Seismic Techniques

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Seismic Techniques

Seismic Refraction Profiling

Seismic Reflection Profiling

Continuous Surface-Wave System (CSWS)

Downhole Seismic Surveys

Crosshole Seismic Surveys

Crosshole Seismic Tomography

Seismic Refraction Profiling

Outline | Detail | Results |

OUTLINE

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source ('shot') located on the surface. Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.

DETAIL

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source ('shot') located on the surface. For shallow applications this normally comprises a hammer and plate, weight drop or small explosive charge (blank shotgun cartridge). Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array (or spread) of geophones spaced at regular intervals. Beyond a certain distance from the shot point, known as the cross-over distance, the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival). Observation of the travel-times of the direct and refracted signals provides information on the depth profile of the refractor.

Shots are deployed at and beyond both ends of the geophone spread in order to acquire refracted energy as first arrivals at each geophone position.

Data are recorded on a seismograph and later downloaded to computer for analysis of the first-arrival times to the geophones from each shot position. Travel-time versus distance graphs are then constructed and velocities calculated for the overburden and refractor layers through analysis of the direct arrival and T-minus graph gradients. Depth profiles for each refractor are produced by an analytical procedure based on consideration of shot and receiver geometry and the measured travel-times and calculated velocities. The final output comprises a depth profile of the refractor layers and a velocity model of the subsurface.

The primary applications of seismic refraction are for determining depth to bedrock and bedrock structure. Due to the dependence of seismic velocity on the elasticity and density of the material through which the energy is passing, seismic refraction surveys provide a measure of material strengths and can consequently be used as an aid in assessing rippability and rock quality. The technique has been successfully applied to mapping depth to base of backfilled quarries, depth of landfills, thickness of overburden and the topography of groundwater.

RESULTS

During data acquisition individual shot records are displayed as variable area wiggle traces displaying travel time against distance (see right above). These enable an initial calculation of overburden and refractor apparent velocities and provide an important check on the quality of the data.Following acquisition wiggle traces are used to display the data during picking of the first-arrivals for each geophone position and shot.

The processed data is normally presented as a series of three plots; time-distance graphs for the picked first-arrivals on each shot, a true depth profile for the identified refractors and a velocity profile for the overburden and refractors. Any existing ground truth information such as borehole and trial pit logs, is overlain on the depth profile in order to help calibrate the seismic results and then provide an indication of the level of correlation along the survey line. The refractor depth is displayed as a series of overlapping arcs that represent the solutions for each geophone in the array. The refractor can lie anywhere on the arcs below the intersections with adjacent arcs (right).

Seismic Reflection Profiling

Outline | Detail |

OUTLINE

DETAIL

Seismic reflection profiling involves the measurement of the two-way travel time of seismic waves transmitted from surface and reflected back to the surface at the interfaces between contrasting geological layers. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these layers. The strength of the contrast in the acoustic impedance of the two layers determines the amplitude of the reflected signal. The reflected signal is detected on surface using an array of high frequency geophones (typically 48-96). As with seismic refraction, the seismic energy is provided by a 'shot' on surface. For shallow applications this will normally comprise a hammer and plate, weight drop or explosive charge. In most reflection surveys shots are deployed at a number of different positions in relation to the geophone array in order to obtain reflections from the same point on the interface at different geophones in the array. Each common point of reflection is termed a common mid-point (CMP) and the number of times each one is sampled determines the 'fold coverage' for the survey. Traces relating to the same CMP are stacked together to increase the signal-to-noise ratio of the survey before being combined with other CMP's stacked traces to produce a reflection profile. In order to stack related CMP traces a stacking velocity is applied to each trace. This accounts for the difference in two-way travel time between the normal incidence reflection (vertical travel path below the shot) and those at increasing offsets from the shot (known as the normal moveout or NMO). The stacking velocity will vary down the trace to take account of the increase in velocity with depth for each reflection event.

The simplest form of seismic reflection profiling is the constant-offset method. This technique uses a single geophone offset from the source by a fixed distance. The two are moved along the survey line in equal steps with a single trace being recorded at each position. The main advantage of this technique is the limited amount of processing that needs to be applied to the data due to the almost vertical orientation of each raypath. However, in order to avoid problems with interference from groundroll and the shot airwave, the offset distance has to be selected with care.

