Machine learning in earth sciences

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Applications of

In Earth Sciences, some data are often difficult to access or collect, therefore inferring data from data that are easily available by machine learning method is desirable.[10] For example, geological mapping in tropical rainforests is challenging because the thick vegetation cover and rock outcrops are poorly exposed.^[17] Applying remote sensing with machine learning approaches provides an alternative way for rapid mapping without the need of manually mapping in the unreachable areas.^[17]

Reduce time costs

Machine learning can also reduce the efforts done by experts, as manual tasks of classification and annotation etc are the bottlenecks in the workflow of the research of earth science.^[10] Geological mapping, especially in a vast, remote area is labour, cost and time-intensive with traditional methods.^[18] Incorporation of remote sensing and machine learning approaches can provide an alternative solution to eliminate some field mapping needs.^[18]

Consistent and bias-free

Consistency and bias-free is also an advantage of machine learning compared to manual works by humans. In research comparing the performance of human and machine learning in the identification of dinoflagellates, machine learning is found to be not as prone to systematic bias as humans.^[19] A recency effect that is present in humans is that the classification often biases towards the most recently recalled classes.^[19] In a labelling task of the research, if one kind of dinoflagellates occurs rarely in the samples, then expert ecologists commonly will not classify it correctly.^[19] The systematic bias strongly deteriorate the classification accuracies of humans.^[19]

Optimal machine learning algorithm

The extensive usage of machine learning in various fields has led to a wide range of algorithms of learning methods being applied. The machine learning algorithm applied in solving earth science problem in much interest to the researchers.

Below are highlights of some commonly applied algorithms.[23]

• 
  Support Vector Machine (SVM)
  In the Support Vector Machine (SVM), the decision boundary was determined during the training process by the training dataset as represented by the green and red dots. The data of purple falls below the decision boundary, therefore it belongs to the red class.^[7]
• 
  K nearest neighbor
  K nearest neighbor classifies data based on their similarities. k is a parameter representing the number of neighbors that will be considered for the voting process. For example, in the figure k = 4, therefore the nearest 4 neighbors are considered. In the 4 nearest neighbors, 3 belong to the red class and 1 belongs to the green class. The purple data is classified as the red class.^[24]
• 
  Decision Tree
  Decision Tree shows the possible outcomes of related choices. Decision Tree can further be divided into Classification Tree and Regression Tree. The above figure shows a Classification Tree as the outputs are discrete classes. For regression Tree, the output is a number. This is a white-box model which is transparent and the user is able to spot out the bias if any appears in the model.^[7]
• 
  Random forest
  In random forest, multiple decision trees are used together in an ensemble method. Multiple decision trees are produced during the training of a model. Different decision trees may give up various results. The majority voting/ averaging process gives out the final result. This method yields a higher accuracy of using a single decision tree only.^[22]
• 
  Neural Networks
  Neural Networks mimic neurons in a biological brain. It consists of multiple layers, where the layers in between are hidden layers. The weights of the connections are adjusted during the training process. As the logic in between is unclear, it is referred to as 'black-box operation'. Convolutional neural network (CNN) is a subclass of Neural Networks, which is commonly used for processing images.^[24]

Usage

Mapping

Geological or lithological mapping and mineral prospectivity mapping

Vegetation cover is one of the major obstacles for geological mapping with remote sensing, as reported in various research, both in large-scale and small-scale mapping. Vegetation affects the quality of spectral image[25] or obscures the rock information in the aerial images.^[5]

Landslide susceptibility and hazard mapping

Landslide susceptibility refers to the probability of landslide of a place, which is affected by the local terrain conditions.^[29] Landslide susceptibility mapping can highlight areas prone to landslide risks which are useful for urban planning and disaster management works.^[7] Input dataset for machine learning algorithms usually includes topographic information, lithological information, satellite images, etc. and some may include land use, land cover, drainage information, vegetation cover^{[7][30][31][32]} according to their study needs. In machine learning training for landslide susceptibility mapping, training and testing datasets are required.^[7] There are two methods of allocating datasets for training and testing, one is to random split the study area for the datasets, another is to split the whole study into two adjacent parts for the two datasets. To test the classification models, the common practice is to split the study area randomly into two datasets,^[7][33] however, it is more useful that the study area can be split into two adjacent parts so that the automation algorithm can carry out mapping of a new area with the input of expert processed data of adjacent land.^[7]

