Cross-borehole seismology

Cross-borehole seismology is a geophysical exploration technique involving the acquisition of data that can be used to image subsurface geology and determine the spatial distribution of the physical properties of geological materials. The data are acquired by placing seismic detectors in one borehole and a seismic source in an adjacent borehole. Seismic energy that propagates from the source travels through or reflects from subsurface geological materials. The total time of travel and amplitude of this energy is recorded by the detectors and is used to construct an image of the subsurface geology between the boreholes.

Cross-Borehole Versus Surface Exploration

Various methods exist that allow geoscientists to produce reasonable images of the earth's subsurface geology. Seismic data can be acquired using surface seismic exploration and cross-borehole seismology. When these data are properly processed and interpreted, an image of the subsurface is constructed.

In seismic reflection profiling, geophysicists typically arrange seismic detectors along a straight line at or near the surface of the earth and then generate sound waves by vibrating the ground. Seismic waves can be generated by detonating charges of dynamite, by dropping a weight on the ground, or by pounding the ground with a sledgehammer. To eliminate environmental risks associated with using explosives, a system called vibroseis is often used, in which a huge vibrator mounted on a special truck repeatedly strikes the earth to produce sound waves.

A seismograph records how long it takes sound waves to travel through or reflect from rock layers and arrive at seismic detectors. In seismic reflection profiling, the recorded data display the amplitudes of the reflected sound waves as a function of travel time. Such a graphic record is called a seismogram. The seismic source, detectors, and seismograph are then moved a short distance along the line, and the experiment is repeated. For seismic reflection profiling, the frequency range of investigation is limited to between 10 and 300 cycles per second, depending on the seismic velocity in the subsurface materials and the depth to the target of interest. This frequency range does not provide the necessary resolution of subsurface features that is typically required when making production decisions.

To help solve this problem, cross-borehole seismology was developed. In this method, a source of seismic energy is placed in one borehole, and appropriate seismic detectors are placed in an adjacent borehole. Boreholes can be vertical or horizontal. Between seven and twenty detectors are placed 2 to 5 meters apart and adjacent wells are at least 100 to 300 meters apart but always less than 1 kilometer apart. The source is fired, and the resulting seismic energy propagates through or reflects from the rock and is detected in the adjacent borehole. The travel times and amplitudes of seismic waves that have been transmitted or reflected through the rock mass between the drill holes are recorded. The source and detectors are then moved to another position in their respective boreholes, and the process is repeated. This procedure is continued until the region of interest is adequately covered by the propagating energy.

In contrast to seismic investigations conducted at the surface, cross-borehole surveys are performed at target depths; therefore, they do not suffer from the low-frequency resolution problems that are pervasive in surface seismic recordings. For cross-borehole seismology, the operating frequency range is from 400 to 30,000 cycles per second. Such data can provide high resolution of the subsurface geology between boreholes.

Cross-Borehole Technology

The idea of cross-borehole seismology has existed for many years. Field experiments were discussed as early as 1953 by Norman H. Ricker. Seismic surveys between boreholes were carried out in France by the Institute Français du Petrole in the early 1970s. Initially, the images produced from the data were fuzzy, filled with artifacts, and generally not worth the costs involved in acquiring them. Some geophysicists, however, saw in these images the proof of cross-borehole concepts and knew that with additional processing effort, coupled with advancements in seismic source and detector technology, the clarity and resolution would make the acquisition and processing of cross-borehole seismic data cost-effective.

Few disciplines affecting exploration geophysics developed more rapidly in the 1980s and 1990s than cross-borehole methods. Interaction among various scientific disciplines, along with advancements in seismic field data acquisition, imaging and inverse-problem theory, and computing speed, accelerated the development of cross-borehole seismology. Borehole source and detector technology, as well as computer-based data analysis algorithms, advanced to the point that routine application of the method became feasible by the late 1990s. Much of the progress in cross-borehole seismology has been driven by the development of powerful, nondestructive borehole seismic sources and by the increasing need for enhanced definition of oil-producing reservoirs.

