For details of our global editorial offices, for customer service, and for information on how to request permission to reuse the copyrighted material in this book, please visit our website at www.wiley.com/wiley-blackwell. All brand and product names used in this book are the trade names, service marks, trademarks, or registered trademarks of their respective owners.
Preface to the 2 nd Edition
Another big difference with this publication is that while all the figures included here are published in black and white gray scale, color versions of many of them are included on a companion website at: www.wiley.com/go/ reynolds/introduction2e, along with the list of . Additionally, the book is also available in electronic form in its entirety and also as e-chapters, all of which are available for purchase through Wiley's website at www.wiley.com.
Acknowledgements
Introduction
What are ‘applied’ and
- Geophysical methods
- Matching geophysical methods to applications
- Planning a geophysical survey
- General philosophy
- Planning strategy
- Survey constraints
- Geophysical survey design
- Target identification
- Optimum line configuration and survey dimensions
- Selection of station intervals
- Noise
- Position fixing
- Data analysis
If this is not sufficiently taken into account in the initial budget, the entire success of the research may be jeopardized. The correct orientation and length of the profile must be taken into account (see below).
Bibliography
Bad data, spikes and the like are now often only discovered in the office when it is too late to recover the data. It is not uncommon for marine geophysics contractors to dump datasets on clients without performing adequate data quality control reviews.
General geophysics texts
The increased use of grid packs means that subtle details in individual profiles can be missed; maxima are reduced and minima are increased through the grid routines. While rapid data gridding and data visualization are important parts of quality control, when used properly they should not replace interpretation, an aspect of increasing concern.
Further reading
In shallow seismic reflection profiling, such as is done for sea scraper surveys, semi-automatic horizon selection software can miss or incorrectly select events. In the absence of a conscious effort to apply reasonable quality control, the end results for the client may be incorrect, incomplete, or both.
Gravity Methods
Introduction
Physical basis
- Theory
- Gravity units
- Variation of gravity with latitude
- Geological factors affecting density
- Sedimentary rocks
- Igneous rocks
- Metamorphic rocks
- Minerals and miscellaneous materials As the gravity survey method is dependent upon contrast in densi-
The value of the gravitational acceleration varies across the surface of the Earth for many reasons, one of which is the shape of the Earth. Centrifugal acceleration, which is equal to the rotational speed (a) to the square of the distance to the axis of rotation (d), serves to reduce the value of the gravitational acceleration.
Measurement of gravity
- Absolute gravity
- Relative gravity
There may come a time when the use of portable absolute weight meters will replace the use of relative weight meters for some applications. Spacing between gravity stations is crucial for subsequent data interpretation.
Gravity meters
- Stable (static) gravimeters
- Askania
- Boliden
- Gulf (Hoyt)
- Unstable (astatic) gravimeters
- Thyssen
- LaCoste-Romberg
- Burris
- Worden
- Vibrating string
- Marine and airborne gravity systems
Changes in gravity affect the weight of the mass which is balanced by the restoring action of the spring. Any displacement of the mass due to a change in gravity is detected by a capacitor transducer and activates a feedback circuit.
Corrections to gravity observations
- Instrumental drift
- Tides
- Latitude
- Free-air correction
- Bouguer correction
- Terrain correction
- Building corrections
- E¨ otv¨ os correction
- Isostatic correction
- Miscellaneous factors
- Bouguer anomaly
One of the main problems with the Bouguer correction is knowing which density to use. The density of the typical house bricks was obtained from brick manufacturers and was found to be 2.1 Mg m−3.
Interpretation methods
- Regionals and residuals
- Anomalies due to different geometric forms
- Depth determinations
- Mass determination
- Second derivatives
- Second vertical derivative (SVD) maps One of the problems inherent within the interpretation of Bouguer
- Downward and upward continuation The effect on gravity of a geological mass at considerable depth is far
- Euler deconvolution
- Sedimentary basin or granite pluton?
It is possible to calculate and plot maps of the second vertical derivative of Bouguer anomaly data. In the first of these, the Bouguer anomaly map appears to have a consistent trend in the direction of the gravity gradient (increasingly positive to the east) with isogalls aligned with a. One of the most important considerations in interpreting regional gravity data in particular is the amount of computer processing.
