Exploration Geophysics - Volume 29, Issue 3-4, 1998

The usage of computer algebra is in its infancy in many fields but is likely to increase as the speed and practicability of computer generated analytical solutions improves. This paper discusses the use of computer algebra to solve ray-tracing problems in simple seismic reflection models. The classical two-point problem, of finding ray travel times as a function of source receiver distances for multi-layer models, does not have a complete explicit solution except in the simplest of cases or where unrealistic assumptions are made. Approximation methods have therefore been adopted to obtain numerical solutions. Such methods, however, have the disadvantage of sometimes requiring remodelling and recalculation if parameters are altered. If analytical solutions can be found, they could incorporate parameters of the model as variables and the expressions only need re-evaluation rather than a complete re-determination. The re-evaluation would be based on exact expressions and not on approximations such as are used now.

Exact parametric solutions can be derived for three-dimensional models with multiple layers having different acoustic velocities, with reflections and refraction at interfaces according with Snell's Law. These solutions have been generated using subroutines for the Maple computer algebra system which simulate three-dimensional ray tracing through acoustic media and for the interactions with interfaces. The solutions result in the generation of long expressions, which evidently could not be reasonably found by hand calculation. The symbolic solutions give travel time, ray position and ray direction as parametric equations using initial ray direction as the parameters. When a numerical receiver position is entered, travel time and ray-path solutions are returned by solution of the parametric equations.

The subroutines are general and could be used to generate mathematical expressions for other simple earth models. For more complex models the analytical solutions may yield very long expressions. Geophysicists will find further applications for computer algebra packages for problems which are currently solved by numerical and approximate methods.

Modern underground coal mining requires detailed knowledge of geological faults and other geological features. A fault of throw greater than a seam thickness of about 3 m can cause enormous delays to mine production. BHP Coal has been using 2D seismic surveying for some time to map structures with throws down to about 5 or 6 m but in an effort to obtain better resolution (mainly through seismic coverage), 3D seismic surveys are now being undertaken.

For this application, the strength of 3D seismic surveying lies in the ability to establish the continuity of subtle features from one bin section to the next. Maps of the interpreted horizons allow very small features to be seen. Attribute analysis adds further structural and lithological information to the interpretation.

The present indications are that 3D seismic surveying is set to become a key tool in coal mine exploration.

The continental margin facing the southern Tasman Sea off southeastern Australia is of high petroleum and geological significance because it contains Australia’s richest oil province, the Gippsland Basin. Exploration activity in the region has concentrated on the shelfal part of this basin. Off Tasmania the upper margin is host to important deep sea trawl fisheries, and regional geophysical data suggest some long-term petroleum potential.

In early 1997, 20,000 km² of the margin were surveyed by the Australian Geological Survey Organisation, in cooperation with Scripps Institution of Oceanography and the University of Sydney. The SeaBeam 2000 multibeam sonar system of the US RV Melville was used to map the topography and character of the seafloor in unprecedented detail. The SeaBeam 2000 is a 12 kHz, 121-beam echo-sounder that collects both bathymetry and backscatter (sidescan) data across a swath of ~3.4 times the water depth. The survey also collected 3500 km of magnetic and gravity profile data.

The margin off Gippsland is dominated by a large embayment 100 km across and floored by a ESE-trending chasm, the Bass Canyon, which is 60 km long and 10-15 km wide. The canyon has cut down about 2 km into the margin and is bounded to the north and south by very steep inner canyon walls 1000 m high. Sediments transported down the canyon debouch at its mouth, at~4000 m depth, and spread onto the abyssal plain via a network of distributary channels and levees. Sediment from the shelf is channelled into the entrance of Bass Canyon by three major, deeply-incised tributary canyons and a number of smaller ones. Bass Canyon has probably acted as a conduit for clastic sediment to the deep sea floor since breakup at ~80 Ma. Seismic profiles over the Bass Canyon complex show that canyon erosion has exposed sections of almost the entire Gippsland Basin sequence down to the Early Cretaceous and underlying basement. Numerous suitable dredge and core sampling sites in the canyons have been identified for a geological sampling program.

Off east Tasmania, the survey showed the margin to be generally steep and rugged, with relatively little sediment having been deposited on the continental slope, except in narrow rifts, since breakup. The adjacent abyssal plain is commonly underlain by 2 km of Late Cretaceous-Cainozoic sedimentary section. The continental slope is cut by an extensive system of canyons, some more than 30 km long and 500 m deep. Large fault blocks on the lower slope show structural trends consistent with a NW-NNW rift direction and NE-ENE transfer direction. The upper slope appears to be underlain by a subsided Miocene carbonate shelf. Volcanics, of probable late Cainozoic age, are fairly widespread on the seafloor off northeast Tasmania. Such volcanic terrain includes a number of scattered cones, the largest 450 m high.

