss of the bubble and the mass transfer of the bubble substance and the change in the internal energy of the bubble is: Where T is the bubble temperature, P is the bubble pressure, m b is the mass of gas