Applications and case histories
- Mineral exploration
- Discovery of the Faro lead-zinc deposit, Yukon
- Pyramid ore body, North West Territories
- Sourton Tors, Dartmoor, SW England This is an example of where gravity did not work at all in association
- Central Mineral Belt Uranium Project, Labrador
- Engineering applications
- Detection of natural and man-made cavities
- Detection of massive ice in permafrost terrain
- Archaeological investigations
- Hydrogeological applications
- Location of buried valleys
- Microgravity monitoring in the oil industry
- Volcanic hazards
- Glaciological applications
- Glacier thickness determinations
- Tidal oscillations of Antarctic ice shelves
Several low values of the residual gravity anomaly with amplitudes of the order of -70 µGal were identified (Figure 2.61). In eruptions, a decrease in gravity due to a decrease in material was observed (Figure 2.67B). The geology of the area is shown in Figure 2.69A, and the distribution of gravity stations in Figure 2.69B.
Geomagnetic Methods
Introduction
Basic concepts and units of geomagnetism
- Flux density, field strength and permeability
- Susceptibility
- Intensity of magnetisation
- Induced and remanent magnetisation
- Diamagnetism, paramagnetism, and ferri- and ferro-magnetism
The ratio between the flux density B and the magnetizing field strength H is a constant called the absolute magnetic permeability (µ). The surface concentration of free poles or total strength m per unit area is a measure of the magnetization strength J (Box 3.3). Intensity of induced magnetization, Jiin rock with susceptibilityκ caused by the Earth's magnetic field F (tesla) in terms of flux density, i.e.
Magnetic properties of rocks
- Susceptibility of rocks and minerals
- Remanent magnetisation and K¨ onigsberger ratios
The coercivity, Hc, is an indication of the 'hardness' or permanence of the magnetisation. The sensitivities depend on the alignment and shape of the magnetic grains scattered throughout the rock. For further details on the magnetic matter method, see e.g. the discussions of Lowrie (1990) and Tarling (1983).
The Earth’s magnetic field
- Components of the Earth’s magnetic field
- The main dipole field
- The non-dipolar field
- Time variable field
Consequently, it is possible to obtain a theoretical value for the field strength of the Earth's magnetic field for any location on Earth (Figure 3.15). Observations of the Earth's magnetic field have been made in London and Paris for over four centuries. In addition, the intensity of the main magnetic field decreases by about 5% per century.
Magnetic instruments
- Torsion and balance magnetometers
- Fluxgate magnetometers
- Resonance magnetometers
- Cryogenic (SQUID) magnetometers
- Gradiometers
- Airborne magnetometer systems
This voltage reaches its maximum when the rate of change of the magnetic field is greatest (Figure 3.19C). In a magnetic field such as Earth's (F), the majority of protons are oriented parallel to this field, while the remainder are oriented antiparallel (Figure 3.22A). Because the precession frequency depends on the Earth's ambient field strength (see Box 3.6), the total field strength can be determined from a measurement of the precession frequency.
Magnetic surveying
- Field survey procedures
- Noise and corrections
- Data reduction
The sensors can be mounted rigidly as booms (from the front of the aircraft nose) or spars (from the tail of the aircraft) (Figure 3.24A) or at the bottom of the aircraft wing tips (especially useful in horizontal gradiometer mode for ensure as much horizontal separation of the sensors as possible). Regional geographic gradients (φ) and longitudinal (θ) can be determined for the areas in question and related to a base value (F0), for example, in the southeast corner of the study area. Consequently, the anomalous value of the total field (δF) can be calculated arithmetically, as shown by the example in Box 3.7.
Qualitative interpretation
- Profiles
- Pattern analysis on aeromagnetic maps
Once you determine that the linear anomaly is due to the cathodically protected tube, the anomaly can be filtered out of the data set. Since the magnetic data have been corrected to remove the effect of the Earth's magnetic field, the anomaly will be due to vectors associated with the source body. Two of the three dikes (A and B) produce large positive anomalies, while the third dike (C) produces a broader low.
Quantitative interpretation
- Anomalies due to different geometric forms
- Simple depth determinations
- Reduction to the Pole (RTP)
- Modelling in two and three dimensions
- Depth determinations and Euler deconvolution
The horizontal distance d between these two tangents is a measure of the depth to the magnetic body (see box 3.8). From the shape of the Fourier spectrum, estimates of the depth of the magnetized bodies can be made (Hahn et al., 1976). A simplified geological map of the study area is given in Figure 3.52 with the aeromagnetic survey data.
Applications and case histories
- Regional aeromagnetic investigations
- Mineral exploration
- Detection of underground pipes
- Detection of buried containers
- Landfill investigations
- Acid tar lagoon survey
- UneXploded Ordnance (UXO)
Problems with ore body identification can be seen from Figure 3.60. The map of the total magnetic field intensity anomaly of part of the site is shown in Figure 3.66A. An example of the analytical signal and total magnetic field extracted from a larger magnetic survey is shown in Figure 3.71.
