Review Article A Review of the Advantages and Limitations of Geophysical Investigations in Landslide Studies - FloRe
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Hindawi
International Journal of Geophysics
Volume 2019, Article ID 2983087, 27 pages
https://doi.org/10.1155/2019/2983087
Review Article
A Review of the Advantages and Limitations of
Geophysical Investigations in Landslide Studies
Veronica Pazzi , Stefano Morelli , and Riccardo Fanti
Department of Earth Sciences, University of Firenze, Via G. La Pira 4, 50121 Firenze, Italy
Correspondence should be addressed to Veronica Pazzi; veronica.pazzi@unifi.it
Received 30 November 2018; Accepted 27 June 2019; Published 14 July 2019
Academic Editor: Pantelis Soupios
Copyright © 2019 Veronica Pazzi et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Landslide deformations involve approximately all geological materials (natural rocks, soil, artificial fill, or combinations of these
materials) and can occur and develop in a large variety of volumes and shapes. The characterization of the material inhomogeneities
and their properties, the study of the deformation processes, and the delimitation of boundaries and potential slip surfaces are not
simple goals. Since the ‘70s, the international community (mainly geophysicists and lower geologists and geological engineers) has
begun to employ, together with other techniques, geophysical methods to characterize and monitor landslides. Both the associated
advantages and limitations have been highlighted over the years, and some drawbacks are still open. This review is focused on
works of the last twelve years (2007-2018), and the main goal is to analyse the geophysical community efforts toward overcoming
the geophysical technique limitations highlighted in the 2007 geophysics and landslide review. To achieve this aim, contrary to
previous reviews that analysed the advantages and limitations of each technique using a “technique approach,” the analysis was
carried out using a “material landslide approach” on the basis of the more recent landslides classification.
1. Introduction recent updating of [4], more reasonable use of geotechnical
material terminology (clay, silt, sand, gravel, and boulders)
Large landslides and smaller-scale mass movements are is starting to spread, although some classical terminologies
natural widespread processes that result in the downward and (mud, debris, earthflow, peat, and ice) are maintained after a
outward movement of slope-forming materials, significantly recalibration of their definitions, because they have acquired
sculpting the landscape and redistributing sediment and a recognized status in landslide science by now. The Hungr
debris to gentler terrain. The rapid population growth and the classification includes aggregations of different materials that
pressure from human activities have strongly influenced their have been mixed by geomorphic processes such as weather-
extension and occurrence so that they have become disasters ing, mass wasting, glacier transport, explosive volcanism, or
causing vast direct and indirect socioeconomic consequences human activity. The use of geotechnical terminology is indeed
[1]. These deformations involve approximately all geological most useful, as it relates best to the mechanical behaviour
materials (natural rocks, soil, artificial fill, or combinations of the landslide as stated by [4] and even to most common
of these materials) and can occur and develop in a large investigation methods. In any case, the distinction between
variety of volumes and shapes [2]. Artificial fills are usually different materials is usually based on interpretation of the
composed of excavated, transported, and placed soil or rock, main geomorphic characteristics within landslide deposits
but they can also contain demolition debris, ash, slag, and but can also be inferred from the geological attributes of
solid trash. The term rock refers to hard or firm bedrock that the involved parent material. The type of material is one
was intact and in place prior to slope movement. Soil, either of the most important factors influencing the movement of
residual or transported material, is used for unconsolidated landslides, which can be categorized as falls, topples, spreads,
particles or poorly cemented rock or aggregates. Soil is slides, or flows according to their behaviour from the source
usually further distinguished on the basis of texture as debris area to the final deposit through distinctive kinematics [2, 3,
(coarse fragments) or earth (fine fragments) according to 5]. Actually, the most common criterion used in landslides
the well-established Varnes Classification [3]. Following the classification is based on the combination of the materials
2 International Journal of Geophysics
with the type of movement, but it is possible to find many measured variations, in fact, could be local anomalies within
other classification criteria, including velocities, volumes, the landslide or caused by the rough topography, and as a
water content, geotechnical parameters, and processes related result, they could be of no or little interest [12]. This is why
to the formation of the mobilized material, among others. according to [11], the references for landslide investigation
This is because, as stated by [5], engineering geology literature purposes are relatively few, and according to [13], there have
on landslides is affected by inconsistent terminology and been few landslides in which geophysical techniques were
ambiguous definitions from older classifications and current very useful. Nevertheless, the application of these techniques
key terms for both specialists and the public. Currently, the has changed over the years thanks to technological progress,
most widely accepted and used classification is that of [2], the availability of cheaper computer electronic parts, and
which enhances the previous system devised by D.J. Varnes the development of more portable and faster equipment and
[3, 6]. Since then, only small improvements for specific cate- new software for data processing [12], allowing the adequate
gories have occurred, such as that for flow-like landslides by investigation of 3D structures, which addresses one of the
[5]. In 2014 Hungr et al. [4], by maintaining the consolidated most ancient geophysical method limitations according to
concepts introduced by [2], redefined some basic elements [11].
(basically typology and material) that still refer to the original This review work, which starts from [11], is focused
characterization of [3] and, consequently, updated the total on the last twelve years of works (2007-2018) published in
amount of categories (from 29 to 32), along with revisiting international journals and available online. The main goal
some of their descriptions. This new landslides classification was to analyse the geophysical community efforts in over-
version (Table 1), which was proposed to simplify landslides coming the geophysical technique limitations highlighted in
studies, is increasingly circulating in the academic world, and the conclusion section of [11]. The drawbacks pointed out
for this reason, it is used as the reference in the present paper. were as follows: (i) geophysicists have to make an effort
Characterizing landslide material inhomogeneities and in the presentation of their results; (ii) the resolution and
their properties, studying the deformation processes, and penetration depth of each method are not systematically
delimiting boundaries and potential slip surfaces are not sim- discussed in an understandable way; (iii) the geological
ple goals. They require the availability of a wide range of data, interpretation of geophysical data should be more clearly and
observations, and measurements (e.g., kinematic, geomor- critically explained; (iv) the challenge for geophysicists is to
phologic, geological, geotechnical, and petro-physical data convince geologists and engineers that 3D and 4D geophysi-
[7]) and the evaluation of geologic and hydrologic conditions cal imaging techniques can be valuable tools for investigating
related to phenomena occurrences [8]. To obtain the needed and monitoring landslides; and finally, (v) efforts should
information, many techniques including both traditional also be made towards achieving quantitative information
methods (detailed geomorphological surveys, geotechnical from geophysics in terms of geotechnical parameters and
investigations, local instrumentation, and meteorological hydrological properties. To reach the aim, contrary to the
parameters analyses) and more recent methods (remote- four geophysics and landslide reviews discussed in section
sensing satellite data, aerial techniques, and synthetic aper- number 2 [8, 11, 12, 14] that analysed the advantages and
ture radar interferometry) can be employed [[9, 10] and limitations of each technique using a “technique approach,”
references within]. Among the latter, geophysical techniques the analysis in this paper was carried out on the basis
are also included, since they are very useful in detecting of a “material landslide approach” according to the recent
the petro-physical properties of the subsoil (e.g., seismic landslide classification discussed above [4]. Finally, since it is
wave velocity, electrical resistivity, dielectric permittivity, and beyond the aim of the work, we do not discuss the theoretical
gravitational acceleration [7]). Even though linking geophysi- principles of the different geophysical techniques nor how to
cal parameters and geological/geotechnical properties should perform field surveys in this paper.