Continuous Surface-Wave System (CSWS)

Outline | Detail | Results |

OUTLINE

The continuous surface-wave method utilises a specific type of seismic wave, known as the Rayleigh wave, in order to determine in situ shear modulus-depth profiles to depths of between 8m to 20m. The velocity of a Rayleigh wave is related to the shear modulus (G) and density of the ground through which it propagates. Unlike crosshole seismic methods, which are routinely used to determine geotechnical parameters such as the shear modulus (and additionally Poisson's ratio), the CSW technique require no boreholes. The system comprises a portable frequency-controlled vibrator and an array of low frequency geophones arranged co-linearly with the source. Rayleigh waves are generated at frequencies of between 5Hz and 100Hz in 0.1-5Hz increments in order to build up a comprehensive stiffness-depth profile.

DETAIL

Rayleigh waves are constrained to propagate within a zone approximately 1 wavelength in depth, such that increasing the wavelength (decreasing the frequency) of the transmitted energy will result in an increase in the depth of investigation. The wavelength and phase-velocity of the Rayleigh waves generated at a particular frequency are calculated by determining the phase shift between the transmitted and measured signals at each geophone location.

Phase-velocities are measured over a range of frequencies in order to build up a dispersion spectrum for the ground below the spread. This is then inverted to determine a velocity-depth profile and finally a stiffness-depth profile.The system comprises a portable frequency-controlled vibrator and an array of low frequency geophones arranged co-linearly with the source. A laptop computer controls both the vibrator and data acquisition. Rayleigh waves are generated at frequencies of between 5Hz and 100Hz in 0.1-5Hz increments in order to build up a comprehensive stiffness-depth profile.

For further information on the Continuous Surface-Wave System see 'The Continuous Surface-Wave System: A Modern Technique for Site Investigation', Menzies, B. & Matthews, M. (1996); Special Lecture: Indian Geotechnical Conference, Madras, December 11-14th 1996.

RESULTS

Raw results are initially presented as a dispersion curve of phase velocity against depth. Inversion of the curve results in a stiffness-depth profile (shear modulus-depth) for the sampling location (see above) that can be compared directly to the results of other methods such as seismic cone penetration (SCPT). Each test takes approximately 2 hours and provides about 50 stiffness measurements at different depths. A preliminary stiffness-depth profile can normally be computed on site using an empirical wavelength /depth inversion routine.

Downhole Seismic Surveys

Outline | Detail | Results |

OUTLINE

Downhole seismic surveys are the simplest and cheapest method in the suite of borehole seismic techniques, as they require only a single borehole. Seismic energy is generated on surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves are measured at regular intervals down the hole using a string of hydrophones or, in the case of S-wave surveys, a single clamped triaxial geophone that is gradually moved down the hole. The P- and S-wave arrival times for each receiver location are combined to produce travel-time versus depth curves for the complete hole. These are then used to produce total velocity profiles from which interval velocities and the various elastic moduli can be calculated (in conjunction with density data from geophysical logging of the borehole).

DETAIL

Downhole seismic surveys are the simplest and cheapest method in the suite of borehole seismic techniques requiring only a single borehole. Seismic energy is generated on surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves are measured at regular intervals down the hole using a string of hydrophones or, in the case of S-wave surveys, a single clamped triaxial geophone that is gradually moved down the hole.

P-wave energy is normally provided by a hammer and plate or weight drop similar to shallow seismic reflection and refraction profiling surveys. Polarised S-waves are generated using a shear wave hammer. This comprises two hammers connected to either end of a plank that is held to the ground using a vehicle or heavy weight. Collecting both positive and negative polarised (so called A and B) S-waves using the two hammers separately, enables the S-wave arrivals on the receiver shot records to be distinguished from those of P-waves and coherent noise.

RESULTS

Crosshole Seismic Surveys

Outline | Detail | Results |

OUTLINE

Crosshole seismic surveys involve measurement of the travel time of seismic energy transmitted between two or more boreholes in order to derive information on the elastic properties of the intervening materials. One hole is used to deploy the source whilst the other hole(s) are used to detect the arrival of the seismic energy. The travel times of the seismic waves are derived from the first-arrivals identified on the seismic trace for each shot-receiver position and are used with the known distance(s) between the shot/receiver boreholes to calculate the apparent velocities (P and S) for each depth interval. This data is then used to derive a vertical profile of the various elastic moduli.

DETAIL

The relationship between the velocity of seismic waves and the density and elastic properties of the materials through which they are travelling means that seismic techniques can be utilised to provide information on various geotechnical properties of the subsurface, such as Poisson's ratio and the shear modulus. The most common method of measuring these properties in engineering studies is through the use of crosshole seismic surveys.

