Using deep reinforcement learning with smooth constraint to invert magnetotelluric data
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摘要:
反演是处理大地电磁测深数据的关键步骤之一,得到了学者的广泛研究。其中基于数据驱动的反演方法主要包括带监督反演和半监督反演,对无监督反演的研究较少。DQN(deep Q-network)是一种经典的深度强化学习算法,作为无监督反演方法最近被用于解决一维大地电磁反演问题。该方法具有不需要训练数据集、对初始模型依赖较小、多次反演能够得到反演结果的概率分布等优点,但存在反演结果不集中的问题。提出了带光滑约束的大地电磁强化学习反演方法(Smooth DQN,简称SDQN)。本方法基于强化学习框架,将反演问题看成马尔可夫决策问题,并分别定义环境、奖励、智能体等术语;然后将正则化反演的模型约束项引入到奖励中来,从而引导智能体不断调整预测模型的电阻率参数以得到更符合模型约束的结果。理论模型反演结果表明,相较于DQN反演和Occam反演方法,在相同反演次数情况下SDQN方法反演不同噪声水平的观测数据时结果更稳定。西藏扎西康矿集区的大地电磁实测数据反演结果与Occam反演结果基本吻合并与已有的地质解释资料一致。SDQN方法具有反演结果更集中、对观测数据的抗噪能力更强的优点,是解决大地电磁反演问题的新工具。
Abstract:Inversion is one of the key steps in processing magnetotelluric sounding data and has been widely studied by scholars. The data-driven approaches mainly include supervised inversion and semi-supervised inversion,
but there is limited research on unsupervised inversion .Objective Deep Q-network (DQN) is a classical deep reinforcement learning algorithm,
which is an unsupervised inversion approach that has recently been applied to invert one-dimensional magnetotelluric data. It has the advantages of not requiring a training dataset,being less dependent on the initial model, and being able to obtain the probability distribution of inversion results through multiple inversions.However, it suffers from the issue that the inversion results are not concentrated.Method To address this issue,
this paper proposes the use of deep reinforcement learning with a smooth constraint to invert magnetotelluric data (Smooth DQN, SDQN) . This method is based on the framework of reinforcement learning,considers the inversion problem as a Markov decision problem, and defines the termsenvironment ,reward ,agent , and so on. Then, the model constraint term of regularized inversion is introduced into the reward function,guiding the agent to continuously adjust the resistivity parameters of the prediction model to obtain results that are more consistent with the model constraints.Results The experimental results of the theoretical model inversion show that, compared with the DQN inversion and Occam inversion methods, the results of the proposed method are more stable when the observed data are inverted with the same number of iterations and different noise levels. The inversion results of the magnetotelluric measured data in the Tashi Kang Mine area of Tibet are
largely consistent with the Occam inversion results and align with the existing geological interpretations.Conclusion The experimental results show that this method has the advantages of more concentrated inversion results and stronger anti-noise capability for the observed data, and it is a new tool for solving the problem of magnetotelluric inversion.
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Key words:
- Deep Reinforcement Learning /
- Magnetotelluric Inversion /
- Smooth Constraints /
- DQN
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表 1 SDQN算法超参数设置
Table 1. Hyperparameters in the network.
超参数 数值 小批量大小 32 经验池大小 2,500,000 约束网络更新频率 500 折扣因子 0.95 预测网络更新频率 5 学习率 0.0013 探索因子 0.1 表 2 5层理论模型
Table 2. Five-layer synthetic model.
层号 电阻率/(Ω·m) 厚度/m 1 200 1820 2 20 1900 3 100 4535 4 10 2150 5 100 − 表 3 10层预测模型3种初始模型参数设置
Table 3. Three initial model parameter settings for the predictive model
层号 1 2 3 4 5 6 7 8 9 10 电阻率/(Ω·m) 10 10 10 10 10 10 10 10 10 10 电阻率/(Ω·m) 100 100 100 100 100 100 100 100 100 100 电阻率/(Ω·m) 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 厚度/m 500 600 720 865 1035 1245 1495 1795 2150 − 表 4 初始模型在100 Ω·m下反演结果的误差和方差
Table 4. Error and variance of the initial model inversion results at 100 Ω·m
反演次数 最终结果的均方误差 所有结果的均方误差的方差 DQN SDQN DQN SDQN 100 99.97 14.23 2532.45 296.60 200 13.355 15.094 3019.32 427.82 300 25.332 12.0845 2134.51 201.47 表 5 8层理论模型电阻率和厚度
Table 5. Eight-layer synthetic model.
层号 1 2 3 4 5 6 7 8 电阻率/(Ω·m) 2500 1000 100 10 100 25 10 2.5 厚度/m 600 1400 2200 3400 7000 9000 11000 − 表 6 20层预测模型三种初始模型参数设置
Table 6. Three initial model parameter settings for the predictive model
层号 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 电阻率/(Ω·m) 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 厚度/m 600 672 752 842 944 1057 1184 1326 1485 1663 1863 2087 2337 2618 2932 3284 3678 4119 4613 − 表 7 5%噪声水平下DQN和SDQN方法的反演时间
Table 7. Inversion time of DQN and SDQN methods at 5% noise level
迭代次数 反演时间/s DQN SDQN 50 3287.29 3562.45 100 4164.09 4520.86 150 4918.86 5144.10 200 5628.15 5745.47 250 6304.54 6299.31 300 6944.33 6841.33 -
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