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                                                                  Exhibit (a)(4)

                       [SALOMON SMITH BARNEY LETTERHEAD]




March 8, 1998

The Board of Directors
3-D Geophysical, Inc.
599 Lexington Avenue
New York, New York 10022

Members of the Board:

You have requested our opinion as to the fairness, from a financial point of
view, to the holders of the common stock of 3-D Geophysical, Inc. ("3-D
Geophysical") of the consideration to be received by such holders pursuant to
the terms and subject to the conditions set forth in the Agreement and Plan of
Merger, dated as of March 8, 1998 (the "Merger Agreement"), among Western Atlas
Inc. ("Western Atlas"), WAI Acquisition Corporation, a subsidiary of Western
Atlas ("Subsidiary"), and 3-D Geophysical. As more fully described in the Merger
Agreement, (i) Subsidiary will commence a tender offer to purchase all
outstanding shares of the common stock, par value $0.01 per share, of 3-D
Geophysical (the "3-D Common Stock" and, such tender offer, the "Tender Offer")
at a purchase price of $9.65 per share, net to the seller in cash (the "Cash
Consideration") and (ii) subsequent to the Tender Offer, Subsidiary will be
merged with and into 3-D Geophysical (the "Merger" and, together with the Tender
Offer, the "Transaction") and each outstanding share of 3-D Common Stock not
previously tendered will be converted into the right to receive the Cash
Consideration.

In arriving at our opinion, we reviewed the Merger Agreement and held
discussions with certain senior officers, directors and other representatives
and advisors of 3-D Geophysical and certain senior officers and other
representatives of Western Atlas concerning the business, operations and
prospects of 3-D Geophysical. We examined certain publicly available business
and financial information relating to 3-D Geophysical as well as certain
financial forecasts and other information and data for 3-D Geophysical which
were provided to or otherwise discussed with us by the management of 3-D
Geophysical. We reviewed the financial terms of the Merger as set forth in the
Merger Agreement in relation to, among other things: current and historical
market prices and trading volumes of 3-D Common Stock; the historical and
projected earnings and other operating data of 3-D Geophysical; and the
capitalization and financial condition of 3-D Geophysical. We considered, to the
extent publicly available, the financial terms of certain other similar
transactions recently effected which we considered relevant in evaluating the
Merger and analyzed certain financial, stock market and other publicly available
information relating to the businesses of other companies whose operations we
considered relevant in evaluating those of 3-D Geophysical. In addition to the
foregoing, we conducted such other analyses and examinations and considered such
other information and financial, economic and market criteria as we deemed
appropriate in arriving at our opinion.

In rendering our opinion, we have assumed and relied, without independent
verification, upon the accuracy and completeness of all financial and other
information and data publicly available or furnished to or otherwise reviewed by
or discussed with us. With respect to financial forecasts and other information
and data provided to or otherwise reviewed by or discussed with us, we have been
advised by the management of 3-D Geophysical that such forecasts and other
information and data were reasonably prepared on bases reflecting the best
currently available estimates and judgments of the management of 3-D Geophysical
as to the future financial performance of 3-D Geophysical. We have not made or
been provided with an independent evaluation or appraisal of the assets or
liabilities (contingent or otherwise) of 3-D Geophysical nor have we made any
physical inspection of the properties or assets of 3-D Geophysical. In
connection with our engagement, we were not requested to, and did not, solicit
third party indications of interest in a possible acquisition of 3-D
Geophysical, nor were we requested to consider, and our opinion does not
address, the relative merits of the Transaction
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The Board of Directors
3-D Geophysical, Inc.
March 8, 1998
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as compared to any alternative business strategies that might exist for 3-D
Geophysical or the effect of any other transaction in which 3-D Geophysical
might engage. Our opinion is necessarily based upon information available to
us, and financial, stock market and other conditions and circumstances existing
and disclosed to us, as of the date hereof.

Smith Barney Inc. and Salomon Brothers Inc (collectively doing business as
Salomon Smith Barney) have acted as financial advisors to 3-D Geophysical in
connection with the proposed Transaction and will receive a fee for such
services, a significant portion of which is contingent upon the consummation of
the Transaction. We also will receive a fee upon the delivery of this opinion.
In the ordinary course of our business, we and our affiliates may actively
trade or hold the securities of 3-D Geophysical and Western Atlas for our own
account or for the account of our customers and, accordingly, may at any time
hold a long or short position in such securities. We have in the past provided
investment banking services to 3-D Geophysical unrelated to the proposed
Transaction, for which services we have received compensation. In addition, we
and our affiliates (including Travelers Group Inc. and its affiliates) may
maintain relationships with 3-D Geophysical, Western Atlas and their respective
affiliates.

Our advisory services and the opinion expressed herein are provided for the
information of the Board of Directors of 3-D Geophysical in its evaluation of
the proposed Transaction, and our opinion is not intended to be and does not
constitute a recommendation to any stockholder as to whether or not such
stockholder should tender shares of 3-D Common Stock in the Tender Offer or how
such stockholder should vote on the proposed Merger. Our opinion may not be
published or otherwise used or referred to, nor shall any public reference to
Salomon Smith Barney be made, without our prior written consent; provided, that
this opinion letter may be included in its entirety in the Solicitation/
Recommendation Statement of 3-D Geophysical relating to the proposed
Transaction.

Based upon and subject to the foregoing, our experience as investment bankers,
our work as described above and other factors we deemed relevant, we are of the
opinion that, as of the date hereof, the Cash Consideration to be received in
the Transaction by the holders of 3-D Common Stock (other than Western Atlas
and its affiliates) is fair, from a financial point of view, to such holders.


Very truly yours,

/s/ Salomon Smith Barney

SALOMON SMITH BARNEY