Geophysical Investigations To Deal With Uncertainties in Difficult Ground Conditions
GEOPHYSICAL INVESTIGATIONS TO DEAL WITH UNCERTAINTIES IN DIFFICULT GROUND CONDITIONS
Dr. Sanjay Rana
Director, PARSAN Overseas Pvt. Ltd., New Delhi, India
For planning and development of any major civil project adequate knowledge of geotechnical conditions at site is important. If not adequately investigated, geological 'surprises' can lead to failures and accidents during execution. Engineering geophysics is an efficient means of subsurface investigation. The continuous information provided by geophysical methods, as against discrete point information provided by conventional means like drilling, makes project execution safer & faster. Technological advancements and development of portable digital data acquisition instrument systems have increased their versatility. A combination of two or more geophysical methods helps in overcoming limitations of individual methods and evaluating underground conditions in a holistic way. Present paper discusses various geophysical techniques to provide unique solutions to subsurface challenges.
Sub-surface imaging by means of geophysical survey is a powerful tool for site assessment and mapping which historically has been under-utilized world-over. Continuing improvements in survey equipment performance and automation have made large area surveys with a high data sample density possible. Advances in processing and imaging software have made it possible to detect, display, and interpret small geological features with great accuracy. Some of the unique advantages of geophysical survey are:
- Geophysical methods are quick to apply, saving time and money.
- Light and portable equipment allows access to remotest of sites.
- Provides information on critical geological features like faults/ fractures/ weak zones/ shear zones, not visible from surface observation.
- Researchers can assess site conditions, and target specific locations for detailed investigations by drilling, while avoiding others.
- Quicker interpretation of subsurface geology avoiding delays during execution due to meeting the unexpected.
- Shear wave profiles can be quickly obtained for ascertaining liquefaction potential and earthquake response.
- Buried utilities, pipes and cables, can be detected before drilling/ excavation, avoiding damage and costly accidents.
- Concrete structures can be quickly scanned to ascertain integrity and detect defects like voids, honeycombing etc.
A detailed survey plan is worked out in consultation with the client, to address critical issues. The right combination of various available tools should be chosen to resolve the problem considering site conditions. Surveys which are both successful and cost-effective must satisfy a number of basic requirements. They must be implemented using appropriate and properly configured survey equipment. The data sampling strategy and density must be matched to the spatial resolution and statistical requirements of the survey. Monitoring the quality of data during acquisition is mandatory. Post survey data processing must be both appropriate and mathematically sound.
The chosen contractor should have experienced personnel and advanced instruments to carry out high resolution geophysical surveys. A technical report describing the work, including high resolution maps with detailed interpretation, should be presented to the client upon completion of analysis.
2 GEOPHYSICAL METHODS
The appropriate techniques for investigations should be chosen based on:
- Objective of investigations
- Resolution required
- Depth penetration required
- Physical property to be defined
- Geology of the area
- Nature of target & host material
It is a proven fact that in most of the cases, use of a single geophysical technique will not reveal unique results, as there is a vast overlap of any physical property for various geological materials. It is therefore recommended to use an integrated approach to uniquely resolve the issues. Description of various geophysical methods commonly used for geotechnical investigations has been provided in following sections.
2.1 Seismic Refraction Surveys
Seismic technique is one of the most developed and widely used geophysical techniques, providing vital information on subsurface, crucial for most of the engineering projects. Based on P-wave velocities from the model, thickness and topography of overburden, weathered rock and fresh bedrock are easily obtained. An example gradient velocity model, with conventional layered model superimposed has been presented in Fig-1 hereunder:
Fig. 1 Example Seismic Refraction Result
Key features of seismic refraction survey are:
- Precise determination of seismic velocities.
- Precise determination of water table in overburden.
- Localization and identification of geological units - overburden, bedrock, etc.
- Great accessibility to rough terrain and remote regions.
Key advantages of seismic refraction survey are:
- Rapid ground coverage.
- Only option in rough remote terrains.
- Provides continuous profile of subsurface, critical for engineering projects.
- Estimation of Dynamic Elastic Moduli like Poisson's Ration, Young's Modulus, Shear Modulus.
Key applications of seismic refraction survey are:
- Bedrock profile, rock quality and depth.
- Thickness of overburden
- Fractures and weak zones
- Topography of ground water
- Rippability assessment in mines
- Slope stability studies
- Pipeline route studies
Key limitations of seismic refraction survey are:
- Velocity increase with depth a pre-requisite
- Hidden layer & Blind Zone anomalies
2.2 Electrical Resistivity Imaging
2D Resistivity Imaging uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, showing variation in resistivity both along the survey line and with depth.
