1) Earth Resistivity Tomography (ERT)

Electrical Resistivity Tomography (ERT) method is a geophysical technique applicable in mapping subsurface conditions for various uses including geologic, geotechnical, hydrogeology, environmental mineral exploration, and archaeological investigation.

The ERT method is used successfully to map anomalous geologic conditions including weak zone detection, void detection, fault/fracture detection, monitor changes in ground water or contaminations, salt water intrusion, contaminant plumes, map geologic conditions (including depth to bedrock, or to water table, stratigraphy, lithology, structure, and fractures or all of these.

Electrical Resistivity Tomography records deeper information and assesses deeply seated sub-surfaces conditions, solution features, watercourses and archaeological targets. This technique also allows geophysical investigation in landscapes, which are not generally amenable for other types of survey such as woodland, marshland and areas of dense vegetation

With this technique, the apparent electrical resistivity distribution of the subsurface is measured using an array of four electrodes. By injecting a dc or very low frequency ac current (I) between a pair of electrodes and measuring the resulting potential difference (ΔV) with a second pair of electrodes, it is possible to calculate the apparent resistivity using a derivation of Ohms law (R=ΔV/I).

We must emphasis that the ERT method is an electrical method and will only provide information regarding the electrical properties of the subsurface material. For this site the saline ground water will be the main controlling factor, and we expect that open or partially filled cavities to be highlighted as zones with extreme low resistivity of less than 1 Ω.m or even as low as 0.1-0.2 Ω.m due to the higher concentration of saline water. Areas with disintegrated rock (weathered and highly fractured) will exhibit lower resistivity values compared to the site norm within each rock formation.

2) Microgravity

The microgravity method, which detects both the lateral and vertical variations in the rock density, will have limitation over this site in terms of depth and resolution. In addition, the analysis and presentation of the results will be in the form of contour plots of the residual Bouguer Anomaly and to convert these into cross-section they need to be modeled but the solution are not unique, i.e. several models for the same anomaly as the cause of the low gravity areas are the cumulative effects of both shallow and deeper low density areas.

3) Multichannel Analysis of Surface Waves (MASW)

Multichannel Analysis of Surface Waves (MASW) method will measure the surface waves and in particular Rayleigh wave velocity, which is frequency dependent. Principally it considers the dispersive characteristic of acoustic waves i.e., a particular frequency would travel with particular velocity. Higher frequencies generated by the source will travel in the shallow layers only, whilst lower frequencies travel in both shallow and deep layers, velocity will vary with frequency.

This is known as geometrical dispersion and to improve on the depth penetration lower frequencies component are desired but from experience working in Doha these are difficult to achieve. The MASW method will provide information to a maximum depth of 25-30m in best conditions.

The acquisition principle of the multichannel method is carried out with the use of 24 or 48 low frequency geophones (4.5Hz) with constant receiver spacing along the line. The geophone array is towed behind a car with the geophones mounted on a land streamer or gliding plate. The seismic energy is produced with a sledge hammer or Propelled Energy Generator (PEG) source .

4) Seismic Refraction

Seismic Refraction surveys are used for varoius purposes to map the bedrock depth and evaluate rock rippability. Seismic refraction measurements are applicable in mapping subsurface conditions for various uses including geologic, geotechnical, hydrologic, environmental, mineral exploration, petroleum exploration, and archaeological investigations.

Seismic Refraction investigations are commonly used to determine layer thickness and/or the subsurface compressional wave velocity of the overburden and underlying bedrock or to water table, stratigraphy, lithology, structure, and fractures. This inherently shallow technique requires that compressional seismic wave velocities increase with depth so wave refraction occurs.

Refraction surveys, the predominant seismic method measures seismic wave velocities of materials. A source (e.g., sledge hammer, gun device, weight, or explosives) is used to create and emit waves into the subsurface. These waves travel at material specific velocities, are refracted at various interfaces, and eventually are received by geophones installed on surface that convert them into an electrical signal that is displayed on a seismograph.

5) Downhole

Dynamic geo-technical properties of soil and rock are determined by various in-situ test methods. Dynamic problems are related to earthquakes (liquefaction, ground response and slope stability), vibrating machinery, pile driving, dynamic compaction and other techniques for ground improvement and also from blast, wind, wave and ice loadings. Apart from these the traffic vibrations also cause dynamic problems in a great extent. Engineering interest is mostly concentrated on those methods that can measure the dynamic properties of soils/rocks in order to design the site specific problems.

The objective of seismic down-hole/up-hole tests is to measure travel times of P and S-waves from the energy source to the receiver/s. These tests can be performed in single borehole. A hole is drilled to the required depth at the testing site and a vibrating source is created to determine shear wave velocity for various soil layers. In this case the waves will be travelling in vertical direction either down or up depending on the location of the source of impulse.
In down-hole test, the generated waves propagate through the soil layers in downward direction. A single wave source is located on the ground surface adjacent to the borehole. In side the hole, either a string of multiple receivers at known depths is fixed or a single receiver is moved to different depths as the test advances. All receivers are connected to high speed recording system and the output is measured as a function of time.

6) Crosshole Seismic

Crosshole Seismic method is uses for 1D profiling of seismic velocity in set of borehole which consist two or three number of borehole for precise assessment of different engineering parameters. The Crosshole Seismic Test makes direct measurements of P-wave velocities, or/and S-wave velocities, in boreholes advanced primarily through soil. At selected depths down the borehole, a borehole seismic source is used to generate a seismic wave train. Downhole receivers are used to detect the arrival of the seismic wave train in offset borings at a recommended spacing of 3 to 6 m [10 to 20 ft]. The distance between boreholes at the test depths is measured using a borehole deviation survey. The borehole seismic source is connected to and triggers a data recording system that records the response of the downhole receivers, thus measuring the travel time of the wave train between the source and receivers.

By the crosshole data, P and S Wave Velocities are calculated and other engineering parameters (i.e., Elastic moduli, Poison’s ration) could be calculated by incorporating density values of the strata.

7) Refraction Microtremor (ReMi)

ReMi can be performed under the same layout as used for seismic refraction, to obtain excellent shear wave velocity profiles of subsurface. ReMi is a new, proven seismic method for measuring in-situ shear-wave (S-wave) velocity profiles. It is economic both in terms of cost and time. 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 40-100m depth efficiently.

8) Ground Penetrating Radar

GPR is an easy-to-deploy geophysical technique that produces high-resolution images of the shallow subsurface in a manner analogous to reflection seismic. It is usually used as a surface technique although borehole radar is used in specialist applications. Like many geophysical techniques, GPR can provide very useful data if applied in the appropriate context. Hence, any sub-surface investigation that requires shallow, high-resolution imaging may benefit by GPR survey.

This technique involves a short duration; high frequency electromagnetic impulse being transmitted by the radar transducer into the subsurface. Whenever a contrast in dielectric properties of the subsurface is encountered, some of the transmitted impulse is reflected back to the surface, where it is detected by a receiver housed in the transducer. The strength of the reflected signal is proportional to the magnitude of the contrast in properties.

9) Electromagnetic (EM) Survey

The electromagnetic method otherwise referred to as the EM method measures the response of induced electromagnetic fields into the ground. The electromagnetic fields comprise alternating electric intensity and magnetic force. Contrary to other conventional resistivity methods, this method does not need ground contact and hence problems related to direct electrical coupling could be avoided. Therefore, this method provides faster and easier data acquisition and is mostly used as reconnaissance tools. It is often employed in detecting anomalies for further investigation.