Many borehole sources are made of piezoelectric ceramic materials that convert varying voltages in the material into mechanical vibrations, typically generating seismic signals in the range of 400 to 2,000 cycles per second. The same materials are also used in the seismic detectors because of their controllability, their high-frequency response, and their good impedance match to hard rock. The sources and detectors are packaged in metallic housings to form borehole probes that can operate in fluid-filled boreholes more than 1,000 meters deep. In the simplest borehole arrangement, the source and the detectors hang freely in the boreholes, but for better coupling or for operation in dry holes, more sophisticated sources and detectors with electrically powered clamping mechanisms lock the seismically active parts firmly against the rock in the borehole. For clamping detectors, the piezoelectric material is oriented to respond to seismic waves in three dimensions, allowing both compressional and shear waves to be recorded.

Another important borehole source was developed by the American oil company Texaco in the 1990s. A nondestructive, broadband air-gun array provided energy transmission over distances exceeding 600 meters. The broadband nature of the source provided the necessary spatial resolution of the subsurface.

Although the source waveform may be a pulse, transmitting a continuous, coded signal is a better way to achieve maximum transmission range. In the mid-1990s, a high-energy, broadband, clamped borehole vibrator was designed and successfully implemented to accomplish this task. Inside the borehole vibrator, a 114-kilogram mass is suspended below a hydraulic piston that is set into axial motion by a hydraulic valve. The vibrator clamp is coupled to the motion of the piston, which transmits stress to the borehole wall and sends seismic energy into the formation. This is analogous to surface vibroseis. The vibrator source transmits a continuous, controlled signal over a long period of time, keeping the stress low while transmitting considerable energy. Thus, a large amount of seismic energy is transmitted into the formation without harming the cement casing in a cased borehole.

By applying the techniques of tomographic imaging, cross-borehole seismic data can be used to image the subsurface and to estimate subsurface physical properties. Tomographic imaging is a highly effective way of condensing and organizing the large amount of information contained in high-density cross-borehole seismograms. The medical community has used computed tomography (CT) scanning since the 1960s to generate imaged cross-sections of different portions of the human body. In medical imaging techniques, such as X-ray tomography, the source and receiver rotate all the way around the object to be imaged. Cross-borehole seismology is similar to the medical case, except the angular coverage is not nearly as great.

In the early 1980s, geophysicists began applying techniques similar to those of medicine to earth science problems. These problems range from estimating the internal velocity structure of the subsurface to formulations that provide a complete three-dimensional image of the subsurface geology. In the early 1990s, tomographic reconstruction became a standard technique for analyzing cross-borehole and surface seismic data.

Obtaining information about subsurface geology from cross-borehole data constitutes a type of inverse problem. That is, measurements are first made of energy that has propagated through and reflected from within the subsurface. The received travel times and amplitudes of this energy are then used to estimate values of the physical parameters of the medium through which it has propagated. The parameters that are typically extracted are velocities and depths, from which a gross model of the subsurface structure can be derived. Initially, gross subsurface structure was considered the ultimate goal of seismic tomography, but it became obvious that an accurate set of velocities versus depth can effectively be used to constrain other types of seismic inversion, including the velocity control necessary for constructing an accurate depth image of the subsurface. During the 1990s, the goal for cross-borehole seismic data evolved into obtaining a reasonable estimate of the properties of the subsurface geology, particularly density, compressibility, shear rigidity, porosity (pore space in rocks), and permeability (ability for fluids to flow in rocks). To accomplish this goal, both compressional and shear wave data must be recorded.

Cross-Borehole Data Processing

Because seismic waves propagating in the earth's subsurface readily spread, refract, reflect, and diffract, algorithms for processing cross-borehole seismic data had to be developed to produce realistic subsurface images. Effective software and interactive graphics are required to pick and process the acquired cross-borehole data into a reasonable image. The received borehole signals are first filtered and digitized. By adding up many waveforms per detector, signal-to-noise ratios are enhanced. When using a vibrator source, impulse seismograms are obtained by cross-correlating the received data with the vibrator waveform, which further reduces random noise in the data. By implementing these signal enhancement techniques, the transmitted power in the borehole can be kept low enough to avoid damage to expensive boreholes and still record high-quality seismograms across distances of more than a few hundred meters in most rock types.

Using a variety of computer programs, cross-borehole seismograms are processed to yield seismic sections that represent the earth's reflectivity in time. However, since wells are drilled in depth, not in travel time, the seismic data need to be converted to depth in the imaging process. Furthermore, since reflectivity is a property associated with subsurface interfaces, a rock sample in the laboratory does not have any intrinsic reflectivity. Therefore, reflectivity is not an actual rock property, and this parameter must be converted to another parameter that really describes the rock. Typically, the chosen parameter is the internal velocity of seismic wave propagation through subsurface materials.

