Co-citation analysis is an important content of bibliometric analysis, employed to identify emerging trends and hot topics from a number of papers. Co-cited articles are considered to have similarities in contents commonly, identified by the clusters of co-cited references which means creating a link between two or more references when they appear simultaneously in the reference lists of citing articles (Raghuram, Tuertscher and Garud 2010).
4.1. Co-citation analysis of authors
Scholars from the same community have been cited together, and authors with high citations also pay attention to the collaboration with authors from other institutions. A co-citation network about authors is shown (Fig. 4). Top-6 most cited authors are Guzzetti(131), Huffman(111), Ferretti(105), Wang(92), Zhang(88), and Voigt(86). The citation frequency is the total citation by our 2,833 papers dataset. It can be found that influential authors with high frequency also put emphasis on the collaboration with others.
A typical article of Guzzetti (Guzzetti et al. 2012) presents satellite-based technologies for landslide mapping, which considers the exploitation of very-high resolution digital elevation models to analyze surface morphology, and the visual interpretation and semi-automatic analysis of different types of satellite images, including panchromatic, multispectral, and synthetic aperture radar images. A typical article of Voigt (Voigt et al. 2011) talks about the activity of the Center for Satellite based Crisis Information of the German Aerospace Center after a devastating Haiti earthquake in 2010. A specific approach including preprocessing procedures and visual interpretation on a grid-basis for damage maps is proposed, to avoid problems generated from the large number and inconsistency of satellite maps internationally.
4.2. Dynamic evolution of co-cited keywords
Keywords are clear symbols of the critical content of research. Before the co-cited keywords analysis, technology-related words, but unrelated to a management issue, have been excluded artificially, such as lidar, persistent scatterer, synthetic aperture radar etc., and some synonyms and abbreviations are merged, such as forecast and prediction, vulnerability and susceptibility, analytic hierarchy process and AHP, etc. And then, a co-occurrence network of keywords is obtained by CiteSpace (Fig. 5). The 10 most frequent words are: model, GI (Geographic Information), imagery, classification, area, climate, earthquake, impact, algorithm, vulnerability.
Moreover, a time-zone map of keywords is generated (Fig. 6), taking five-year period as a slice, and the whole interval is divided into six parts. For clarity, only keywords with high node centrality or high frequency are reserved. Through the time-zone analysis, hot issues in recent years and future directions are analyzed. The graph is a collection of nodes with the similar occurrence time in the same time zone. If there are less keywords in a time zone, fewer influential results exist in the period. And the connection of nodes between time zones indicates the inheritance of research. The evolution trend of hotspots in each period are summarized (Table 5), reflecting the main problems and influential methods.
Table 5
Evolution of research trends.
Year
|
Main keywords
|
1991–1996
|
Imagery, dynamics, vegetation, identification
|
1997–2001
|
GI (Geographic Information), system, model, NDVI (Normalized Difference Vegetation Index), temperature, rainfall
|
2002–2006
|
Prediction, classification, vulnerability, deformation, accuracy, earthquake
|
2007–2011
|
Risk, damage, recovery, segmentation, simulation, logistic regression, landslide
|
2012–2016
|
Map, optimization, time series, support vector machine, AHP (Analytical Hierarchy Process)
|
2017–2021
|
Uncertainty, urbanization, ocean, UAV (Unmanned Aerial Vehicle), extreme, CNN (Convolutional Neural Network)
|
Over 30 years, the main methods involved in applications of satellites in emergency management are: model, system, simulation, logistic regression, optimization, time series, support vector machine, AHP, and CNN. Satellite-based technologies are applied in the problems of identification, classification, segmentation of satellite images, as well as risk assessment. The key stages in emergency management of discussions have gradually shifted from disaster identification to prediction and vulnerability assessment, further to damage evaluation and recovery.
From the view of the type of disaster, literatures about satellite monitoring for the rainfall and temperature occur early, in 1997–2001, as well as researches on flood and fire management. During 2002–2006, the focus on earthquake enhances rapidly, and 2008 Wenchuan earthquake and 2010 Haiti earthquake are extensively studied by scholars. After that, the landslide gets more attention of scholars. In recent years, disasters related to extreme weather conditions such as extreme precipitation, or disasters occurring on the ocean become the hotspots, and management problems considering uncertainty or urbanization are discussed, worthy of further study.
4.3. Cluster analysis of co-cited articles
In order to visualize the co-citation relationship of sample literatures, CiteSpace is used to conduct a cluster analysis on cited literatures. According to the topic of cited reference, several clusters are generated, whose labels are produced based on the text analysis of titles, keywords and abstracts of cluster members, reflecting the most frequent contents. Therefore, the cluster labels represent hot topics of relevant research.
The sample articles are arranged into 13 highlighted research clusters (Table 6), and the top five hot topics are Sentinel-1, deep learning, landslide, Analytical Hierarchy Process/AHP and earthquake, with cluster size 80, 55, 55, 53, and 51 respectively, and year labels of these clusters are 2016, 2016, 2015, 2017, and 2008 respectively. The representative articles of each cluster are also provided in the software (Fig. 7). As the cluster labels are produced by CiteSpace automatically, a careful check for the papers that be clustered into each label is conducted. Owing to the research topic belongs to an interdisciplinary subject, the clustering results of co-cited articles contain a degree of dispersion. To get more effective and rapid emergency management, an integrated methodology or framework needs to be established. Also, there may be overlaps between different cluster labels that focus on disaster issues, technologies, or both.