always be supported with direct information (e.g., data
from drillings), geophysical methods can provide the layered 2. Geophysical Techniques and Landslides:
structure of the soil and certain mechanical parameters [11]. The State of the Art of Review Papers
Therefore, because almost all of the advantages of geophysical
methods correspond to disadvantages of geotechnical tech- One of the first papers related to the application of geophys-
niques and vice versa, the two investigation techniques can be ical techniques for the investigation of landslides, defined
considered complementary. Finally, the geophysical inversion as a pioneering work by [11], is [8]. Herein, “landslides” are
data, and, therefore, the creation of a reliable subsoil model, defined as a sudden or gradual rupture of rocks and their
is a complex and nonlinear problem that must be evaluated movement downslope by the force of gravity. In this paper,
by taking into account all the available data on the site [11]. the main advantages of applying geophysical methods are as
It is to be noted that the success of geophysical methods follows: (a) the rapid investigation of vast areas, collecting
is mostly dependent on the presence of a significant and a larger number of sample points than those acquired by
detectable contrast in the physical properties of different geologic engineering techniques; (b) the determination of
lithological units. However, in landslide characterization, the mechanical properties of wet and dry soils based on the
geophysical contrast (i.e., differences in mechanical and phys- measurements of large rock volumes directly involved in the
ical properties) cannot be associated only with a boundary processes; (c) the measured parameters reflect the combined
in mechanical properties (i.e., landslide boundaries) and geological and hydrological characteristics, which sometimes
therefore be of interest relative to the slope stability. These cannot be identified separately; and (d) the measurements
International Journal of Geophysics 3
Table 1: Nomenclature of the newly proposed landslide classification version according to [4] based on the Varnes classification system.
Words divided by / (slash symbol) have to be used alternatively. In italic movement types that usually reach extremely rapid velocities as
defined by [2], while for the others, the velocity varies between extremely slow to very rapid (for details, refer to [4]).
TYPE OF
ROCK SOIL
MOVEMENT
Fall Rock/ice fall Boulder/debris/silt fall
Rock block topple
Topple Gravel/sand/silt topple
Rock flexural topple
Rock rotational slide Clay/silt rotational slide
Rock planar slide Clay/silt planar slide
Slide Rock wedge slide Gravel/sand/debris slide
Rock compound slide
Clay/silt compound slide
Rock irregular slide
Sand/silt liquefaction spread
Spread Rock slope spread
Sensitive clay spread
Sand/silt/debris dry flow
Sand/silt/debris flowslide
Sensitive clay flowslide
Debris flow
Flow Rock/ice avalanche Mud flow
Debris flood
Debris avalanche
Earthflow
Peat flow
Mountain slope
Soil slope deformation
deformation
Slope Deformation
Soil creep
Rock slope deformation
Solifluction
can be repeated any number of times without disturbing the methods, (iii) the calibration of the acquired data by means
environment. Four main goals can be reached by applying of geological/geotechnical data, and finally, (iv) the signal-
vertical electric sounding (VES), seismic refraction (SR), to-noise ratio. In the paper, several case studies are shown
self-potential (SP), and electromagnetic measurements (EM), wherein the SR was successfully employed to determine the
listed as follows: (i) the investigation of the landslide geo- lower landslide boundary.
logic configuration, (ii) the investigation of the groundwater Ten years later, the SR, seismic reflection (SRe), electrical
(determining the level and its fluctuation with time) as a land- resistivity (ER), SP, EM, and gravimetry were discussed by
slide formation factor, (iii) the study of the physical properties [12] as the most frequently used methods in landslide charac-
and status of the landslide deposits and their changes with terization. For each method, the author gives (i) the theoreti-
time, and (iv) the investigation of the landslide displacement cal principles, (ii) how to perform the measurements, (iii) the
process. Reference [8] also showed how electrical resistivity sources for those which are active techniques, and, finally, (iv)
values and seismic waves velocities decrease between the some expected results. Moreover, he presents some summary
bedrock and the rocks in the landslide body. Finally, in the tables with the physical property ranges (e.g., those of the P-
conclusion section of [8], microseismic noise (SN) analysis is wave velocity, density, and electrical resistivity) of the most
mentioned as a valuable method by which to characterize the common soil and rock masses in their crude form (without
slope soil strata. taking into account variations caused by different clay con-
Reference [14] conducted a review of the geophysical tents, weathering, saturation, etc.). Finally, for each discussed
methods employed in landslide investigations. They high- method, [12] synthesizes in one table its suitability for use
lighted that the selection of the method/s to be applied in landslide characterization, human artefact (like pipes and
depends on its/their suitability for solving the problem. To foundations) identification, and physical properties determi-
estimate this adequacy, there are four main control factors: nation for geotechnical purposes. Overall, the SP method
(i) the definition/understanding of the geophysical contrasts results are not or only marginally suitable in all fields. Never-
that have to be investigated, (ii) the evaluation of the charac- theless, in the same year, [15] and, later, [16–18] showed how
teristics (penetration depth and resolution) of the geophysical the SP method could be helpfully employed. From the table
4 International Journal of Geophysics
in [12], the seismic tomography and 2D and 3D geo-electric 3. Geophysical Techniques and Landslides:
results correspond to the best methods for use in landslide A (Landslide Approach) Analysis
characterization.
Reference [11] presents the state of the art of the geophys- As mentioned in Introduction, this review work is based
ical techniques applied in landslide characterization based on a “material landslide approach” analysis on the basis of
on papers after 1990. According to this review, the methods the more recent landslide classification presented by [4] and
could be divided into seldom, widely, and increasingly used discussed in Introduction. Even though this classification is
categories. Among the first methods they enumerate are not widely employed (only 20% of the analysed papers from
SRe, ground penetrating radar (GPR), and gravimetry, while the years 2015-2018 adopted it, and these papers are marked
among the second group are SR, ER VES, or tomographies with # in Tables 2 and 3), we decided to use it considering
(ERT), and SP, and, finally, among the third group are SN, that the same landslide could assume different names from
surface waves (SW), and EM. Moreover, they indicate seismic paper to paper, though the authors could be more or less the
tomography (ST) as method useful only for limited site con- same. Among the analysed papers, examples are the Super
ditions (rock slides). They synthetize in a table (a) the main Sauze landslide and the La Vallette landslides (marked in
geophysical methods used, (b) the measured geophysical Table 2 with (∘ ) and (∘∘ ), respectively) or the Randa landslide
parameters and information type, (c) the geological context, (marked with (∘ ) in Table 3). This means that the analysed
(d) the landslide classification following [2], (e) the geomor- works are clustered and discussed in two groups, “soil” and
phology, and (f) the applications (targets). According to the “rock,” respectively, on the basis of the material landslide type
review in [11], there are three main advantages and three main (columns 2 and 3 of Table 1).