The technique is extremely useful to get information on weak zones or buried channels under the rock interface, which go undetected in seismic refraction that terminates at rock interface. Resistivity imaging can be effectively used to determine rock profile along dam axis across high current shallow rivers where deployment of hydrophones is not possible restricting use of seismic refraction. Deep penetration seismic refraction techniques require use of explosives, which are not always easy to deploy especially in sensitive areas. In such cases resistivity imaging can be effectively used to get detailed information of deeper layers. Following is an example from work of resistivity imaging carried out at HEP site.
Fig.2 Example Electrical Resistivity Imaging Result
Key applications of Electrical Resistivity Imaging are:
- Determination of underground water resources
- Bedrock quality and depth measurements
- Mineral prospecting
- Dam structure analysis
- Contamination source detection
Key advantages of Electrical Resistivity Imaging are:
- Excellent 2-dimensional display of ground resistivity.
- Delineation of features like cavity, contamination plumes, weak zones in structures like dams etc.
2.3 ReMi (Refraction Micro-Tremor)
ReMi (Refraction Micro-tremor) is a new, proven seismic method for measuring in-situ shear-wave (S-wave) velocity profiles. It has distinct edge over MASW and SASW in terms of logistics, execution and results. ReMi can be performed under the same layout as used for seismic refraction, to obtain shear wave velocity profiles of subsurface. Testing is performed at the surface using the same conventional seismograph and vertical P-wave geophones used for refraction studies. Depending on the material properties of the subsurface, ReMi can determine shear wave velocities down to a minimum of 40 meters (130 feet) and a maximum of 100 meters (300 feet) depth.
The ReMi method is economic both in terms of cost and time, and offers significant advantages. In contrast to borehole measurements, ReMi tests a much larger volume of the subsurface. ReMi is non-invasive and nondestructive, and uses only ambient noise as a seismic source and no permits are required for its use. ReMi seismic lines can be deployed within road medians, at active construction sites, or along highways. There is no need to close a street or shut down work for the purpose of data acquisition. ReMi survey usually takes less than two hours, from setup through breakdown. The method provides more accurate results compared to conventional effort of picking up shear waves from records which more often than not lead to errors.
Typical 2D ReMi profile constructed using various 1D profiles is shown hereunder showing shear wave velocities up to a depth of 100m:
Fig.3 Example ReMi Result
The shear wave information obtained through ReMi is used for:
- Earthquake site response
- Liquefaction analysis
- Soil compaction control
- Mapping the subsurface and estimating the strength of subsurface materials
- Finding buried cultural features such as dumps and piers
2.4 Cross-hole Seismic Surveys
The primary purpose of obtaining cross-hole data is to obtain the most detailed in situ seismic wave velocity profile. Cross-hole velocity data are valuable for assessing man-made materials, soil deposits, or rock formations. The seismic technique determines the compressional (P) and/or shear (S) wave velocity of materials and the data can be used in problems related to soil mechanics, rock mechanics, foundation studies, and earthquake engineering. The set-up includes a source hole & two receiver holes as shown hereunder:
Fig.4 Typical Cross-hole Setup
The method allows precise determination of P and S wave seismic velocities which leads to determination of Dynamic Elastic Moduli like Poisson's Ration, Young's Modulus, Shear Modulus. Cross-hole method detects even thin anomalous zones in subsurface.
2.5 Cross hole Seismic Tomography
The latest technique of seismic refraction tomography provides much more realistic and accurate subsurface velocity model compared to typical layered models obtained through conventional seismic refraction surveys. It is based on generation of elastic energy using various sources at predetermined depths in one bore hole and detecting it in another borehole through a chain of hydrophones. Velocity analysis involves estimation of time required to cross the distance between source and receiver depending on variations in elastic properties of material crossed. Deviation survey is carried out prior to tomography for determining alignment of bore holes. The set-up includes a source hole & a receiver hole as shown hereunder:
Fig.5 Typical seismic tomography Setup
A tomographic section generated from survey between two non planar holes for a hydro project for detection of a cavity.
Fig.6 Tomographic section showing cavity
2.6 Ground Penetrating Radar
Ground Penetrating Radar, also known as GPR, Georadar, Subsurface Interface Radar, Geoprobing Radar, is a non-destructive technique to produce a cross section profile of subsurface without any drilling, trenching or ground disturbances. GPR profiles are used for locating buried objects and investigating subsurface features.
Key application areas of GPR are:
- Geological and hydro-geological investigations including mapping of bedrock topography, water levels, solution features, glacial structures, soils and aggregates.
