H. F. Morrison

Borehole-to-surface electrical resistivity monitoring of a salt water injection experiment

A field experiment was conducted at the University of California Richmond Field Station to demonstrate the sensitivity of borehole‐to‐surface resistivity measurements in groundwater investigations. A quantity of saline water was injected into a fresh water aquifer while the resistivity was monitored using a multichannel borehole‐to‐surface system. Two experiments were conducted using pole‐pole and pole‐dipole receiver electrode arrays. The data from the pole‐pole experiment were superimposed to simulate a dipole‐pole array and the data from the pole‐dipole array were superimposed to simulate a dipole‐dipole array. This superposition of the data was done to enhance the anomaly and facilitate interpretation. A numerical modeling study was performed in conjunction with the field experiment in order to interprete the results. A three‐dimensional modeling program was used to simulate the geological setting of the field experiment and the salt water injection. This modeling revealed that an asymmetric displacem...

A hybrid three‐dimensional electromagnetic modeling scheme

We present an efficient numerical method for computing electromagnetic (EM) scattering of arbitrary three‐dimensional (3-D) local inhomogeneities buried in a uniform or two‐layered earth. In this scheme the inhomogeneity is enclosed by a volume whose conductivity is discretized by a finite‐element mesh and whose boundary is only a slight distance away from the inhomogeneity. The scheme uses two sets of independent equations. The first is a set of finite‐element equations derived from a variational integral, and the second is a mathematical expression for the fields at the boundany in terms of electric fields inside the boundary. The Green’s function is used to derive the second set of equations. An iterative algorithm has been developed to solve these two sets of equations. The solutions are the electric fields at nodes inside the finite‐element mesh. The scattered fields anywhere may then be obtained by performing volume integrations over the inhomogeneous region. The scheme is used for modeling 3-D inho...

Geophysical exploration with audiofrequency natural magnetic fields

Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers. After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field. This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. ...

A theoretical study of surface‐to‐borehole electromagnetic logging in cased holes

A new electromagnetic logging method, in which the source is a horizontal loop coaxial with a cased drill hole and the secondary axial fields are measured at depth within the casing, has been analyzed. The analysis, which is for an idealized model of an infinite pipe in a conductive whole space, has shown that the casing and formation are uncoupled at the low frequencies that would be used in field studies. The field inside the casing may be found by first finding the field in the formation and then using this field as an incident field for the pipe alone. This result permits the formation response to be recovered from the measured field in the borehole by applying a correction for the known properties of the casing. If the casing response cannot be accurately predicted, a separate logging tool employing a higher frequency transmitter could be used to determine the required casing parameters in the vicinity of the receiver. This logging technique shows excellent sensitivity to changes in formation conduct...

Audio‐frequency electromagnetic tomography in 2-D

The mathematical formulation of acoustic diffraction tomography is applied to the problem of low frequency, diffusive electromagnetic (EM) fields. EM tomographic inversion, in two-dimensional (2-D) Cartesian geometry, is illustrated for a crosshole source-receiver configuration. The object function of the conductivity distribution is related to the transformed and filtered data by an inverse Fourier transform in the vertical direction and an inverse Laplace transform in the lateral direction. The reconstructed conductivity image is found to be a band-limited version of the actual conductivity distribution. To stabilize the inversion, a regularized least-squares method is used for image reconstruction. As in the seismic case, the inversion quality can be understood by inspecting the wavenumber domain coverage of the object function. Numerical experiments show that the resolution is better in the vertical direction than in the horizontal and it is also a function of source operating frequency. The position and attitude of the target are recovered well.

Transient fields of a current loop source above a layered earth

The electric field induced within four layered models by a repetitive current wave form in a circular loop transmitter is presented along with the resulting magnetic fields observed on the surface. The behavior of the induced electric field as a function of time explains the observed sign reversal of the vertical magnetic field on the surface. In addition, the differences between magnetic field responses for different models are explained by the behavior of the induced electric fields. The pattern of the induced electric field is shown to be that of a single “smoke ring,” as described by Nabighian (1979), which is distorted by layering but which remains a single ring system rather than forming separate smoke rings in each layer.