limitations in employing geophysics for the subsurface map- Moreover, we decided to analyse the works starting from
ping of landslides. As benefits of the geophysical methods, the 2007 because the review in [20] is focused only on the
author enumerates (i) the flexibility and the relative efficiency ERT technique application; nevertheless, we do not analyse
on slopes; (ii) the noninvasiveness and the generation of in detail all references already discussed therein, but we
information on the internal structures of soil or rock masses; synthetize the results. The results of the review analysis are
and (iii) the allowance of examining large volumes of soil. summarized in Tables 2 and 3, where for each work, we
As drawbacks, he highlights that (i) the resolution, which is specify: (a) the landslide typology according the authors of
dependent on the signal-to-noise ratio, decreases with depth; the paper (i.e., how they refer to the landslide in the text) and
(ii) the solution for a set of data is nonunique, and the results (b) according to the classification from [4] (where possible,
must be calibrated; and (iii) these methods yield indirect since sometimes it is not easy to identify the landslide classes
information on the subsoil, such as physical parameters from [4] on the basis of only the text); (c) the materials
rather than geological or geotechnical properties. One of the involved in the landslides; (d) which geophysical methods
main conclusions of the review is that in landslide char- and (e) which other traditional techniques were employed;
acterization, the geophysical survey design is still a much- and (f)-(l) how many efforts were performed to overcome the
debated question, and no unique strategy has arisen from the five drawbacks highlighted by [11] and listed in Introduction.
literature. To quantify these efforts, a three-level scale was employed,
where +, -, and n.d. mean, respectively, that many/some,
Reference [11] is the last review published in an interna-
insufficient, and nondiscussed efforts were made to overcome
tional journal and available online that focused on the advan-
the limitations. Unfortunately, we know that the evaluation
tages and limitations of the geophysical methods applied in
of how many efforts were performed could seem subjective.
landslides characterization. Reference [19], in fact, discusses, Therefore, in Table 4, for each drawback, we summarize how
by means of case studies, benefits and drawbacks of the we evaluated the efforts.
most common geophysical techniques (GPR, ER, and SR)
in geomorphological applications. Therefore, in this paper 3.1. “Soil” Landslides. “Soil” landslides, with respect to “rock”
landslides are just one of the possible fields of application. landslides, are the typology most studied with geophysical
Two more recent reviews about geophysics and landslides techniques. Among the 120 analysed papers, more than
are [20, 21]. The first is focused only on the ERT tech- half (e.g., 66 papers, which means 75 landslides analysed
nique applied in landslide investigations and analyses the without considering those reported in [20]) were about “soil”
advantages and limitations of 2D-, 3D-, and 4D-ERT (or landslides, and among them, more than half were on the
time-lapse ERT: tl-ERT) surveys based on papers of the flow type. As summarized in Table 5, in fact, no one was
period from 2000 to 2013. The second is a review of the focused on falls, topples, or spreads, while 28 landslides
current state of the art and the future prospects of the (the 37.3%) were analysed focused on the slide (6 clay/silt
near surface geophysical characterization of areas prone to rotational slides, 8 clay/silt planar slides, 11 rotational and
natural hazards (e.g., landslides, rockfalls, avalanches and planar slides, 1 debris slide, and 2 clay/silt compound slides),
rock glaciers, floods, sinkholes and subsidences, earthquakes, 41 (the 54.6%) on the flows (5 sensitive clay flowslides, 9
and volcanos) published in a book series (and, therefore, debris flows, 5 mud flows, and 22 earthflows), and 6 (the 8.1%)
not freely available online for download), wherein the anal- on the slope deformations (soil slope deformation). Only two
ysis of the geophysical techniques applied in landslides of the analysed landslides were marine landslides [33, 35],
characterization is limited to subsections of the case study indicating that it is not easy to conduct geophysical surveys
section. to characterize landslides that dive into the sea. It is also
Table 2: This table summarizes the analysed scientific papers from the last twelve years (2007-2018) focused on “soil landslide”. The landslide typology and materials are defined as in
the papers themselves. Moreover, where possible, we added the landslide classification according to [4]. Papers marked with # already adopted this classification. Papers marked with (∘ )
and (∘∘ ) focus on the Super Sauze and La Vallette landslides, respectively. Drawbacks 1 to 5 are the limitations of the geophysical techniques applied to landslide characterizations pointed
out by [11]. They are, respectively, as follows: Drawback 1: geophysicists have to make an effort in the presentation of their results; Drawback 2: the resolution and the penetration depth
of each method are not systematically discussed in an understandable way; Drawback 3: the geological interpretation of geophysical data should be more clearly and critically explained;
Drawback 4: the challenge for geophysicists is to convince geologists and engineers that 3D and 4D geophysical imaging techniques can be valuable tools for investigating and monitoring
landslides; and Drawback 5: efforts should also be made towards obtaining quantitative information from geophysics in terms of geotechnical parameters and hydrological properties. +, -,
and n.d. mean, respectively, that many/some, insufficient, and non-discussed efforts were made to overcome the limitations (see Table 4), while +∗ means that the interpretation is linked to a
numerical model or landslide feature identification. VES: vertical electrical sounding, ERT: electrical resistivity tomography, IP/SIP: induced polarization/spectral induced polarization, SP:
International Journal of Geophysics
self-potential, SPT: self-potential tomography, F/TDEM: frequency/time domain electro magnetism, VLF-EM: very low-frequency electro magnetism, EM: electro magnetism, RMT: radio
magnetotelluric, AE: acoustic emission, SN: seismic noise, SR: seismic refraction, SRe: seismic reflection, SW: surface waves, DH: downhole, CH: crosshole, GPR: ground penetrating radar,
MG: micro gravity, DTM: digital terrain model, SAR: synthetic aperture radar, GB-InSAR: ground based interferometric SAR, GPS: global position system, TS: total station, TLS: terrestrial
laser scanners, CPT: cone penetration test, TDR: time domain reflectometry.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
GPS, geodetic and
2007 [22] (∘ ) mudslide earthflow clay formation SN (local) strain instrument, - n.d. n.d. n.d. n.d.
piezometer
earthflow (the oldest
clay body over SAR data,
movement), clay ERT, SR, SW,
2007 [23] earthflow mudstone-shale inclinometers, + + - n.d. n.d.
planar slide SN (local), DH
basement stratigraphy
(reactivations)
mudstone,
colluvium,
2007 [24] / / SN (regional) / - n.d. - n.d. +
limestone and
carbonate breccia
intra-material SR, ERT, SW,
2007 [25] (∘ ) earthflow clay formation boreholes + - + n.d. +
mudslide SN (local)
rock and debris rock avalanche and
2007 [26] / SN (local) / - n.d. +∗ n.d. n.d.
flows debris flows
(a) geology,
(a) soft-rock
geomorphology,
(mudslide or clayey (a) earthflow
SN (local), geotechnics and
2007 [27] (∘ ) flow-like landslide) (b) clay planar clay formation + + - - n.d.