- Engineering investigations to evaluate dams, sea walls, tunnels, pavements, roadbeds, railway embankments, piles, bridge decks, river scour, buildings and monuments.
- Location and evaluation of buried structures including utilities, foundations, reinforcing bars, cavities, tombs, archaeological artifacts, and animal burrows.
- Site investigations: location of buried engineering structures and underground storage tanks.
- Subsurface mapping for cables, pipes and other buried structures prior to trench-less operations.
Fig.7 GPR field work
Key advantages of GPR are
- Rapid ground coverage- Antenna towed either by hand or from a vehicle.
- High-resolution coverage of the survey area, detecting even small objects.
- On-site interpretation possible due to instant graphic display.
Key limitations of GPR are:
- Data acquisition may be slow over difficult terrain.
- Depth of penetration is limited in materials with high electrical conductivities, clays.
- Energy may be reflected and recorded from aboveground features, walls, canopies, unless antennae are well shielded.
- Near surface artifacts (reinforcing bars, boulders, components of made ground) may scatter the transmitted energy and complicate the received signal and/or reduce depth of penetration.
- Working on principle of reflection, GPR detects the utilities and provides information on depth and location. Classification of utility any further can be done only with availability of background data and is not a deliverable of GPR survey.
2.7 Micro-Gravity Survey
Gravimetry technique measures variations in Earth's gravitational field caused by density contrasts in the near surface rock and sediment. Gravimetric surveys are carried out using extremely sensitive instruments capable of measuring tiny variations in the gravitational field. Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations. Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows are associated with shallow low density bodies. Thus, cavities and voids can be readily detected using micro-gravity surveys. These surveys are always carried out in conjunction with a precise topographic survey, to enable terrain corrections to be carried out.
Typical applications are:
- Regional geological mapping;
- Oil and gas exploration;
- Mineral exploration;
- Sediment thickness studies;
- Archaeological surveys;
- Void detection.
3 INTEGRATING GEOPHYSICAL METHODS
Various geophysical methods have their individual limitations. As an example, seismic refraction is an excellent tool for obtaining information on rock depth, but suffers from the limitation that velocity must increase with depth. In few typical geological conditions, the assumption does not hold good, and in such cases electrical tomography provides an excellent complimentary tool.
Another example is use of shear wave velocities. The P-wave velocity in a material is mostly dependent on compressive strength. Experience (along with a little common sense and some helpful tables) allows us to deduce nature of material once the velocity is known. For example, if the P-wave velocity is 600 m/s, then we may deduce that the material is a compacted soil. A sudden increase to 1500 m/s suggests that we have hit the water table. A velocity above 3000 m/s is almost certainly a fairly competent bed rock. A refraction analysis will tell us the depth from the surface to each of these materials and this result is adequate for many applications such as finding the depth to ground water or the excavation costs. However for major applications, like nuclear power plants, the complexity is much more. For a layer with velocity of 1500m/s, we can no longer assume that this is just a saturated alluvial material. We have to consider some more materials that might exhibit this same compressional wave velocity: saturated gravels, clay deposits, weathered rock, coal, or even quick sand. In such cases, shear wave velocities may be used to distinguish materials as they show wide variations and help us resolve the problem.
4 CASE STUDY
Extensive seismic refraction was carried out for a hydroelectric power project. At one location seismic refraction indicated presence of rock at shallow depth. The geology of the area was complex, with various fold patterns. It was therefore suggested to carry out additional investigations using electrical tomography, to ascertain continuity of rock strata. The result obtained from ERI is shown hereunder:
Fig.8 Electrical Resistivity Imaging Results
The profile indicates the presence of higher resistivity geoelectrical layer at the top which may be the hard quartzite layer. It is also obvious from the section that the resistivities of geoelectrical layers are decreasing downward in the section. The low resistivity zones are seen within the depth range of ~38 - 43 mbgl which may be feeble rock formation.
Engineering geophysics is an efficient means of subsurface investigation. The merit of application of this low cost aid lies in its ease of deployment and rapidity in providing reliable knowledge of the underground over a large area, substantiating the requisite geotechnical evaluation studies thereby. They are helpful towards optimizing need of the conventional direct exploration methods thereby affecting economy of the projects.
A single geophysical method cannot resolve all the problems associated with subsurface investigations. There is therefore need for integrated application of various geophysical techniques like seismic refraction, resistivity imaging, Microgravity, GPR, ReMi, Crosshole/ downhole/ uphole seismic and seismic reflection, to determine various properties of subsurface like bedrock quality and depth, low velocity zones (even under rock interface), fault/ fracture/ shear zones, water lenses, tunnel route geology, voids etc.