Internal velocity structure is estimated from cross-borehole and surface seismic data using seismic tomographic methods. Because subsurface velocities vary according to the physical properties of the rock through which the wave travels, geophysicists can use these velocities to determine the depth and structure of rock formations. In addition, since seismic waves change in amplitude when they are reflected from rocks that contain gas and other fluids, the fine details of amplitude changes in the seismograms can be used to infer the type of rocks in the subsurface. Thus, conventional seismic sections can yield far more information about the subsurface geology by using cross-borehole data and tomographic techniques to produce subsurface images as a function of depth and to estimate rock properties from the images.

The basic procedure for processing both cross-borehole and surface seismic data using tomography is iterative seismic tomography. The procedure involves picking transmitted or reflected events on raw seismograms and associating these events with the structure of a proposed subsurface geological model. Section of rock are then divided and imaged into a grid of pixels with local rock and fluid parameter values. The laws of physics are used to trace raypaths of seismic waves through the proposed geological model from the seismic source down through or from a subsurface boundary and back to the seismic detectors. Ray-traced travel times and amplitudes from the model are then compared with the travel times and amplitudes recorded on the seismogram. The medium parameters or geometry of the geological model are then updated to make the ray tracing consistent with the observed data. Corrections are made to the medium parameters or geometry systematically to reduce the differences between the observed and modeled travel times and amplitudes. After several iterations, the differences between the recorded and modeled data become acceptably small, and a “best” image of the subsurface geology that is self-consistent with the acquired data emerges.

Since the source-detector coverage of the object or area of interest is far from complete in cross-borehole seismology, non-uniqueness of solutions and lowered resolution of images result. Thus, tomographic images based on limited view angles must be interpreted with care. However, the tomographic parameter determination is still very useful, especially in areas of significant lateral velocity variations in the subsurface. By including cross-borehole, seismic reflection profiling, well-log, and any other available geophysical data, such as gravity, electrical, or radar, in the tomographic process, the resolution and certainty in describing the subsurface geology are greatly improved.

Applications

Cross-borehole seismology has been used in a number of applications. The greater the degree of angular coverage around the rock mass of interest, the greater the reliability of the constructed subsurface image. By making numerous measurements for various source-detector positions in adjacent boreholes and analyzing the travel times and amplitudes for these source-detector locations, the velocity, elastic parameters, and attenuation of the intervening rock can be estimated from the transmitted and reflected energy. Cross-borehole seismology has been used for hydrocarbon exploration, mineral exploration, fault detection, stress monitoring in coal mines, delineation of the sides of a salt dome, investigation of dams, mapping dinosaur bone deposits, and nuclear waste site characterization.

In 1985, a research team from the Southwest Paleontology Foundation began excavating a giant sauropod dinosaur (45 to 60 meters long), later named Seismosaurus. The Seismosaurus skeleton was discovered after eight tailbones were exposed by weathering and erosion. By 1987, almost the entire tail had been excavated. To remove the rest of the skeleton, the location of the rest of the skeletal remains needed to be determined. In 1989, a number of boreholes were drilled in the area, and cross-borehole seismology provided data to construct vertical cross-sections of where the Seismosaurus was located in the subsurface.

The Engineering Geoscience group at the University of California, Berkeley, used cross-borehole seismology in the 1990's to search for buried treasures, specifically the Victorio Peak treasure in southeastern New Mexico, said to consist of antique Spanish weapons, coins, and refined gold bars. The technique was also used to search for Yamashita's treasure of gold and jewels in the Philippines. The searchers' goal was to detect and delineate underground voids or cavities that could possibly contain the buried treasures. Although neither study resulted in the discovery of lost treasure, the ability of cross-borehole seismology to detect buried channels and voids was clearly demonstrated at both sites.

In 2009, the Federal Lands Highway Program of the US Department of Transportation used cross-borehole seismic tomography to investigate an active sinkhole causing damage to property in a residential neighborhood of central Florida. Personnel were able to obtain images that showed the “throat” of the sinkhole at a depth of 24 meters.