ERT hydrology
(b) translational landslide
(b) geomorphology
landslide
and geotechnics
ERT, SR, SN
2008 [28] earthflow earthflow loess DEM - n.d. + n.d. +
(local)
5
6
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
mudstone,
colluvium,
2008 [29] / / SN (local) / - n.d. - n.d. +
limestone and
carbonate breccia
metasediments,
2009 [30] debris flow debris flow SN (local) meteorological data + n.d. +∗ n.d. n.d.
gneisses, quartzites
flysch, calcareous
2009 [31] earthflow earthflow SRe boreholes, DEM - n.d. + n.d. n.d.
Alps
coarse components
2009 [32] earthflow earthflow SR, SRe boreholes - n.d. n.d. n.d. n.d.
in silty-clayey matrix
clay rotational and
2009 [33] / / SRe / - - - n.d n.d.
planar slide
sandy-clay piezometer, field
2010 [34] translational clay planar slide marly limestone ERT observations, - n.d. + n.d./+ +
landslide boreholes
marine and boreholes, CPT,
clay rotational slide, seismic
translational slope glaciomarine clay geotechnical test,
2010 [35] sensitive clay sub-bottom + n.d. + n.d. n.d.
landslide (Fjord-deltaic trenches,
flowslide profile
sediments) bathymetric data
clay/silt rotational glaciolacustrine inclinometers,
2010 [36] deep landslide SN (local) + n.d. + +/n.d. n.d.
slide, mud flow clays boreholes, GPS
2010 [37] debris slide debris slide schist and gneiss VLF-EM, VES boreholes, TS - n.d. - n.d. n.d.
composite multiple
mudstone,
2010 [38] earth slide – earth earth flow ERT GPS + + - n.d./+ n.d.
sandstone
flow
deep-seated soil slope coarse colluvium
2011 [39] SN (local) / - n.d. + n.d. +
landslide deformation over mudstone
International Journal of Geophysics
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
meteorological data,
2011 [40] / clay compound slide sandy, silty clay SN (local) extensometers, SR, - n.d. + n.d. -
International Journal of Geophysics
robotic TS
geometrical model,
2012 [41] clayey landslide earthflow black marls SR + - + n.d. n.d.
DEM
2012 [42] many / black marl ERT, SR, SW / - n.d. + n.d. +
rotational and
intra-material
2012 [43] (∘∘ ) planar slide, mud clay-shale deposits ERT, SR, SW / - - + n.d. +
landslide
flow
2012 [44] debris flow debris flow / SN (regional) / - n.d. +∗ n.d./- n.d.
complex
retrogressive rotational and TDR, weather
2012 [45] schist, flysch ERT + n.d. - n.d./+ -
rot-translational planar slide station
slide
rotational and
2012 [46] (∘∘ ) flow-like landslide planar slide, mud black marls 3D-SR boreholes, SR, ERT + n.d. + +/n.d. n.d.
flow
flow-like landslides SR, ERT, GPR,
2012 [7] (∘ ) earthflow / many + + + n.d n.d.
(mudslides) EM, MG
deep-seated soil slope
2012 [47] / SN (regional) / - n.d. +∗ n.d. n.d.
landslide deformation
sensitive clay resistivity CPT,
2013 [48] quick clay landslide clay ERT, IP - n.d. + n.d n.d.
flow-like geotechnical test
clay-rich formation
deep-seated soil slope
2013 [49] and colluvial SN (local) / - - + n.d. n.d.
landslide deformation
deposits
silty sand,
planar
2013 [50] shallow landslide marlstone, ERT TRD, tensiometer - - + n.d./+ +
slide-earthflow
sandstone
7
8
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
2D and 3D SR,
SRe, SN
boreholes, CPT,
earth flow (quick sensitive clay (local), 2D and
2013 [51] quick clay LiDAR, geotechnical - n.d. + -/n.d. n.d.
clay landslide) flowslide 3D ERT, EM,
test
GPR, RMT,
MG
boreholes, CPT,
earth flow (quick sensitive clay
2013 [52] quick clay SRe LiDAR, geotechnical - n.d. + -/n.d. n.d.
clay landslide) flowslide
test, ERT, IP
gravel, sand, and
plastic and
clay/silt rotational non-plastic fines
2013 [53] / ERT, IP, MG geotechnical analysis - n.d. - n.d. n.d.
slide from quartzite,
phyllite, slate, and
limestone
boreholes,
geotechnical
surficial failures
clayey sand, silty analysis,
2013 [54] along a planar clay/silt planar slide ERT - n.d. - n.d./- +
sand, sandstone meteorological
soil/rock interface
station, TDR.
piezometer
(a) black marls, (b)
2013 [55] muddy landslide debris flow, rock fall SN (local) / - n.d. +∗ n.d. n.d.
flysch and clay-shale
mudslide and LiDAR, TLS, field
2013 [9] rotational/planar planar slide black marls, flysch SR investigations, GPS, + n.d. + n.d. n.d.
slide (∘∘ ) DEM
deep-seated soil slope
2013 [56] / SN (local) LiDAR - n.d. +∗ n.d. n.d.
landslide deformation
clay sandy 2D and 3D
2014 [57] transitional slide clay planar slide calcarenitic ERT, SR, SN boreholes + n.d. + +/n.d. n.d.
succession (local), SW
2014 [58] / / / SN (local) / - n.d. n.d. n.d. +
field investigation,
composite rotational aerial
clay rotational
2014 [59] landslide (complex marls, chalks ERT, SR orthophotographs, + n.d. + n.d. n.d.
landslide
landslide) LiDAR, boreholes,
DEM
International Journal of Geophysics
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
complex, composite,
boreholes, LiDAR,
successive clay/silt rotational
2014 [60] mudstone ERT aerial photos, DEM, - n.d. + +/n.d. -
earth-slide earth slide, earthflow
GPS, inclinometer
flow
ERT (ERT
International Journal of Geophysics
2014 [20] many / many technique many + - - +/+ n.d.
review paper)
deep-seated
boulders, gravel and
gravitational clay/silt rotational
coarse sand over inclinometers,
2014 [61] deformation and planar slide, SRe, ERT + n.d. - n.d. n.d.
argillites and boreholes, SR
dismantled into earthflow, creep
limestone
slides and flows
inclinometer, 3D
(a) and (b) displacement
(a) and (b) clay (a) colluvium, (b)
rotational- measures,
2014 [62] rotational/planar colluvium and ERT + + + n.d./+ n.d.
translational piezometric water
slide amphibolite
landslide level, soil
temperature
LiDAR, GIS,
glacial till, outwash
2015 [63] (#) debris flow debris flow SN (regional) numerical + n.d. +∗ n.d. n.d.
sediments
modelling
2015 [64] debris flow debris flow SN (regional) / - n.d. +∗ n.d. n.d.
sandstone and
2015 [65] earth-flow earthflow ERT GPS + n.d. n.d. + n.d.
limestone
marl, claystone,
2015 [66] earthflow earthflow mudstone, alluvium, ERT, SP, / - n.d. + n.d. n.d.
limestone
GPS, aerial photo,
boreholes, auger and
penetration,
shallow clay planar plastic blue marls,
2016 [67] (#) clay planar slide ERT geotechnical + - + n.d. n.d.
slide limestone, blue clays
analysis, rainfall and
groundwater level
data
2016 [68] (∘ ) clayey landslide earthflow black marls ERT / + + + n.d./+ -
9
10
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
clayey silt, mild clay
and gravelly sand,
shallow creeping soil slope covered by grass
2016 [69] ERT, GPR boreholes - n.d. - n.d. n.d.
landslide deformation peat and turf.