Crosshole seismic surveys involve measurement of the travel time of seismic energy transmitted between two or more boreholes. One hole is used to deploy the source whilst the other hole(s) are used to detect the arrival of the seismic energy. In order to obtain properties such as Poisson's ratio, both P-wave (compressional) and S-wave (shear) data has to be acquired. This normally requires the use of two separate sources. P-wave energy is detected using a string of between 10-24 hydrophones suspended in water in the borehole. As shear waves are unable to travel through water or air they are detected using a single triaxial geophone clamped to the inside of the borehole using a hydraulic system. Data is collected at fixed intervals down the hole (normally 0.5-2m) by moving the shot and detector(s) in parallel. In the case of an array of hydrophones the string is kept fixed until the shot has passed below the depth of the last hydrophone.

The travel times of the P and S waves are derived from the first-arrivals identified on the seismic trace for each shot-receiver position and used with the known distance(s) between the shot/receiver boreholes to calculate the apparent velocities (P and S) for each depth interval. This data is then used to derive a vertical profile of material stiffness properties. Where the borehole separation is small the calculated apparent velocities will equate to the true velocities for each depth, as the energy travels direct from source to receiver. However, where a refracted or reflected wave arrives at the receiver first (first-arrival), this will not be the case, leading to spurious calculations of the material stiffness properties. Where refraction/reflection is considered to be a problem computer modelling of the raypaths should be utilised to help derive true interval velocities.

RESULTS

First arrival times for the P- and S-wave data are picked in a similar manner to conventional surface seismic techniques and other borehole methods using wiggle traces. Identification of the S-wave arrivals is aided by the collection of opposite polarity shots during data acquisition. During picking the shot record pairs are initially viewed side by side. Subtraction of one record from the other results in cancellation of P-waves and stacking of the S-wave data.

Following picking of the arrival time data for each shot-receiver location and calculation of velocity (based on the borehole separation) the data is displayed as a plot of velocity against depth (above). Calculation of the various elastic moduli is carried out using additional information on the density of the various geological strata in the holes. This is normally obtained from geophysical logging of the boreholes. Final results are generally displayed as tables or profiles of the calculated moduli such as Poisson's ratio and the bulk shear modulus.

Crosshole Seismic Tomography

Outline | Detail | Results |

OUTLINE

Borehole seismic tomography involves the measurement of the travel times of seismic raypaths between two or more boreholes in order to derive an image of seismic velocity in the intervening ground. Data is collected using one hole for the seismic source (normally a sparker) and measuring first-arrival times using strings of hydrophones in the others. Travel times are collected at regular intervals (usually 0.5m to 2m) all the way down the hole(s) for each shot position. This results in a network of overlapping raypaths that can then be used to model the velocity profile. The resulting velocity image is termed a tomogram and enables identification of anomalous velocity zones lying between the boreholes as well as imaging individual velocity layers.

DETAIL

Borehole seismic tomography involves the measurement of the travel times of seismic raypaths between two or more boreholes in order to define an image of seismic velocity in the intervening ground.

Data is collected in a similar manner to crosshole seismic surveys by using one hole for the seismic source (normally a sparker) and measuring first-arrival times using strings of hydrophones in the others. However, unlike crosshole, travel times are collected at regular intervals (usually 0.5m to 2m) all the way down the hole(s) for each shot position.

Measurement of arrival times for each shot, at each position in the receiver borehole, results in a network of overlapping raypaths which can then be used to model the velocity profile (see figure). The plane separating the source and receiver holes is divided into a mesh of grid cells known as finite elements. Each element in the mesh is assigned a starting velocity and the synthetic travel time for the portion of each raypath passing through it is calculated. In this way the total travel time for each raypath is built up and then compared to the measured travel time. The velocities assigned to the various elements are then adjusted iteratively until the calculated and measured travel times for the raypaths are the same. As many of the cells are intersected by a number of raypaths the process can result in very accurate estimates of the velocity for each cell.

The resulting velocity image is termed a tomogram and enables identification of anomalous velocity zones lying between the boreholes as well as imaging individual velocity layers. The primary application of borehole seismic tomography is in engineering studies for the identification of features such as fault zones and voids. When combined with an S-wave survey, the data can additionally be used to provide information on material stiffness properties (see crosshole seismic surveys).

RESULTS

During data acquisition individual shot records are displayed as variable area wiggle traces indicating travel time against downhole distance for each shot position. Following acquisition wiggle traces are used to pick the first-arrivals for each source/receiver pair. The image displays a shot gather for 40 receiver locations down a hole.

Following picking the raw travel-time data is input into the modelling software and the ray coverage between the two holes is displayed as a ray density profile (right). This provides an initial indication of the amount of data within different sections of the profile and helps illustrate the resolution of the final model.

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