The Australian stress map project has compiled 357 quality ranked stress orientation analyses for the Australian Continent (including New Guinea). Of these, 206 provide reliable stress orientations, approximately doubling the number from the 1992 world stress map compilation. Most new data are from borehole breakouts. Regionally, maximum horizontal stress (@) is oriented northeast-southwest from New Guinea along most of the North West Shelf, rotating to 100°N in the Carnarvon Basin. An 010°N-020°N @ orientation is observed in the Amadeus and Bowen Basins. However, in the southern half of the continent a broad east-west @H trend is observed in the Yilgarn Block, Cooper-Eromanga Basins, and from a number of isolated indicators. In the Otway and Gippsland Basins @ is oriented 130°N. The rotation of stresses along the North West Shelf and from east-west in the Yilgarn Block to north-south in the Amadeus Basin can be explained in the context of the heterogeneous plate boundary forces acting along the convergent plate boundary to the north of Australia. However, plate boundary forces can not explain the rotation of @ from east-west in the Cooper-Eromanga Basins to approximately north-south in the Amadeus Basin, which may be linked to second order influences on the stress field.

The vertical stress (σv) gradient in the Bonaparte and Cooper-Eromanga Basins increases with depth, and is around 20 MPa/km at 1 000 m, attaining 23 MPa/km around 3 000 m. The Amadeus Basin displays an overburden gradient of 25 MPa/km that is little affected by depth. In situ measurements in hard rock terranes suggest a higher average overburden stress gradient of 27 MPa/km. Leak-off pressures suggest that the minimum horizontal stress (σh) is the least principal stress (60-70% of σv) in the Bonaparte and Cooper-Eromanga Basins. Hence in neither basin is the stress regime one associated with reverse faulting (where σH σh σv). Consideration of the frictional limits to faulting suggests that, if in a state of incipient faulting, the stress regime is approximately on the boundary between normal (σv σh σh) and strike-slip (σh σv σh) faulting environments in the Bonaparte Basin and strike-slip in the Cooper-Eromanga Basins. Applications of the stress data include assessing both natural and induced fluid flow directions in the subsurface. For example, hydraulic fractures induced for geothermal exploitation of hot-dry-rock in the Cooper-Eromanga Basins would tend to be vertical, and not, as previously suggested horizontal.

The quiet daily magnetic variation, denoted Sq, occurs as a background signal during magnetic surveying. The morphology of Sq is dependent on a number of factors, particularly latitude. Type curves that describe the latitudinal dependence of the horizontal and vertical components of Sq on a global scale appear in many texts. This paper first describes a recent compilation of global Sq curves for the total magnetic field, which is the component of most relevance to magnetic mapping.

An analysis is then made of total-field variations from a north-south line of stationary recording magnetometers which operated across central Australia as part of the AWAGS experiment of 1989-1990. These data are analysed for information on the magnetic daily variation across the Australian continent, particularly its latitudinal variation. Observations cover a full year and their analysis is divided into four seasons to show variation with season as well as with latitude. The observed data show a minimum in the total-field signal in the geomagnetic latitude band 20°–30°, in support of the global Sq total-field curves. There is also clear evidence in the AWAGS data for the path of the Sq focus across Australia, identified by an amplitude minimum in the daily variation of the horizontal magnetic north component. The band in which the total-field Sq variations are generally reduced is termed the total-field “doldrums”; this band is on the equator-side of the path of the Sq focus.

In both the domestic and overseas mineral exploration projects of the Metal Mining Agency of Japan (MMAJ), many holes have been drilled in order to core for geological logging and laboratory analysis. However drill holes themselves are seldom used to obtain additional information except for occasional geophysical logging such as resistivity, self-potential and induced polarisation (IP). Therefore the MMAJ has been developing an IP tomography data acquisition system since 1992 for the purpose of effectively utilising drill holes, reducing exploration cost and improving accuracy of geophysical exploration.

This system is based on time-domain measurement and adopts a pole-dipole electrode configuration to avoid coupling effects compared to a pole-pole array and to deploy the electrodes more easily than a dipole-dipole array. It consists of a transmitter, receiver, personal computer, controller, non-polarisable electrodes, ground cables and borehole cables, and has the following features:

It acquires resistivity and chargeability data at 10 sites simultaneously.