(bedrock: mudstone,
sandstone)
sandstone, claystone,
gamma-ray, soil
siltstone, volcanic ERT, EM, SP,
2016 [70] (#) debris flow debris flow Radon, boreholes, - n.d. - n.d. n.d.
rocks, Quaternary MG
GPS
sediments
boreholes,
silty clay with
inclinometers,
translational gravels over
2016 [71] clay/silt planar slide ERT piezometers, + n.d. + n.d./- n.d.
landslide sandstone and
pluviometer,
mudstone
osmometer
LiDAR, geotechnical
sensitive clay FDEM, ERT,
2016 [72] quick-clay landslide quick clay test, resistivity-CPT, + n.d. + n.d. n.d.
flowslide SR, SW
boreholes
clay/silt rotational sandy and clayey
2016 [73] loess landslide ERT / - - + n.d. +
slide loess
multiple earth mudstone, 3D-ERT,
2016 [74] (#) earth flow SR + - + n.d. -
slide-earth flow sandstone geotechnical analysis
SW, SN (local),
2016 [75] complex landslide clay compound slide clays, sands, gravels borehole + - + n.d. n.d.
VES
(a) clay/silt
rotational and
(a), (b) complex planar slide
boreholes,
2017 [10] roto-translational (b) clay/silt limestone SN (local) + n.d. - +/n.d. n.d.
inclinometers,
landslide rotational and
planar slide,
earthflow
schists, dolobreccias,
2017 [76] debris flow debris flow SN (local) flow stage sensors - n.d. +∗ n.d. n.d.
quartzites
boreholes,
sandstone,
Sackung-like soil slope ERT, SR, SN geotechnical
2018 [77] colluvium + n.d. + n.d. n.d.
movement deformation (local) analysis, DEM,
Quaternary deposits
remote images, GPS
International Journal of GeophysicsInternational Journal of Geophysics
Table 2: Continued.
Landslide typology
Landslide typology Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference (as defined by the Material Other available data
(according [4]) Technique/s 1 2 3 4 5
author/s)
LiDAR, GPS,
rotational and
boreholes
2018 [78] flow-like landslide planar slide, mud sandstone, limestone IP, SIP + - + n.d. n.d.
geotechnical
flow
analysis, ERT, SR
roto-translational clay/silt rotational
flysch, metamorphic SN (local),
2018 [79] slide, earth slide and and planar slide, DTM, aerial images - n.d. + n.d. n.d.
rocks ERT
flows earthflow
boreholes,
sandstone, shale,
geotechnical
2018 [80] / earth flow (?) coal shale, marl, clay, ERT + - + n.d./+ +
analysis, SPT, field
silt
investigations
(a) mudflow, (b) (a) mud flow, (b) ultrasonic gauge,
2018 [81] / SN (local), AE + n.d. +∗ n.d./- n.d.
debris flow debris flow video cameras
clay/silt rotational clayey and sandy
2018 [82] loess landslide ERT pressure pore - n.d. + n.d. n.d.
slide loess
(a), (b) clay-rich
clay-rich matrix,
2018 [83] (#) debris slide (clayey (a), (b) earthflow SN (local) / + n.d. +∗ n.d. n.d.
marls and limestone
landslide)
1112
Table 3: This table summarizes the analysed scientific papers from the last twelve years (2007-2018) focused on “rock landslide”. Landslide typology and materials are defined as in the
papers themselves. Moreover, where possible, we added the landslide classification according to [4]. Papers marked with # already adopted this classification. Papers marked with (∘ ) focus
on the Randa landslides. Drawbacks 1 to 5 are the limitations of the geophysical techniques applied to landslide characterizations pointed out by [11]. They are, respectively, as follows
Drawback 1: geophysicists have to make an effort in the presentation of their results; Drawback 2: the resolution and penetration depth of each method are not systematically discussed in an
understandable way; Drawback 3: the geological interpretation of geophysical data should be more clearly and critically explained; Drawback 4: the challenge for geophysicists is to convince
geologists and engineers that 3D and 4D geophysical imaging techniques can be valuable tools for investigating and monitoring landslides; and Drawback 5: efforts should also be made
towards obtaining quantitative information from geophysics in terms of geotechnical parameters and hydrological properties. +, -, and n.d. mean, respectively, that many/some, insufficient,
and non-discussed efforts were made to overcome the limitations (see Table 4), while +∗ means that the interpretation is linked to a numerical model or landslide feature identification. VES:
vertical electrical sounding, ERT: electrical resistivity tomography, IP/SIP: induced polarization/spectral induced polarization, SP: self-potential, SPT: self-potential tomography, F/TDEM:
frequency/time domain electro magnetism, VLF-EM: very low-frequency electro magnetism, EM: electro magnetism, RMT: radio magnetotelluric, AE: acoustic emission, SN: seismic noise,
SR: seismic refraction, SRe: seismic reflection, SW: surface waves, DH: downhole, CH: crosshole, GPR: ground penetrating radar, MG: micro gravity, DTM: digital terrain model, SAR:
synthetic aperture radar, GB-InSAR: ground based interferometric SAR, GPS: global position system, TS: total station, TLS: terrestrial laser scanners, CPT: cone penetration test, TDR: time
domain reflectometry.
Landslide
Landslide
typology (as Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference typology Material Other available data
defined by the Technique/s 1 2 3 4 5
(according [4])
author/s)
Rockfall/debris rock fall, debris
2007 [84] basaltic SN (local) / - n.d. +∗ n.d. n.d.
flows flow
2007 [85] rock fall rock fall limestone GPR boreholes, mining - - + n.d. n.d.
rock and debris rock avalanche
2007 [26] / SN (local) / - n.d. +∗ n.d. n.d.
flows and debris flows
boreholes, GPR,
geodetic,
heterogeneous
2007 [86] (∘ ) rock fall rock fall SN (local), 3D-SRT geotechnical, + n.d. - +/n.d. n.d.
gneisses
meteorological
monitoring system
TLS,
2008 [87] rock fall rock fall limestone ERT, GPR + - +∗ n.d. n.d.
photogrammetry
rock fall and
rock fall and limestone,
2008 [88] rock-fall SN (regional) / - n.d. +∗ n.d. n.d.
rock avalanche amphibolite, granite
avalanches
2008 [89] rockfall rock fall lava dome SN (regional) / - n.d. +∗ n.d n.d.
boreholes,
rockslide (any geotechnical test,
rotational specific airborne EM, ERT, hydrophysical logs,
2008 [90] marl and schist - - + n.d/- n.d.
sliding typology geophysical logs, SP DEM, gamma ray
emerged) spectrometer,
inclinometers
International Journal of GeophysicsTable 3: Continued.