Applied Seismology: Introduction and Principles
Introduction
There are many books available that deal extensively with exploration seismology, such as that by Claerbout McQuillin et al. It will provide a brief introduction to the shallow refraction and reflection methods (emphasis added), and briefly to the processes used in the seismic industry for hydrocarbon exploration. In addition to hydrocarbon exploration, seismic methods have a significant number of other applications (Table 4.1), ranging from crude depth-to-rock determinations to more subtle but fundamental information about the physical properties of subsurface media, and from the obvious applications such as the suitability of the site to seemingly obscure uses such as in forensic investigations into land-based aircraft crashes, such as the Lockerbie air disaster in Scotland in 1989, and the location of miners trapped after roof collapses.
Seismic waves
- Stress and strain
- Types of seismic waves
- Body waves
- Surface waves
- Seismic wave velocities
All the frequencies in body waves travel through a given material at the same speed, subject to the consistency of the elastic moduli and density of the medium through which the waves propagate. Rayleigh waves travel along the free surface of the Earth with amplitudes that decrease exponentially with depth. In the case of laboratory measurements, ultrasonic transducers are used to send a pulse through a sample of the material in question.
Raypath geometry in layered ground
- Reflection and transmission of normally incident rays
- Reflection and refraction of obliquely incident rays
- Critical refraction
- Diffractions
In reality, it is generated from a limited area of the reflector surface as defined by the first Fresnel zone (Figure 4.6). The curvature of the diffraction tails is a function of the velocity of the host medium (Figure 4.10). In the case of an isolated cause, such as a boulder, where the shot is located above the source of the diffraction, a hyperbolic traveltime response is obtained (Figure 4.10; . see also Box 4.9).
Loss of seismic energy
- Spherical divergence or geometrical spreading
- Intrinsic attenuation
- Scattering
The amplitude, which is proportional to the square root of the energy density, therefore varies proportionally to 1/r. One of the outstanding problems receiving much attention in hydrocarbon exploration is the effect of clay on the petrophysical properties of sandstone and shale. The attenuation coefficient is a measure of the fractional energy loss per unit distance, and 2π/Q is the fractional loss per wavelength.
Seismic energy sources
- Impact devices
- Sledge hammer
- Drop-weight devices
- Accelerated weight drop (AWD)
- Impulsive sources
- Explosive sources
- Non-explosive sources
- Airguns and sleeve guns
- Water guns
- Gas guns/sleeve exploders
- Steam gun and Starjet
- High-resolution waterborne sources
- Vibrators .1 Vibroseis
- Small-scale land vibrators
- Marine vibrators
- Animals
A schematic cross-section of the Superseis charge used in the Maxipulse system: (a)=rimfire percussion cap; (b)=delay column; (c)=amplifier; and (d)=nitrocarbonitrate main charge. The respective bubble pulse periods (T) and amplitudes (P) are indicated as a function of time. B) The corresponding bubble pulse train. The weight of the vehicle is transferred to the jack in the middle of the vehicle when in use.
Detection and recording of seismic waves
- Geophones and accelerometers
- Hydrophones and streamers
- Seismographs
The geophone is driven into the ground with a nail attached to the base of the case (Figure 4.34A) to ensure a good ground connection. The maximum sensitivity of any geophone occurs when the coil axis is parallel to the direction of maximum ground motion. The geophones are connected to common multi-core receiving cables that are secured to the top of the grid.
Seismic Refraction Surveying
Introduction
General principles of refraction surveying
- Critical refraction
- Field survey arrangements .1 Land surveys
- Waterborne surveys
The simplest case is to position the shot at the beginning and end of the spread (end-on shot). By careful operation of the boat and positioning of the bottom tow cable, long seismic fault lines can be obtained. The offset between the shock and the sonobuoy is determined by the travel time of the direct wave through the water.
Geometry of refracted raypaths
- Planar interfaces .1 Two-layer case
- Three-layer case
- Multilayer case
- Dipping-layer case
This takes the form of the general equation of a straight line, y=mx+c, where m=gradient and c=intercept on the y-axis in a time-distance graph. The analysis works by determining V1, V2, t1 and t2 from the traveltime graph for the top two layers, and therefore the thicknesses of the first two refractors can be calculated using the equations in Box 5.4. The form of the equations and of the analysis of the travel time graphs follow the procedures.