Feature identification and detection

Discontinuity analyses

The Rock Mass Rating (RMR) System^[39] a world-wide adopted rock mass classification system by geomechanical means with the input of six parameters. The amount of water inflow is one of the inputs of the classification scheme, representing the groundwater condition. Quantification of the water inflow in the faces of a rock tunnel was traditionally carried out by visual observation in the field, which is labour and time consuming with safety concerns.^[40] Machine learning can determine the water inflow by analyzing images taken in the construction site.^[40] The classification of the approach mostly follows the RMR system but combining damp and wet state as its difficult to distinguish only by visual inspection.^[40][39] The images were classified into the non-damage state, wet state, dripping state, flowing state and gushing state.^[40] The accuracy of classifying the images was about 90%.^[40]

Classification

Soil classification

The most popular cost-effective method for soil investigation method is by Cone Penetration Testing (CPT).^[41] The test is carried out by pushing a metallic cone through the soil and the force required to push at a constant rate is recorded as a quasi-continuous log.^[4] Machine learning can classify soil with the input of Cone Penetration Test log data.^[4] In an attempt to classify with machine learning, there are two parts of tasks required to analyse the data, which are the segmentation and classification parts.^[4] The segmentation part can be carried out with the Constraint Clustering and Classification (CONCC) algorithm to split a single series data into segments.^[4] The classification part can be carried out by Decision Trees (DT), Artificial Neural Network (ANN), or Support Vector Machine (SVM).^[4] While comparing the three algorithms, it is demonstrated that the Artificial Neural Network (ANN) performed the best in classifying humous clay and peat, while the Decision Trees performed the best in classifying clayey peat.^[4] The classification by this method is able to reach very high accuracy, even for the most complex problem, its accuracy was 83%, and the incorrectly classified class was a geologically neighbouring one.^[4] Considering the fact that such accuracy is sufficient for most experts, therefore the accuracy of such approach can be regarded as 100%.^[4]

Geological structure classification

Exposed geological structures like

Laboratory earthquakes are produced in a laboratory setting to mimic real-world earthquakes. With the help of machine learning, the patterns of acoustical signals as precursors for earthquakes can be identified without the need of manually searching. Predicting the time remaining before failure was demonstrated in a research with continuous acoustic time series data recorded from a fault. The algorithm applied was Random Forest trained with about 10 slip events and performed excellently in predicting the remaining time to failure. It identified acoustic signals to predict failures, and one of them was previously unidentified. Although this laboratory earthquake produced is not as complex as that of earth, this makes important progress that guides further earthquake prediction work in the future.[43]

Streamflow discharge prediction

Real-time streamflow data is integral for decision making, for example, evacuations, regulation of reservoir water levels during a flooding event.^[44] Streamflow data can be estimated by information provided by streamgages which measures the water level of a river. However, water and debris from a flooding event may damage streamgages and essential real-time data will be missing. The ability of machine learning to infer missing data^[10] enables it to predict streamflow with both historical streamgages data and real-time data. SHEM is a model that refers to Streamflow Hydrology Estimate using Machine Learning^[45] that can serve the purpose. To verify its accuracies, the prediction result was compared with the actual recorded data and the accuracies were found to be between 0.78 to 0.99.

Challenge

Inadequate training data

An adequate amount of training and validation data is required for machine learning.^[10] However, some very useful products like satellite remote sensing data only have decades of data since the 1970s. If one is interested in the yearly data, then only less than 50 samples are available.^[46] Such amount of data may not be adequate. In a study of automatic classification of geological structures, the weakness of the model is the small training dataset, even though with the help of data augmentation to increase the size of the dataset.^[20] Another study of predicting streamflow found that the accuracies depend on the availability of sufficient historical data, therefore sufficient training data determine the performance of machine learning.^[45] Inadequate training data may lead to a problem called overfitting. Overfitting causes inaccuracies in machine learning^[47] as the model learns about the noise and undesired details.

Limited by data input

Machine learning cannot carry out some of the tasks as a human does easily. For example, in the quantification of water inflow in rock tunnel faces by images for Rock Mass Rating system (RMR),^[40] the damp and the wet state was not classified by machine learning because discriminating the two only by visual inspection is not possible. In some tasks, machine learning may not able to fully substitute manual work by a human.

Black-box operation

In many machine learning algorithms, for example, Artificial Neural Network (ANN), it is considered as 'black box' approach as clear relationships and descriptions of how the results are generated in the hidden layers are unknown.^[48] 'White-box' approach such as decision tree can reveal the algorithm details to the users.^[49] If one wants to investigate the relationships, such 'black-box' approaches are not suitable. However, the performances of 'black-box' algorithms are usually better.^[50]

References