It acquires very accurate data because of an operational amplifier at each electrode in the borehole cable and the relay box.

It acquires the data automatically through the controller, computer and relay boxes.

In 1997, the MMAJ conducted an IP tomography test survey in order to examine the applicability of the system for mineral exploration in the Barrier Main Lode, located northeast of Broken Hill, NSW, Australia. The quality of the acquired data was excellent because of high sensitivity of the system and absence of culture noise such as power lines. The data was analysed with the inversion technique based on the alpha centers method and the non-linear least square method. In the reconstructed resistivity and chargeability images, the ore body appears as a low resistivity zone and as a high chargeability zone. The reconstructed images are consistent with the result of IP logging although the reconstructed resistivity is lower than that obtained by IP logging. It is confirmed that the reconstructed chargeability image can delineate a massive sulphide ore deposit such as the Barrier Main Lode more clearly than the reconstructed resistivity image.

The effect of clay content on elastic wave velocity is an important factor which should be considered in porosity estimation from sonic log data. Over the past ten years several investigators have reported experimental results of ultrasonic P-wave velocity measurements in clastic rocks and a number of empirical relationships have been proposed for velocity as a function of porosity and clay content for sandstones. These studies provide a better understanding of the complex nature of elastic wave velocities in reservoir rocks. However the practical aspects of applying the empirical equations derived from these studies have not been examined. This paper briefly reviews some of the most often-cited empirical velocity/porosity equations and compares them with commonly used sonic porosity methods for a series of gas-bearing shaly sandstones from the Cooper Basin in South Australia. None of these empirical equations present a globally acceptable method for porosity estimation from the sonic log. The shortcomings of these equations are more noticeable when they are used for gas-bearing reservoirs in shaly sandstones with low to moderate porosities. The application of some of these equations to field data in the Cooper Basin implies that their reliability when used on other datasets should be considered suspect until a good calibration can be achieved.

Compressional and shear wave velocities were measured as a function of effective pressure on a set of cores from gas producing reservoirs in the Cooper Basin in South Australia. The aim of this study was to investigate the relationship between the stress sensitivity of acoustic velocities and geological and petrophysical characters of low to medium porosity shaly sandstones. The suite of samples consists of 22 core plugs from depths between 1917 m and 2564 m with helium porosity ranging from 2.6% to 16.5%. The samples are mainly fine to medium-grained sublitharenite. The echo-pulse technique is used to measure ultrasonic velocity and attenuation in dry and fully water-saturated cores under effective pressures of 5 MPa to 60 MPa. Experimental results show that compressional and shear wave velocities increase non-linearly with effective pressure. The influence of pressure was more pronounced below 40 MPa for both compressional and shear wave velocities. The rapid increase in wave velocities with effective pressure between 5 MPa and 40 MPa is attributed mainly to the closure of low aspect ratio pores such as micro-cracks and loose grain contacts within the rock framework. Compressional wave velocity in dry samples shows stronger pressure sensitivity, whereas there is no significant difference between the stress dependency of shear wave velocity in dry and water saturated samples. There is a positive correlation between change in velocity with pressure and core porosity and permeability. However, this association is weak and diminishes with increasing effective pressure. The pressure dependence of acoustic velocity in the Cooper Basin samples is not related to total clay content, but samples with a greater amount of pore filling and grain-coating clay particles appear to be less pressure sensitive at dry condition. Cooper Basin sandstones show higher pressure sensitivity compared to data from other studies.

Hydrocarbon production may cause changes in dynamic reservoir properties including pressure and fluid saturation. Understanding the magnitude of such variations is essential for the exploration of new reserves and optimising the performance of existing fields. Laboratory measurements of acoustic properties of representative rock samples, simulating in situ pressure and fluid saturation, provide a useful guide for calibrating and interpreting seismic and sonic log data. A petro-acoustic study was carried out to investigate factors influencing acoustic properties of Cooper Basin sandstones. This study integrated field data and petrography with laboratory measurements of acoustic properties of representative rock samples. In a previous paper we highlighted experimental data; here we elaborate on implications of these results in predicting rock property and pore fluid type from sonic logs and seismic reservoir monitoring. Analysis of data from acoustic measurements at ultrasonic frequencies on samples from the Cooper Basin reveals that the pressure dependency of the Cooper Basin rocks is very large. The velocity-pressure relationship obtained from laboratory data is consistent with the sonic log anomaly observed in partially pressure-depleted reservoirs in the Cooper Basin. Neglecting the pressure effect on velocity results in the overestimation of rock porosity by the sonic log in overpressured formations, and underestimation of porosity in pressure depleted zones. A method is proposed to correct the sonic log reading for pressure variation in the study area. At in-situ reservoir pressure Vs and Vp are strongly correlated and dry and water-saturated samples show significantly different velocity ratios (Vp/Vs). The Vp/Vs ratio is not affected by porosity and clay content and therefore has potential as a gas indicator in the region. The strong stress sensitivity and the distinct Vp/Vs. values for dry and water saturated Cooper Basin cores suggest that the dynamic changes in pressure and saturation of the reservoir rocks may also be detectable from acoustic impedance or travel time at seismic and sonic log frequencies.