Landslide
Landslide
typology (as Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference typology Material Other available data
defined by the Technique/s 1 2 3 4 5
(according [4])
author/s)
International Journal of Geophysics
rockfall
2008 [91] rock fall / SN (local) videos, photos + n.d. +∗ n.d. -
(artificial)
heterogeneous boreholes, field
2008 [92] (∘ ) rockslide rock wedge slide SR, GPR, + - + -/n.d. n.d.
gneisses mapping
metalliferous
2009 [93] rockfall rock fall SN (local) TS, TLS + + -∗ n.d. n.d.
limestone
2009 [94] rock-fall rock fall chalk SN (local) + n.d. -∗ n.d. n.d.
2010 [95] rockfall rock fall othogneisses SN (local) thermometer - n.d. -∗ n.d. n.d.
rock block heterogeneous
2010 [96] rockslide SN (local) GPS + n.d. +∗ n.d. n.d.
toppling gneisses
rock compound meta-granodiorite GPS, meteorological
2010 [97] rockslide SN (local) - n.d. + n.d. n.d.
slide and two-mica gneiss data
rock slice rock rotational
2010 [98] limestone SN (local) extensometer - n.d. +∗ n.d. n.d.
collapse slide
meteorological
2010 [99] rockslide rock fall micaschists SN (local) + n.d. +∗ n.d. n.d.
station, GPS
limestone and marly
2010 [100] rock fall rock fall SN (local), SR extensometer + n.d. +∗ n.d. n.d.
limestone
rock-ice rock/ice
2010 [101] plutonic rocks SN (regional) DEM - n.d. +∗ n.d. n.d.
avalanche avalanche
deep-seated
rock slope
2010 [102] with several flysh and evaporites SR, SRe / + n.d + n.d. n.d.
deformation
debris flow
2011 [103] rockslide many many SN (regional) / - n.d. +∗ n.d. n.d.
2011 [104] rockfall rock fall volcanos SN (local) / - n.d. +∗ n.d. n.d.
2011 [105] rockslide rock fall micaschists SN (local) GPS, boreholes + + +∗ n.d. n.d.
2011 [106] rockfall rock fall limestone SN (local), SR - n.d. +∗ n.d n.d.
1314
Table 3: Continued.
Landslide
Landslide
typology (as Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference typology Material Other available data
defined by the Technique/s 1 2 3 4 5
(according [4])
author/s)
regional
rock block paragneiss and
2011 [107] rock slope SN (local) earthquakes, fibre + n.d. +∗ n.d. n.d.
toppling schists
optic strain sensors
radiocarbon and
dendrogeomorpho-
flysch (clay-
rockslide logical analysis,
2011 [108] rock planar slide stone/mudstone ERT + n.d. + n.d. n.d.
(long-runout) geomorphic
sequence)
mapping, GPS,
geotechnical analysis
deep-seated
field survey,
gravitational mountain slope
2011 [109] flysch ERT kinematic analysis, + - + n.d. n.d.
slope deformation
trenches
deformation
rock fall
2012 [110] (precondition rock fall granitic gneiss AE meteorological data - - +∗ n.d. n.d.
for)
rock fall/rock
2012 [111] / orthogneiss SN (local) / + n.d. +∗ n.d. n.d.
block topple (?)
2012 [112] rock-fall rock fall gneiss and gabbro SN (local) thermometers, GPS - n.d. +∗ n.d./- n.d.
rockfall and limestone, clay
2012 [113] rock fall SN (local) + n.d. + n.d. n.d.
lateral spreading formation
rockfall in the
extensometers,
2012 [114] source area of a rock fall black marls SN (regional) + n.d. +∗ n.d. n.d.
DEM
mudslide
meteorological and
flysch, marls and
2012 [115] rockfall rock fall SN (local) hydrological data, + n.d. +∗ n.d./- n.d.
limestone
local seismicity
International Journal of GeophysicsTable 3: Continued.
Landslide
Landslide
typology (as Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference typology Material Other available data
defined by the Technique/s 1 2 3 4 5
(according [4])
author/s)
rockslide-debris rock irregular rhyodacite breccias,
2013 [116] SN (regional) / - n.d. +∗ n.d. -
flow slide, debris flow tuff
International Journal of Geophysics
(a) argillites meteorological
(a), (b), and (d) (a), (b), and (d)
(b) and (d) station
toppling/basal rock block SN (local)
2013 [117] limestone (a) displacement - n.d. + n.d. n.d.
sliding, (c) rock topple, (c) rock (a) SR, (c) ST
(c) shale- sandstone measures, (b) and
compound slide compound slide
series (c) extensometers,
many: rock fall,
many: rockfall, debris avalanche meteorological data,
SN
2013 [118] debris (any specific soil many (not specified) satellite images, + n.d. +∗ n.d. n.d.
(regional/catchment)
avalanche, slides or rock aerial photos
emerged)
plan and mainly claystone
2013 [119] / VES DEM - n.d. - n.d. n.d.
toppling failures and siltstone
debris flow, rock (a) black marls, (b)
2013 [55] muddy landslide SN (local) / - n.d. +∗ n.d. n.d.
fall flysch and clay-shale
LiDAR,
photogrammetry,
2014 [120] rockfall / limestone SN (local) video cameras, + n.d. +∗ n.d. n.d.
extensometer,
tiltmeter
2014 [121] rockfall rock fall volcanos SN (local) / - n.d. +∗ n.d. n.d.
deep-seated mountain slope
2014 [122] flysch, sandstone ERT field survey + n.d. + n.d. n.d.
landslide deformation
argillaceous
materials
2015 [123] rockslide / ERT, VES, SR - n.d. + n.d. n.d.
intercalated with
mudstone/limestone
tilting and rock rock slope
2015 [124] (#) dolomite ERT TLS, GPS + n.d. + +/n.d n.d.
fall spread
1516
Table 3: Continued.
Landslide
Landslide
typology (as Geophysical Drawback Drawback Drawback Drawback Drawback
Year Reference typology Material Other available data
defined by the Technique/s 1 2 3 4 5
(according [4])
author/s)
geotechnical
analysis,
2016 [125] rockfall rock fall granite CH, SR crackmeters, - n.d. + n.d. +
temperature probes,
inclinometers, SN
21 landslides: 12
rock falls, 8 rock
21 landslides: 12
slides (any
rock falls, 8 rock
2016 [126] (#) specific many SN (local) / - n.d. +∗ n.d. n.d.
slides and 1 rock
typology
avalanche
emerged) and 1
rock
planar rockslide,
2016 [127] rockslide rock fall, debris dolomite SN (regional) / - n.d. +∗ n.d./- n.d.
avalanche
2017 [128] rockfall rock fall limestone SN (local) TLS + + +∗ n.d. n.d.