One of the primary applications of cross-borehole seismology is the enhanced characterization of petroleum reservoirs in existing fields. Fewer frontier fields remain to be discovered, and existing fields may still hold up to two-thirds of their petroleum. The geologic detail needed to properly exploit most hydrocarbon reservoirs substantially exceeds the detail required to find them. For effective planning, drilling, and production, a complete understanding of the lateral extent, thickness, and depth of the reservoir is absolutely essential. This can be accomplished only from detailed seismic interpretation of three-dimensional cross-borehole data integrated with three-dimensional seismic reflection profiling.

A common practice in three-dimensional seismic reflection profiling is to place the seismic detectors at equal intervals and collect data from a grid of lines covering the area of interest. In addition, since many adjacent boreholes are typically available in existing petroleum fields, cross-borehole seismic data can be collected between numerous adjacent boreholes across the site. Integration of cross-borehole and surface seismic data using tomography-based imaging algorithms yields seismic depth sections and parameter characterization of the subsurface geology. The resulting depth sections assist in interpreting the structure (geometry), stratigraphy (depositional environment), and lithology (rock and fluid types) of established hydrocarbon reservoirs. A repeated sequence of cross-borehole surveys as a function of time can aid in monitoring the effectiveness of enhanced oil-recovery methods. Based on integrated tomographic models of the subsurface geology generated from the cross-borehole and surface seismic data, more wells can be drilled in the field at strategic locations, allowing a three-dimensional view of the subsurface to eventually emerge. These data provide petroleum companies with a continuously utilized and updated management tool that impacts reservoir planning and evaluation for years after the surface and cross-borehole seismic data were originally acquired and processed.

Significance

The concept of estimating material properties from data collected around an object has a broad variety of applications. Cross-borehole seismology provides spatially continuous, high-resolution data that are necessary to image reservoir-scale features large distances from a well. These include faults, stratigraphic boundaries, unconformities, porosity, and fracturing. Coupled with seismic tomography, cross-borehole seismology provides an important methodology for investigating subsurface geology.

In addition to measuring one-way transmitted energy in the subsurface, seismic reflection waves can be recorded in cross-borehole surveys. These reflections are significantly better in resolving rock layers than are surface seismic reflections because of the fact that the cross-borehole frequencies are one to two orders of magnitude greater than those of surface surveys. Integration of cross-borehole and surface seismic surveys is especially valuable, particularly for analyzing subsurface velocity structure. Furthermore, the seismic problem of imaging subsurface geology in depth and estimating physical rock parameters can be solved by correlating and integrating cross-borehole and surface seismic data using tomographic algorithms. Numerous geophysical, engineering, and environmental applications have emerged.

Evaluation and exploitation of existing petroleum reservoirs is a major application of cross-borehole seismology. Reservoir complexity produced by spatial heterogeneities in porosity, permeability, clay content, fracture density, overburden pressure, pore pressure, fluid-phase behavior, and other related factors leads to large uncertainties in estimated total recovery. The most feasible approach for mapping these spatial variabilities comes from surface and cross-borehole geophysical measurements that are integrated through seismic tomographic processing. There is little doubt that cross-borehole seismology has played a major role in helping to solve not only exploration problems but also production and recovery problems in the petroleum industry.

In many areas of natural resource exploration and exploitation, numerous drill holes often exist, making cross-borehole seismology an ideal investigation tool. At these sites, a full suite of well-log data, rock-core analyses, and three-dimensional cross-borehole and surface seismic data can be acquired. Employing seismic tomography, these data can be processed, integrated, and interpreted to yield a geological model of subsurface lithology that closely approximates reality. The data acquired using cross-borehole seismology have been successfully processed and interpreted to yield three-dimensional images and lithological estimates of the subsurface geology.

Principal Terms

amplitude: the maximum departure (height) of a wave relative to its average value

imaging: a computer method for constructing a picture of the subsurface geology from acquired seismic data

inversion (inverse problem): using measured data to construct a geological model that describes the subsurface and is consistent with the measured data

lithology: the description of rocks, such as rock type, mineral makeup, and fluid in the rock pores

reflectivity: the ratio of the amplitude of the reflected wave to that of the incident wave

resolution: the ability to separate two features that are very close together

seismic reflection profiling method: measurements made of the travel times and amplitudes of events attributed to seismic waves that have been reflected from interfaces where seismic properties change

seismic tomography: a processing technique for constructing a cross-sectional image of a slice of the subsurface from seismic data

seismology: the study of seismic waves

travel time: the amount of time it takes seismic energy to travel from the source into subsurface geology and arrive back at a seismic detector

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