Aeromagnetic surveys have been flown for fifty years as one of the methods providing information for hydrocarbon exploration. The method was generally regarded as a reconnaissance tool looking for major structures. Major advances have been made in the last decade in the quality of aeromagnetic data so that low amplitude anomalies arising from magnetic bodies within the sedimentary section can now be measured with sufficient precision to provide information about the geology of the sedimentary rocks associated with hydrocarbon accumulations. This presents a challenge to the interpreter to develop better interpretation procedures to take full advantage of the high quality magnetic data. Recent developments in computer technology, instruments, processing and interpretation of the earth’s magnetic field have significantly extended the scope of aeromagnetic surveys as a tool in the exploration for hydrocarbons. The new approach used in the analysis and interpretation of aeromagnetic survey data over sedimentary basins allows interpreters to fully utilise information carried by the magnetic field data. The high quality experimental aeromagnetic survey flown over part of the Eromanga and Cooper basins by Mines and Energy South Australia and Santos, shows that magnetic layers in the sedimentary rocks make this an appropriate area for exploration by detailed aeromagnetic surveys. By applying new interpretation techniques it is now possible to make fuller use of the high quality digital magnetic data. It is possible to delineate major structures within a weakly magnetic basement, to follow magnetic horizons within the sedimentary section, to pick out fault and joint patterns within the sediments, to detect abnormal magnetic regions which may be the result of alterations caused by hydrocarbon seepages and to compute the thickness of the sediments. The aeromagnetic method is very effective for interpolating between widely spaced seismic lines that are used to tie the magnetic interpretation to the stratigraphy. The depth to basement estimates made in this way are comparable with those derived from seismic survey data. Since the completion of the experimental research the methods developed have been successfully applied to several basins including ones in low latitudes, rugged terrains and volcanic provinces.

The new approach to interpretation of aeromagnetic survey data is providing petroleum companies with quick, comprehensive and cost-effective tools for first-pass exploration. The non-invasive nature of the surveys makes them ideal for use in difficult terrains and in politically and environmentally sensitive areas.

The revival in gravity methods stimulated by the availability of reliable positioning and, more importantly, elevation estimation, using relatively low cost GPS methods has transferred the focus on survey accuracy and precision from elevation errors to those implicit in the complete topographic correction. The terrain correction has normally been ignored in low relief terrain since the error in doing so has usually been much less than that related to errors in elevation accuracy. It is now possible to specify and observe gravity surveys that can yield a precision of 0.01 to 0.03 mGal in the Bouguer anomaly in production practice. Unfortunately even quite minor aberrations in local topography near the gravity meter may generate errors in excess of 0.1 mGal and no tight specification is justified unless it also demands detailed description of the local terrain, outlines how to avoid problems, and requires calculation of such corrections as are necessary. This will require a change in observer culture and some suggestions of the scale of the problem and means for avoiding it are given in this paper. There is no doubt that the terrain correction is now the major source of deficiency in gravity survey data bases. The methods used for calculation of the terrain correction also need review and care since digital terrain models are not necessarily of adequate precision or reliability for calculation of the effects close to the observation point.

The giant gold deposit of Porgera, located in the Enga Province in highland Papua New Guinea, had a 1997 reserve of 82.6 Mt at 4.4g/t, or 11.6 Moz. The high-grade mineralisation, most of which has been mined out, is associated with quartz-roscoellite veining along the Romane Fault Zone (Zone VII).

Early prospecting in the late 1930s discovered alluvial gold in the Porgera valley. From 1964 to 1983 the Waruwari deposit was defined by drilling east-oriented holes. In 1983 the Romane Fault mineralisation was discovered using drill holes oriented to the south.