Airborne electromagnetic bathymetry

Airborne electromagnetic techniques provide an efficient means for determining the bathymetry of shallow seas and coastal regions. Offshore time‐domain data collected with the INPUT® system may be interpreted quickly and inexpensively using the simple table look‐up algorithm described here. This interpretation algorithm is based on two dimensionless parameters which depend on the six channel amplitudes of the recorded INPUT data. These amplitudes are computed and tabulated for a suite of simple two‐layer models in which seawater depth is the only variable quantity. The experimental data can then be interpreted by comparison with the tables of theoretical values to obtain the bathymetry beneath the flight path. The inversion algorithm has proven accurate and stable. In the range of 0–40 m of seawater, the representative difference between the interpreted depths and the charted depths is about 2 m. Bottom gradients lower than 5 percent were resolved accurately beneath 0–25 m of water. Independent interpreta...

Self-potential studies at the cerro prieto geothermal field

A reconnaissance self-potential survey conducted at the Cerro Prieto geothermal field revealed a dipolar anomaly of about 160 mV maximum peak-to-peak amplitude and covering an area of more than 400 km2. The anomaly is centered close to the present production zone, and its axis runs nearly due north-south, passing roughly through the location of the present 75-MW power plant. Preliminary theoretical analysis shows that a horizontally dipolar subsurface charge distribution, whose intensity increases with depth along a vertical plane, produces anomalies similar to that seen at Cerro Prieto. Such a charge distribution may be produced by the vertical flow of either heat or fluid parallel to a vertical discontinuity, if either the thermoelectric or electrokinetic coupling coefficients change across the discontinuity. Stratigraphic and geophysical evidence indicate that the trace of a fault, which extends to a depth of about 3 or 4 km and is thought to provide a major conduit for the geothermal fluids, coincides with the axis of the self-potential anomaly a few hundred meters north of the power plant. Thus, a vertical flow of heat and/or fluid along this fault could provide an explanation for the observed self-potential anomaly if the magnitudes of the flows and the coupling coefficient differences are large enough. We are presently measuring thermoelectric and electrokinetic coupling coefficients of core samples from Cerro Prieto, and will use these values in conjunction with temperature, fluid flow and electrical-resistivity data from the field to calculate anomaly amplitudes to be compared with the observed self-potential data.

Electric field response of two-dimensional inhomogeneities to unipolar and bipolar electrode configurations

A quantitative comparative analysis of some bipolar and unipolar resistivity arrays is made in terms of perturbation effects due to buried lateral inhomogeneities. The Schlumberger, the collinear dipole‐dipole, the pole‐dipole, and the unipole electrode configurations of approximately the same dimensions are employed. A “focusing effect” in the vertical component of the current density distribution is demonstrated in the unipole configuration. Characteristic diagrams illustrating the configuration responses to lateral inhomogeneities located at different depths and under various overburden layer conductivities indicate that, for comparable geometric spreads, the unipole and the pole‐dipole configurations are significantly more effective.

A SOLUTION FOR TM-MODE PLANE WAVES INCIDENT ON A TWO-DIMENSIONAL INHOMOGENEITY

Quantitative interpretation of magnetotelluric (MT) surveys depends at present on the availability of an efficient forward modeling algorithm. To date, two major numerical techniques have been used to obtain the scattered fields from buried inhomogeneities in plane-wave fields: methods solving the governing differential equation which generally uses a finite-element or finite-difference approach, and methods which solve an integral-equation formulation of the problem. For two-dimensional (2-D) inhomogeneities a solution for incident fields with the electric field parallel to the strike of the inhomogeneity (TE mode solution) was developed by Hohmann (1971) using the integral-equation approach. For a perfect conductor an integral formulation, for surface scattering currents, for the TM mode (magnetic field parallel to the strike of the inhomogeneity) was developed by Parry (1969). General 2-D solutions in the presence of an arbitrary mode plane wave (mixed TE-TM) were obtained by Ryu (1970), Swift (1971), and Rijo (1977) using either a finite-element or finite-difference technique.