Aldo T. Mazzella

Geophysics and solvents: The Borden experiment

The clearing in the central Ontario pine forest was hot, sandy, and busy as the two visitors stepped from the rental car and looked dubiously at the scene. A large metal‐clad building dominated the east side of the clearing which was about 50 m across. A white trailer, a tent, a variety of cars and trucks, 45 gallon drums, scaffolding, raised wooden platforms, PVC pipe of all sizes, hoses, electrical cables, flood lights, tool chests and people filled the rest.

Phase and amplitude inversion of crosswell radar data

Phase and amplitude inversion of crosswell radar data estimates the logarithm of complex slowness for a 2.5D heterogeneous model. The inversion is formulated in the frequency domain using the vector Helmholtz equation. The objective function is minimized using a back-propagation method that is suitable for a 2.5D model and that accounts for the near-, intermediate-, and far-field regions of the antennas. The inversion is tested with crosswell radar data collected in a laboratory tank. The model anomalies are consistent with the known heterogeneity in the tank; the models relative dielectric permittivity, which is calculated from the real part of the estimated complex slowness, is consistent with independent laboratory measurements. The methodologies developed for this inversion can be adapted readily to inversions of seismic data (e.g., crosswell seismic and vertical seismic profiling data).

Frequency-domain Green’s functions for radar waves in heterogeneous 2.5D media

Green’s functions for radar waves propagating in heterogeneous 2.5D media might be calculated in the frequency domain using a hybrid method. The model is defined in the Cartesian coordinate system, and its electromagnetic properties might vary in the x - and z -directions, but not in the y -direction. Wave propagation in the x - and z -directions is simulated with the finite-difference method, and wave propagation in the y -direction is simulated with an analytic function. The absorbing boundaries on the finite-difference grid are perfectly matched layers that have been modified to make them compatible with the hybrid method. The accuracy of these numerical Green’s functions is assessed by comparing them with independently calculated Green’s functions. For a homogeneous model, the magnitude errors range from −4.16% through 0.44%, and the phase errors range from −0.06% through 4.86%. For a layered model, the magnitude errors range from −2.60% through 2.06%, and the phase errors range from −0.49% through 2....

Numerical study of electromagnetic waves generated by a prototype dielectric logging tool

To understand the electromagnetic waves generated by a prototype dielectric logging tool, a numerical study was conducted using both the finite-difference, time-domain method and a frequency-wavenumber method. When the propagation velocity in the borehole was greater than that in the formation (e.g., an air-filled borehole in the unsaturated zone), only a guided wave propagated along the borehole. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave radiated electromagnetic energy into the formation, causing its amplitude to decrease. When the propagation velocity in the borehole was less than that in the formation (e.g., a water-filled borehole in the saturated zone), both a refracted wave and a guided wave propagated along the borehole. The velocity of the refracted wave equaled the phase velocity of a plane wave in the formation, and the refracted wave preceded the guided wave. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave did not radiate electromagnetic energy into the formation. To analyze traces recorded by the prototype tool during laboratory tests, they were compared to traces calculated with the finite-difference method. The first parts of both the recorded and the calculated traces were similar, indicating that guided and refracted waves indeed propagated along the prototype tool.

Frequency domain, waveform inversion of laboratory crosswell radar data

A new waveform inversion for crosswell radar is formulated in the frequency-domain for a 2.5D model. The inversion simulates radar waves using the vector Helmholtz equation for electromagnetic waves. The objective function is minimized using a backpropagation method suitable for a 2.5D model. The inversion is tested by processing crosswell radar data collected in a laboratory tank. The estimated model is consistent with the known electromagnetic properties of the tank. The formulation for the 2.5D model can be extended to inversions of acoustic and elastic data.