While geochemistry and drilling have been credited with the discovery of zone VII, geophysics has played an integral part in defining the extent of the Porgera Intrusive Complex. If used earlier in the exploration process, geophysics may have led to the discovery of zone VII before more than 200 holes were drilled into the deposit.

Geophysics has been used extensively in the exploration program at Porgera to target buried intrusives. Airborne magnetics has successfully mapped the intrusives, along the contacts with which mineralisation is associated. IP and radiometrics have defined the extent of hydrothermal alteration and a combination of 3D magnetic modelling and Audio MagnetotelluricsAMT has defined the system at depth in the latest round of work.

Numerical modelling studies have been used to devise a way to recover the average P-wave velocity field and the following anisotropic elastic parameters for layered transversely isotropic media: the vertical P-wave velocity (α0), the vertical S-wave velocity (ß0), the P-wave anisotropy (ε)and the near-vertical P-wave anisotropy (δ).

Horizontally layered models comprising transversely isotropic materials with vertical symmetry axes (VTI materials) and isotropic materials were used in computer simulation experiments. It is difficult, even under ideal conditions, to obtain average values of ε and δ experimentally from multi-layered media. Hence in our numerical simulations, these were obtained by inversion of transmission data. A new double precision inversion code has been developed to invert travel time data to recover the average elastic parameters ε and δ. The average vertical P and S-wave velocities, α0, ß0directly determined from travel time data. Subsequently, using the average parameters to the top and to the bottom of a layer of interest, the interval parameters of that layer were recovered using a new least-squares algorithm. From the individual parameters for each layer, we may also compute the overall average velocity field and parameters for the whole multilayer model.

Comparison of the inversion results with directly calculated averages indicated that such multi-layered media can be described as a single layer VTI medium except at large incident angles. Simple relationships between the individual and overall average layer parameters were found. A good knowledge of both the individual layer and the overall layer anisotropic parameters, and velocity field may lead to improved seismic data processing and hence, more accurate data interpretation. We expect that this will result in a significant enhancement in seismic resolution and delineation of reservoir volume estimates.

In many applications of electrical geophysics, the electrodes used to detect signal seem to be an important source of noise. To identify which electrodes were most stable for electric field measurements; an international campaign to evaluate electrodes for long-term monitoring of the telluric field was held at Garchy, France from April 1995 to April 1996. More than fifty different electrode pairs from researches around the world were tested in the campaign. Most of electrodes were metal-electrolyte designs, but with different construction details. The inter-comparison was carried out through laboratory and field experiments. The results of the experiment showed a surprising variation between the performances of different electrodes. Stability as determined in a brief laboratory experiment proved to be an excellent predictor of long-term field stability. Instabilities of differing qualitative character were observed during the year-long experiment. Linear drift may be caused by gradual changes in ground contact, leaking of electrolyte, or changes in chemical concentration of electrolyte within the electrodes. Sudden onset transient noise, which could be as large as 20 mV, is likely to be an electrochemical process such as reaction charge release and charge accumulation within the electrodes. Other physical causes of drift could be due to mechanical contact problems. Of metal electrodes, stainless steel is by far the best.

The conclusions from the Garchy experiment are relevant to exploration geophysics: self-potential surveys are strongly affected by the long-term stability of electrodes and some electrodes may also have internal noise exceeding 1 mV per Hz in the frequency band around 1✓Hz, thus potentially affecting measurements of Induced Polarisation data.

To characterise the geomechanical conditions and responses to underground coal mining in different geological settings, we have undertaken microseismic monitoring at two longwall mines. Our work has concentrated on major concerns of the mines such as the patterns of induced fracturing in the roof and floor, the effect of geological faults and the possibility of water and methane gas inflows. Our results have comprehensively defined the fracture patterns. Applications for microseismic monitoring also exist for the mapping of hydraulic fracture growth in reservoir engineering and the data can be used for the study of shear wave anisotropy in fractured media.

Controlled source audio-frequency magnetotelluric (CSAMT) surveys have found frequent application in mineral exploration, and there have also been a number of attempts to use magnetotelluric (MT) soundings in the same application. The advantages of CSAMT over conventional electromagnetic methods lies in its ability (through measurements of the electric field) to discriminate resistive as well as conductive zones, and in its depth-of-penetration capability.