2017 [129] rockfall rock fall limestone SN (local) GPS, photo camera + n.d. +∗ n.d. n.d.
rockslide (any
specific
2017 [130] rock slide paragneiss SW, SN / - n.d. + n.d. n.d.
typology
emerged)
2017 [131] rockfall rock fall black marls SN (local) / - n.d. +∗ n.d. n.d.
deep-seated mountain slope sedimentary rocks
2017 [132] ERT, GPR, SR, MG field survey, GPS + - + n.d. n.d.
landslide deformation and flysch
weather station,
2018 [133] rockfall rock fall limestone SN (local) - n.d. +∗ n.d. n.d.
DEM
rock compound extensometers,
amphibolitic and
2018 [134] rock slide slide, rock fall, SN (local) meteorological + n.d. +∗ n.d. n.d.
augen gneiss
debris avalanche station, GPS
extensometer,
2018 [135] rockfall rock fall limestone SN (local) + n.d. - n.d./- n.d.
meteorological data
International Journal of GeophysicsInternational Journal of Geophysics 17
Table 4: For each drawback, this table explains how the three-level scale (+, -, and n.d., which mean that many/some, insufficient, and
non-discussed efforts were made to overcome the limitations) was applied.
+ - n.d.
(i) B&W figures
(i) Coloured figures
(ii) Non-interpreted
(ii) 3D figures
Drawback 1 figures /
(iii) Figures with
(iii) Figures too small
interpretations
(iv) Only raw data
There is wide discussion There are only some
There are no mentions of
about the technique/s mentions of the technique/s
Drawback 2 the technique/s penetration
penetration depth and/or penetration depth and/or
depth and/or resolution
resolution resolution
There is wide discussion There are only some There are no mentions of
about the geological mentions of the geological the geological
Drawback 3
interpretation of the interpretation of the interpretation of the
geophysical data geophysical data geophysical data
3D/4D data are presented
3D/4D data are presented No 3D/4D data are
Drawback 4 but they are not discussed
and discussed presented or discussed
in depth
There is wide discussion on There are only some There are no mentions of
how to link geophysical mentions of how to link how to link geophysical
Drawback 5 data with geotechnical geophysical data with data with geotechnical
and/or hydrological geotechnical and/or and/or hydrological
properties hydrological properties properties
Table 5: For each type of movement and “soil” landslide typology, the table summarizes how many papers are focused on it. In italic movement
types that usually reach extremely rapid velocities as defined by [2], while for the others, the velocity varies between extremely slow to very
rapid (for details, refer to [4]).
TYPE OF MOVEMENT Number of papers SOIL Number of papers
Fall / Boulder/debris/silt fall /
Topple / Gravel/sand/silt topple /
Clay/silt rotational slide 6
11
Clay/silt planar slide 8
Slide 28
Gravel/sand/debris slide 1
Clay/silt compound slide 2
Sand/silt liquefaction spread /
Spread /
Sensitive clay spread /
Sand/silt/debris dry flow /
Sand/silt/debris flowslide /
Sensitive clay flowslide 5
Debris flow 9
Flow 41 Mud flow 5
Debris flood /
Debris avalanche /
Earthflow 22
Peat flow /
Soil slope deformation 6
Slope Deformation 6 Soil creep /
Solifluction /
important to point out that in our analysis, we do not consider applied to landslides, concerning either how to formulate the
papers focused on the geophysical characterization of quick- inversion problem [41, 46, 52, 55, 68, 83] or how to combine
clay that could evolve into a sensitive clay flowslide but only data from different surveys [7, 42]. All the other papers deal
papers focused on those that already occurred [35, 51, 52, 72]. with the discussion of a case study.
In only 8 works (12.1% of the analysed “soil” landslide A detailed analysis of the applied techniques is discussed
works), it is possible to find a detailed discussion of the theory in Section 4. Below, we present only the main considerations18 International Journal of Geophysics
from some papers. ERT is an active geophysical method that not listed in Table 3 because it was already analysed by [20]].
can provide both 2D and 3D images of the subsoil. A wide In [137], there is another limitation in applying geophysics
review of this technique applied to landslides is provided for rock deposits: the presence of a shallow geophysical
in [20]. Therefore, here, we limit discussion to saying that contrast caused by the subsoil water table that could mask
in most papers (29 of 33 that present ERT applications, i.e., deeper interfaces. Nevertheless, this limitation also has to be
88.0%), 2D ERTs are shown, while only in 6.0% (2 papers of considered for “soil” landslides.
33), 3D ERTs are shown, and in the remaining 6.0% (2 papers More recently, to overcome these limitations, rock slope
of 33), both 3D and 2D applications are presented. stability characterization and monitoring has been carried
Since the ‘60s, passive seismic techniques have been out using passive seismic techniques (see also the discussion
developed to monitor and characterize signals triggered session), implemented initially in open-mine monitoring
by landslide dynamics and related changes in the material [98]. These techniques, in fact, could help in (i) understand-
mechanical properties (i.e., (i) material bending, shearing, or ing the seismic responses of rock to slope deformation (e.g.,
compression; (ii) fissure opening; (iii) slipping at the bedrock the release of stored elastic energy under particular condi-
interface; and (iv) debris flows or mudslides) [22, 55]. They tions) [135, 138], (ii) detecting and locating microearthquakes
are of great interest in (a) detecting debris flows [30], (b) generated by fracturing within unstable rock masses (major
assessing site effects [24, 29], (c) detecting landslide slip sur- effort is required for classifying seismic signals and extracting
faces [10], and (d) estimating the thickness of a material that those related to landslides [86, 99, 129]), and (iii) identifying
could be mobilized by a landslide [136]. Another advantage remote events that could otherwise go unnoticed for weeks or
of this method is its ability to detect remote events that months. Therefore, these methods are applied to avalanches
might otherwise go unnoticed for weeks or months. The main [26, 84, 101, 126], rock topplings [107, 111, 117, 134], rockslides
difficulties arise from two issues: (i) the seismic signatures [55, 96–99, 103, 116, 126, 127, 130], and rock falls or cliff failures
of landslides and mud/debris flows are very complex and [86, 88, 89, 91, 93–95, 100, 104–106, 112–115, 118, 120, 121, 126,
cannot be effectively identified without a detailed waveform 128, 131, 133]. Finally, some works are focused on finding
analysis and (ii) the epicentres of landslides and mud/debris the relation among “rock” landslides, displacement rate mea-
flows cannot be confidently determined by conventional surements, and meteorological (i.e., rain and temperature)
earthquake-locating methods, mainly due to the lack of clear parameters [95, 99, 100].
arrivals of P and S phases [44].