Table 6
Summary of cluster analysis of co-cited articles
Cluster ID
|
Cluster label
|
Cluster size
|
Silhouettea
|
Year(mean)
|
#0
|
Sentinel-1
|
80
|
0.815
|
2016
|
#1
|
Deep learning
|
55
|
0.971
|
2016
|
#2
|
Landslide
|
55
|
0.960
|
2015
|
#3
|
AHP (Analytical Hierarchy Process)
|
53
|
0.955
|
2017
|
#4
|
Earthquake
|
51
|
0.981
|
2008
|
#5
|
ANFIS (Adaptive Neuro-Fuzzy Inference System) model
|
44
|
0.974
|
2011
|
#7
|
Fuzzy Logic
|
28
|
0.997
|
2011
|
#8
|
GPM (Global Precipitation Measurement)
|
28
|
0.932
|
2015
|
#10
|
Normalized difference vegetation index
|
26
|
0.950
|
2003
|
#11
|
Climate change
|
26
|
0.987
|
2003
|
#14
|
GIS
|
16
|
0.961
|
2010
|
#15
|
Landslide’s susceptibility
|
12
|
0.982
|
2012
|
#21
|
Satellite communication
|
8
|
0.975
|
2008
|
a: Silhouette coefficient represents an appreciation of the relative quality of clusters. |
The interest for satellite imagery can be explained by the availability of many open-source data with high-resolution images and regular information updates (Lissak et al. 2020). And GMES Sentinel-1 mission (Cluster #0), NASA GPM (Cluster #8), GIS (Cluster #14) provide abundant integrated information from satellites for disaster response and related studies. GIS integrates digital elevation models, satellite images, hazard maps and vector data on natural and artificial features (energy supply lines, strategic buildings, roads, railways, etc.). Dieu Tien et al. (2019) propose a hybrid approach based on Extreme Learning Machine and Particle Swarm Optimization for flash flood susceptibility mapping. A geospatial database is constructed with 654 flash flood locations and 12 factors.
The most concerned natural disasters include landslide (Cluster #2 and #15), earthquake (Cluster #4), and flood. Flood mapping or flood monitoring occur many times in keywords of literatures in Cluster #0, as well as flood risk management in cluster #7. Ostir et al. (2003) review the remote sensing applications in natural hazard monitoring, and take Mount Mangart landslide as a case study. Lissak et al. (2020) review the spaceborne, aerial and terrestrial remote-sensing methods employed for landslide assessment and processing, and point out that the key for understanding landslides is the complementarity of methods and the automation of the data processing. Building damage detection after natural hazards, especially earthquake, would help to rapid relief and response of disaster, always combined with technologies of image segmentation, and Haiti earthquake (2010 Port-au-Prince) provides a classical database for the related research. Cooner, Shao and Campbell (2016) evaluate the effectiveness of multilayer feedforward neural networks, radial basis neural networks, and random forests in detecting earthquake damage caused by Haiti earthquake. The pre- and post-flood satellite images, coupled with hydrological (river water level) and meteorological (rainfall) data, are contributive for risk management. For emergency response to floods, access to timely and accurate data is essential, and satellite imagery offers a rich source of information which can be analyzed to help determine regions affected by a disaster (Nemni et al. 2020).
AHP (cluster #3) for vulnerability assessment, ANFIS model (cluster #5) for automatic detection, and Fuzzy Logic (cluster #7) for disaster mitigation are the popular methods in emergency management. In recent years, more articles focus on deep learning (cluster #1) to implement automatic detection (including disaster prediction, monitoring and rescue), by dealing with numbers of images or other forms of data obtained from satellites. Ji, Liu and Buchroithner (2018) use the CNN to identify collapsed buildings from post-event satellite imagery. Producer accuracy, user accuracy, and overall accuracy are used as evaluation metrics, and the network performs well in classifying collapsed and non-collapsed buildings. Bai et al. (2021) present new deep learning algorithms and a multi-source data fusion driven flood inundation mapping approach by leveraging a large-scale publicly available Sen1Flood11 dataset consisting of roughly 4,831 labelled Sentinel-1 SAR and Sentinel-2 optical imagery gathered from flood events worldwide. Ahmed et al. (2021) attempt to assess flood susceptibility of the Brahmaputra floodplain of Bangladesh using Deep Boost, Deep Learning Neural Network, and Artificial Neural Network.
Satellite communication (Cluster #21) also plays an important role in the emergency management, to replace or support terrestrial infrastructures due to the facility damage or the lack of power supply (Asuquo et al. 2018; Tani et al. 2020).
4.4. Burst analysis of co-cited articles
Burst analysis is used to detect sudden changes in topics, authors or references of sample articles, based on citation or word frequency. A citation burst of references is conducted to detect a surge of citations, representing an active research area in a given period. The information of top-10 references with strongest citation bursts are listed in Table 7. The burst period of each reference is highlighted in red in the timeline from 1991 to 2021. Among these 10 references, 6 papers are published in the journals about remote sensing to detect the damage of natural disasters, and the earthquake has the most attention in different types of disasters. 2 papers are published in the journals about Earth science, which focus on the semi-automatic detection and inventory maps of landslide. Another 2 papers are published in conferences associated with computer science, studying the convolutional networks. Guzzetti et al. (2012) present satellite-based technologies for landslide mapping, with the highest burst strength, 10.37. Gorelick et al. (2017) become more popular recently, summarizing the databases, system architecture, data distribution models and applications of Google Earth Engine, to help monitor, track and manage the Earth's environment and resources. And this paper receives citation bursts from 2019 to 2021.
Table 7 Top-10 references with strongest citation bursts
a: The strength represents the change in the word frequency that triggered the burst, presented by Kleinberg (2003).