“Static shift” however can be a common problem in many MT and CSAMT soundings. It severely distorts any depth calculations and degrades the resolution of conductivity structure inferred from scalar CSAMT measurements. Static shift also significantly restricts the MT-method in its use for geological studies of the upper and lower crust; and its application to mineral, hydrocarbon, and geothermal resource exploration. Theoretical investigation using modelling has shown that measurements of an Inductive Source Telluric Field (ISTF) may closely predict and thus potentially remove the static shift at MT and CSAMT stations.

Applications of the method in the field require the additional setup of a low-power transmitter near the survey line. The main survey electrodes, cables and receiver are then used to collect additional data for corrections. This process can be run in parallel with the MT or CSAMT data acquisition, and requires little additional survey time. Initial tests of the concept in field studies near Sydney and over a skarn prospect in the Parkes vicinity proved very encouraging. The ‘along-line variation’ indicative of static shift was reduced by the correction by a factor of about 5 in the Parkes case, and the ‘corrected’ CSAMT section appeared smooth and regular as opposed to having the vertical striping typical of static problems.

This paper describes the effect of source/receiver geometrical arrangements on the lateral resolution when implementing the diffraction stacking process in a crosswell seismic survey. Diffraction stacking with a stacking velocity analysis is equivalent to the Kirchhoff prestack time migration except for performing stacking velocity analysis.

The resolving power of seismic data primarily depends on the dominant frequency of the recorded wavelet and the average velocity of the subsurface. It is proposed that lateral resolving power is also controlled by source/receiver geometrical arrangements. To obtain a better understanding of the concept of the seismic lateral resolution, the problem of seismic imaging is approached from the viewpoint of interference of equi-traveltime planes. Suitable source/receiver geometrical arrangements in homogeneous media are discussed. These studies are illustrated on a numerical simulation model, in which a zero phase Ricker wavelet is used and homogeneous and isotropic media is assumed. As a result, we indicate that source/receiver geometrical arrangements can influence the lateral resolution.

Magnetic effects of the extensive hot-spot related Hart Dolerite, Carson Volcanics, and related feeders that occur within the sedimentary section complicate interpretations of aeromagnetic data in defining the nature of the basement beneath the sediments of the Kimberley Basin, of Northwestern Australia. The magnetic effects of these sources can however be suppressed by upward continuation and stabilised downward continuation to reveal what appears to be a largely granitic basement to the area. Computer modelling has defined the geometry, density, and magnetic characteristics of what may be a remnant of an ophiolite slice emplaced under the southeastern margin of the Kimberley Block during subduction related to the convergence of a Kimberley “microcontinent” with the main Australian continent. Linear gravity lows over the Kimberley Block may define granites formed as a result of the subduction process. Five northeast trending zones of differing lithologies can be identified in the basement on the basis of differing characteristic in their associated magnetic and gravity fields. A conjugate dyke-filled fracture system, apparently related to Devonian-Carboniferous rifting processes, has been superimposed on the block. Suites of these fractures are located over areas interpreted as junctions between zones of different lithological character in the basement.

Dynamic neutron radiography is a method to image fluid flow in porous media, based on the tendency of neutrons to be preferentially attenuated by hydrogen. Fast neutrons with energies of several MeV can be produced by particle accelerators, and are suitable for detecting fluids in rock samples of 0.15 to 0.30 m (15 to 30 cm) thick. Slow (thermal) neutrons with energies of about 0.03 eV are produced by nuclear reactors, and are ideal for imaging fluid flow in rock samples in the order of 0.01 m (1 cm) thick.

Thermal neutrons are attenuated by an exponential law, which relates the incident intensity of a neutron beam (I0) to the transmitted neutron beam (I): I = I0 exp(−ρμh), where ρ is the bulk density of the rock in the neutron path, μ is the bulk neutron attenuation coefficient in the neutron path, and h is the thickness of the rock in the neutron path. The exponent term (ρμh) can be linearly related to the components making up the rock:

ρμh = (ρ1μ1V1 + ρ2μ2V2 + ρmμmVm)h, where V1; is the volume fraction of fluid 1, V2 is the volume fraction of fluid 2, Vm is the volume fraction of the rock matrix, the subscript “m” refers to the rock matrix, and “1” and “2” refer to fluid 1 and fluid 2, respectively.

Knowing that V1 + V2 = ø, which is the rock’s porosity, an estimate of the relative fluid saturations can be made. These are the principal economic quantities required for either a hydrogeological or petroleum reservoir. Examples of water infiltration and petroleum imbibition into the Visingsö Sandstone, Sweden, are provided to illustrate the application of the technique.