4. Discussion
3.2. “Rock” Landslides. Among the 120 analysed papers, less
than half (e.g., 54) were about “rock” landslides, and the Most studies focused on geophysical surveys are applied
majority discussed were of the rock fall type. As summarized (a) to explore the subsoil for mineral deposits or fossil
in Table 6 the landslide typology is divided as follows: 41 (the fuels, (b) to find underground water supplies, (c) for engi-
54.6%) falls, 5 (the 6.7%) topples (5 block topples), 18 (the neering purposes, and (d) for archaeological investigations
24.0%) slides (1 rotational, 2 planar, 1 wedge, 3 compound, 1 [19]. Technological progress and the availability of cheaper
irregular), 1 (the 1.3%) spread (rock slope spread), 6 (the 8.0%) computer electronic parts has allowed the improvement of
flows (avalanches), and 4 (the 5.4%) slope deformations (3 more portable equipment and the development of 2D and 3D
mountain slope deformations and 1 rock slope deformation). geophysical techniques [11, 12]. Therefore, the applicability of
In all the works that discuss the application of seismic geophysical methods in landslide characterization has grown
techniques [26, 55, 84, 86, 87, 89, 91, 93–101, 103–107, 111– over the years. Starting from the state of the art of the
118, 120, 121, 126–128, 130, 131, 133, 134], it is possible to find a geophysical techniques applied in landslide characterizations
more- or less-detailed discussion on the theory of the seismic pointed out in [12], this review focused on the papers from
wave analysis carried out to find the “rock” landslide features. the last twelve (2007-2018) years and tried to understand how
“Rock” landslides are well-known phenomena but are many efforts have been made by the international scientific
poorly understood. Contrary to other landslide types, rock- community to overcome the drawbacks. These geophysical
falls are usually sudden phenomena with few apparent pre- techniques limitations are listed in Introduction. To reach
cursory patterns observed prior to the collapse. A key point the goal of this paper, contrary to the four reviews discussed
in the prediction of rock slope failure is better knowledge in Section 2 [8, 11, 12, 14], the analyses of the geophysical
of the internal structure (e.g., the persistence of joints), method advantages and limitation were carried out on the
which requires an interdisciplinary research field among basis of the latest landslide classification, which is mainly
rock mechanics, rock engineering, and mining [98]. This based on the involved materials and geotechnical properties
is why in 64.8% of the analysed papers, the geophysical [4]. Therefore, the 120 analysed papers were divided into two
technique is carried out along with more traditional methods classes: “soil” (in red in the following figures) and “rock” (in
(i.e., boreholes, mining, extensometers, and inclinometers). green in the following figures), which account for 66 and 54
Moreover, there are at least two limitations in applying works, respectively.
geophysical methods for rock deposits: (a) the difficulty of Even though it is well known that it is better to integrate
deploying sensors (i.e., ER electrodes, geophones, or GPR more than one geophysical technique because of the intrinsic
antennas) on sharp and blocky ground with a high void ratio limitations of each approach, in 68.3% of the analysed
and (b) the low geophysical contrast between the rock deposit papers (Figure 1), only one geophysical method is presented
and the underlying layers with comparable properties [[137], and discussed. However, in 64.6% of these works (whichInternational Journal of Geophysics 19
Table 6: For each movement type and “rock” landslide typology, the table summarizes how many papers are focused on it. In italic movement
types that usually reach extremely rapid velocities as defined by [2], while for the others, the velocity varies between extremely slow to very
rapid (for details, refer to [4]).
TYPE OF MOVEMENT Number of papers ROCK Number of papers
Fall 41 Rock/ice fall 40
Rock block topple 5
Topple 5
Rock flexural topple /
Rock rotational slide 1
Rock planar slide 2
Slide 18 Rock wedge slide 1
Rock compound slide 3
Rock irregular slide 1
Spread 1 Rock slope spread 1
Flow 6 Rock/ice avalanche 6
Mountain slope deformation 3
Slope Deformation 4
Rock slope deformation 1
correspond to 44.1% of the total analysed papers, as indicated 90
68.3%
by the bottom/darker part of the blue bar in Figure 1), the 80
geophysical results are interpreted on the basis of other 70
techniques. This means that only in 24.2% of the analysed 60
works (the top/lighter part of the blue bar in Figure 1) is 50 77.8%
60.6%
just one technique presented, and in 80% of these 24.2% 40 31.7%
(which means four works out of five), the employed method 30 39.4%
is a passive seismic technique. This is probably because these 20
22.2%
techniques (a) require quite light equipment, (b) can be 10
employed to both monitor and characterize seismic signals 0
One technique More than one technique
triggered by landslide dynamics [55, 133, 134], and (c) can
SOIL ROCK Total
be useful for overcoming the unpredictable occurrence of
rockfalls [128], even though it is not easy to correlate seismic Figure 1: For each landslide typology (“soil” in red, “rock” in green,
signal features with landslide geological properties [120, 134]. and total in blue), the bar graph shows the number of papers focused
In general, active and passive seismic methods are the on just one technique or on more than one. Numbers on the top of
most employed in landslide characterization and monitoring the bars are the percentage values with respect to the total number of
(Figure 2). In “soil” landslides, the three most employed analysed papers. The darker colours of the “soil” and “rock” bars of
the “one-technique” group indicate in how many works the passive
techniques are ERT, SN (at local and regional scales), and
seismic technique was employed alone. The dark blue portion of
SR. The last, together with SRe and SW, is largely used in
the “one-technique total bar” indicates in how many works other
this kind of landslide typology, and in general, it is easier nongeophysical techniques were employed.
to find papers focused on “soil” landslides that integrate the
abovementioned seismic techniques with other less-common
techniques (e.g., MG, IP, SP, and EM). Our analysis of “soil” In Figure 3, for each drawback, the percentages and the
landslides confirms the conclusions of [20]; i.e., (a) ERT and numbers of papers (numbers on the top of the bars) that fall
SR integration proves to be the most effective, (b) the joint into each level of the three-level scale (+, -, and n.d., which
application of ERT, SR, and GPR seems to solve and overcome mean that many/some, insufficient, and nondiscussed efforts
the resolution problems of each single method, and (c) in the were made to overcome the limitations, as shown in Table 4)
literature, there are very few examples of ERT combined with are summarized. In general, it is possible to observe that
IP to distinguish clayey material or to better interpret ERT. great efforts were made (95 papers out of the 120 analysed,
In “rock” landslides, the three most employed techniques are which is 79.1%, are on the + level of the scale) to improve
SN (at local and regional scales), ERT, and SR, indicating that the geological interpretation of the geophysical data and to
passive seismic techniques are preferred over electrical ones. explain it more clearly and critically (drawback 3). In contrast,
As mentioned above, this is probably because they can be very few efforts were made to (a) systematically discuss,
employed to both monitor and characterize seismic signals in an understandable way, the resolution and penetration
triggered by landslide dynamics [55, 133, 134]. At the fourth depth of each method (drawback 2: 91 papers out of the
position is GPR, although the authors highlight both the 120 analysed, which are 75.8%, are on the n.d. level of the
difficulty of deployment on cliffs and the limitation of its scale), (b) to convince geologists and engineers that 3D
applicability to only highly resistive rock slopes [87, 88, 92, and 4D geophysical imaging techniques can be valuable
132]. tools for investigating and monitoring landslides (drawbackYou can also read
