An important step in the interpretation of magnetotelluric (MT) data is the extraction of scalar parameters from the impedance tensor Z, the transfer function which relates the observed horizontal magnetic and electric fields. The conventional approach defines parameters in terms of elements of a coordinate‐rotated tensor. The rotation angle is chosen such that Z′(θ) approximates in some sense the form for a two‐dimensional (2-D) subsurface conductivity distribution, with zero elements on the diagonal. There are two major problems with this approach. (1) Apparent resistivities, defined from the off‐diagonal elements of the rotated tensor, are independent of the trace of Z. It is problematic that apparent resistivities, the parameters for which we have physical analogs and which are most heavily used in interpretation, are insensitive to the addition of an arbitrary constant on the diagonal of Z. (2) The conventional parameter set is incomplete; there are two degrees of freedom in Z which are transparent to all parameters. Through a variation of the classical eigenstate formulation of a matrix, it is shown that in general there exist two, and only two, polarization states for which the electric and magnetic fields have the same polarization at perpendicular orientations. For each eigenstate the magnetic and electric fields are related by a scalar, the eigenvalue for that state. This scalar relationship between fields is of identical form to the solution for transverse electromagnetic (TEM) waves in a homogeneous medium and thus provides a physically more satisfactory basis for defining apparent resistivity than the conventional approach using the off‐diagonal elements of the coordinate‐rotated impedance tensor. The eigenstate and coordinate‐rotation methods yield identical results in the limited cases of 1-D and 2-D subsurface conductivity distributions. The eigenstates provide the basis for new definitions of parameters as concise, closed expressions which are complete and more amenable to interpretational insight. The polarization ellipses defined by the eigenstates provide a concise display in real space of all the information contained in the impedance tensor.

Problems and misunderstandings arise with the concept of apparent resistivity when the analogy between an apparent resistivity computed from geophysical observations and the true resistivity structure of the subsurface is drawn too tightly. Several definitions of apparent resistivity are available for use in electromagnetic methods; however, those most commonly used do not always exhibit the best behavior. Many of the features of the apparent resistivity curve which have been interpreted as physically significant with one definition disappear when alternative definitions are used. It is misleading to compare the detection or resolution capabilities of different field systems or configurations solely on the basis of the apparent resistivity curve. For the in‐loop transient electromagnetic (TEM) method, apparent resistivity computed from the magnetic field response displays much better behavior than that computed from the induced voltage response. A comparison of “exact” and “asymptotic” formulas for the TEM method reveals that automated schemes for distinguishing early‐time and late‐time branches are at best tenuous, and those schemes are doomed to failure for a certain class of resistivity structures (e.g., the loop size is large compared to the layer thickness). For the magnetotelluric (MT) method, apparent resistivity curves defined from the real part of the impedance exhibit much better behavior than curves based on the conventional definition that uses the magnitude of the impedance. Results of using this new definition have characteristics similar to apparent resistivity obtained from time‐domain processing.

A large unexplored tectonic basin with the potential for significant hydrocarbon accumulations was identified in north‐central Oregon using a variety of geophysical techniques. The basin, informally named after the local town of Heppner, is covered by several thousand feet of Miocene Columbia River Basalt Group (CRBG) but is readily identified by a gravity low against the Blue Mountains Uplift. The Paleocene/Eocene Herren Formation (Pigg, 1961), which outcrops on the Blue Mountains Uplift south of the Heppner Basin, offered good source and reservoir potential. Based on lateral extent, thickness and paleocurrent structures in the Herren Formation, the unit was expected to be present in the basin. Gravity modeling produced nonunique interpretations, thus magnetotelluric (MT) information was used to constrain the CRBG thickness. Static shifts in the MT data were removed using transient electromagnetic (TEM) data before MT data inversion. After extensive experimentation, adequate seismic data were obtained for structural mapping, but the seismic data were interpretable with confidence only after MT determinations of the CRBG thickness. As a result of the favorable geologic and geophysical information, the ARCO Hanna ♯1 well was drilled to 9100 ft (2800 m) near Heppner, Oregon in section 23, T2S, R27E in 1988. The thickness of the CRBG and Oligocene John Day Formation were accurately predicted by the geophysical interpretations. An unanticipated thickness of Eocene Clarno Formation was encountered and drilling ceased in this unit. No Herren Formation was penetrated during drilling. Geophysical well logs indicate the Clarno Formation has densities and resistivities sufficient to account for the gravity and electrical anomalies defining the prospect. Poor seismic quality was explained by the heterogeneous nature of the pre‐CRBG volcanic section encountered in the well.

This study examines the ability of a marine gradiometer to produce diurnal‐free total magnetic intensity data. Using Gulf’s research vessel HOLLIS HEDBERG, an experimental program was carried out in the Santa Barbara channel offshore California. A shore‐based station was operated continuously during the survey period to monitor the temporal magnetic activity. The gradiometer data are adversely affected by the magnetic field of the towing vessel. An analysis of gradiometer bias as a function of ship’s heading and deployment distance shows remarkable agreement with a model of the ship’s magnetic field proposed by Bullard and Mason in 1961. Based upon this model, analytic azimuth correction factors are developed and applied to the gradiometer data. Comparisons are made among single‐sensor, ship‐minus‐shore, and integrated total magnetic intensities. Residual mistie statistics, contour maps, and profile plots form the basis of the comparison. At this location, the ship‐minus‐shore and integrated total intensities produce maps with standard residual misties of 0.55 and 0.62 gammas, respectively. Because of the gradiometer’s operational simplicity and the uncertainty of spatial coherency of diurnal activity, the gradiometer is recommended for routine use in the marine environment. The gradiometer is very effective in eliminating diurnal activity, but it requires moderately more sophisticated data analysis procedures.

The comments made by N. C. Steenland address issues of the aeromagnetic interpretation and gravity data inconsistencies. These comments will be addressed individually. The authors are very familiar with the method of aeromagnetic exploration having used it in various countries, geologic environments and applications. The technique is extremely powerful when used appropriately, and magnetics were not discounted summarily. In any paper on exploration we believe it is important that all results be discussed, and a worse disservice to the industry would have been to ignore aeromagnetics. Indeed, we would have expected criticism had we not reported on our findings about the applicability of magnetics on the Columbia River Basalts (CRB). It is our experience that working with other basalts of different ages and sources, such as the Snake River Basalt example discussed by the reviewer, may not be a guarantee of obtaining appropriate interpretations elsewhere.