Helicopter geophysical survey to detect brine, Brookhaven oil field, Mississippi

Brine produced along with oil at the Brookhaven oil reasons: 1) low altitude flying requires a high degree of maneuverability to avoid numerous populated areas, 2) slow field has contaminated near surface and deeper (at least 100 flight speed provides optimum spatial resolution of m) aquifers. Sources of cultural noise, typical of many oil fields, can interfere with ground and airborne geophysical geophysical signals, 3) mapping of both shallow ( lOO m) subsurface features, and 4) provision of measurements used to map subsurface brine. The first phase of this study, reported here, is to evaluate application of ancillary geophysical measurements uch as VLF (Very Low Frequency) and high resolution magnetic field sensors. airborne electromagnetic (AEM) methods to determine In planning the HEM survey, we used a set of 44 whether any useful AEM data can be collected under Wenner DC resistivity soundings made in and near the oil extremely adverse conditions of heavy cultural noise and a field (Nacht and Barrows, 1985):Interpretation of soundings low resistivity earth. using algorithms of Zohdy and Bisdorf (1975) along a NorthFor this survey a helicopter electromagnetic (HEM) South profile paralleling section C-C’ (Figure 1) are system with a horizontal coil configuration was employed presented as a depth/resistiy 1) 900, resistivity associated with known brine contamination and 7,200, and 52,000 Hz active EM system using horizontal coil suggest areas for additional study. pairs in order map the broad range of resistivities anticipated, 2) high precision total magnetic field sensor, 3) nominal flight BACKGROUND line spacing of 220 meters at an altitude of 60 meters, 4) a The Brookhaven oil field (Figure 1) is one of the high precision UHF navigation system with a backup oldest fields in Mississippi (Kalkhoff, 1986). Since 1943, photographic record of the flight path, and 5) VLF and approximately 54.2 million barrels of brine have been power line monitoring (vertical and horizontal coils) passive pumped to the surface from at least 75 wells. At first, the EM systems. brine was disposed of by pumping it onto the ground or into a nearby stream. Later it was pumped into evaporation pits. TOTAL MAGNETIC FIELD DATA Since 1978 when the above disposal practices were prohibited, The magnetometer system used in this survey consisted brine has been re-injected by Class II wells into the deep oil of a high sensitivity (0.01 nT) airborne sensor. During the producing formations at least 1.5 Km deep. Past and present survey, the magnetic sensor produced a 2-5 nT sinusoidal disposal methods pose a threat to the quality of near surface noise envelope with a wavelength of about 8 seconds along water supplies. Saline waters might seep to the surface the flight path in part caused by motion of the cesium sensor through improperly cased or plugged wells or through within the low inclination (600) local magnetic field. subsurface vertical fractures. The IGRF and a strong east to west regional gradient Gravels of the basal Citronelle Formation and sand were removed from the data in order to enhance local layers in the Hattiesburg Formation are the main aquifers in anomalies. The contour map (Figure 4) shows many short the study area (Figure 2). AI1 of the aquifers in the oil field wave-length anomalies (on the order of 100 m) having have been contaminated by brine to a depth of at least 100 positive amplitudes ranging from 10’s to 100’s of nT. Many m (Kalkhoff, 1986). Contamination outside of the oil field is of these anomalies are associated with oil tanks (100-200 nT unknown. For example, brine can move laterally from the anomalies in the center of the oil field) and steel well Citronelle aquifer to discharge into nearby streams and can casings (several of the smaller anomalies). Frischknecht and move vertically into underlying Hattiesburg aquifers. others (1985) discuss applications of airborne magnetics to locate abandoned cased oil. wells. Certain other cultural DESIGN OF AIRBORNE GEOPHYSICAL SURVEYS features (steel buildings, large machinery, and pipelines) cause Both fixed wing time domain (INPUT) and helicopter some of the magnetic anomalies. frequency domain AEM systems were considered for this Several small circular anomalies are distributed in a survey (Becker and Morrison, 1987). A helicopter way that suggesting a regular drilling pattern (Figure 4) that

Reply to the discussion

We wish to address the concerns raised by Bulnes et al. regarding the magnetic dipole and the associated magnetic current density. These entities are mathematical constructs for calculating electromagnetic fields.

Evaluation of Geophysical Methods For the Detection of Subsurface Tetrachloroethylene In Controlled Spill Experiments

A controlled Tetrachloroethylene (PCE) spill experiment was conducted in a multi-layer formation consisting of sand and clayey-sandlayers. The purpose of the work was to determine the detection limits and capability of various geophysical methods. Measurements were made with ten different geophysical techniques before, during, and after the PCE injection. This experiment provided a clear identification of any geophysical anomalies associated with the presence of the PCE. During the injection period all the techniques indicated anomalies associated with the PCE. In order to quantify the results and provide an indication of the PCE detection limits of the various geophysical methods, the tank was subsequently excavated and samples of the various layers were analyzed for residual PCE concentration with gas chromatography (GC). This paper presents some of the results of five of the techniques: cross borehole complex resistivity (CR) also referred to as spectral induced polarization (SIP), cross borehole high resolution seismic (HRS), borehole self potential (SP), surface ground penetration radar (GPR), and borehole video (BV).

Electrical resistivity variations associated with controlled gasoline spills

Summary A number of geophysical surveys were conducted over two controlled releases of about 100 gallons each of gasoline. In order to clearly identify the responses associated with the gasoline plume, measurements were made before, during and after the injection. The two experiments were conducted about three years apart in the same geologic cell at the controlled tank facilities at the Oregon Graduate Institute in Beaverton , OR. The results of high resolution (2.5 cm spacing) downhole electrical resistivity measurements are presented in this paper. An increase in the electrical resistivity in the capillary zone above the water table was observed with the injected gasoline plume in both experiments. However, the magnitude and shape of the anomaly were considerably different in the two experiments and appear to be associated with the properties of the capillary zone and history of water infiltration prior to the gasoline injection. In order to study longer term responses, measurements were taken 7 months later in the second experiment. These results proved difficult to interpret because considerable rainfall in the area flooded the cell a number of times, causing movement and smearing of the gasoline plume.

Reflections from geologic interfaces shallower than 30 m at the Pittman Lateral, Henderson, Nevada

20 m at the Pittman Lateral in Henderson, Nevada. The single 12-fold, 260 m long reflection profile was acquired with a 0.6 m station spacing. Appropriate recording well numbers equipment and parameters were critical to the success of the study. Parameter selection during data processing 631 / 629 627 625 enhanced reflection energy difficult to identify on un•j t?J Ki q processed shot gathers. Three reflection events are I I t -----IL I I interpreted on the CDP stacked section with vertically c